1956 - Section 12 - Chassis Electrical

SECTION 12 CHASSIS ELECTRICAL

The 12-volt electrical system, used on 1956 Chevrolet cars is basically the same as that used on 1955 Chevrolet passenger cars. Changes that affect service procedures and specifications are as follows:

The master light switch on the instrument panel has been improved through the addition of a fuse which protects the instrument panel light circuit. Mounting of the switch is the same as previous model. Fuse is located on top rear of switch.

The wiring harness has been divided into two -parts to facilitate assembly and service. The main section covers all wiring from the dash panel to the instrument panel and body, and the other section, or "front extension", covers wiring to the headlights, parking lights, horns and voltage regulator. The main harness begins with an 8-way connector held securely in a grommet in a hole at the left side of the dash panel. The "front extension" plugs into this connector, thus servicing electrical units on the engine side of the dash panel. Service procedures on the wiring system are greatly simplified because the front section can be easily unplugged from the rest of the harness.

On vehicles with a factory installed heater, the wiring harness also includes a connector for the radio power source, whether or not the radio is factory installed. This facilitates radio installation by the dealer. Indexed connectors make the wiring operation to the radio, heater and directional signals (directional signals are standard equipment in 1956) foolproof.

The taillamp assembly is enlarged to give better illumination of the rear of the vehicle. The left taillamp also provides a means of concealment for the gas filler neck. The headlamp bulbs are the new T-3 Safety-Aimer Headlamp. This headlamp bulb features better visibility through a brighter beam pattern in both beams. Full advantage of the driving beam is accomplished by the addition of an opaque metal shield built over the front of the lower beam filament which prevents stray light from escaping upward and reflecting back into the driver's eyes.

The T-3 lamp can be identified by the three molded aiming lugs protruding from the face of the lamp and located in triangular fashion around the outer edge. The tops of these lugs are precision ground to give a base for the Safety-Aimer to seat upon.

Due to the entirely different method of adjusting T-3 headlamps, the T-3 Safety-Aimer has been developed to be used exclusively with the T-3 headlamp. In "Safety-Aimer and Headlamp Adjustment" following this description, is a more concise description of the Safety-Aimer and its use.

The Type A Safety-Aimer incorporates an adjustable dial feature which permits a downward adjustment in excess of "2" such as required by the States for truck loading allowances. This dial reads in "inches of down aim" required for "2" for passenger cars to a maximum of 18" for commercial vehicles. A Type A conversion package is available through the Kent Moore Organization under No. J-6412 for adding the adjustable dial to aimer without this feature. All service operations below are performed with the Type A Unit installed on the Aimer.

The sealed beam headlamp features the T-3 headlamp bulb unit and a redesigned retaining and mounting ring. The retaining ring is held to the lamp body by a spring eliminating the screw attachment previously used. This feature simplifies sealed beam replacement as well as eliminates confusion that existed between retaining ring screws and adjusting screws.

Wiring

Two separate wiring harnesses, body wiring harness and chassis wiring harness, are provided in all passenger car models. Multiple connectors, which are located behind the left end of the instrument panel at the cowl, are provided to join the body and chassis harnesses. An optional accessory junction block, which is located on the upper inside of the dash panel to the left of the steering column, provides convenient power takeoffs and fuse clips for all accessories.

The wiring diagrams (fig. 1 and fig. 2) show all electrical units and circuits as well as positions of electrical units which are shown diagramatically. The plastic insulation of each wire is distinctively colored to assist in tracing circuits and making connections.

Additional wiring diagrams on body wiring, convertible top lift and electric window lift circuits are contained in Section 1. Circuits peculiar to the overdrive transmission are illustrated in Section 7.

Lighting System Description and Operation

The lighting and wiring units include the lighting switch, stop light switch, dimmer switch, headlamps and parking lamps, tail and stop lights, instrument and indicator lights, wiring harness and all other wiring except that covered under other circuits.

The lighting switch (fig. 3) incorporates 13 ampere dual thermal circuit breakers to protect the entire lighting system. One circuit breaker protects the headlamps and parking lamps, while all other lights on the vehicle are directed through the remaining circuit breaker. This permits the use of the headlights to be retained if an overload or short develops in another part of the system. A current flow of more than 13 amperes will cause the points of this current limit relay to open and close as they warm and cool. Thus the current to the affected circuit is rapidly interrupted and restored until the short is located and corrected. Protection against destruction of the wiring is thereby provided and possible rapid discharge of the battery averted. Fig. 4 Location of Adjusting Screws shows a circuit diagram for the lighting switch.

When the switch control knob is pulled out to the first position, an internal circuit is established to the switch terminals for the parking lamps, instrument lights and tail lights. When the knob is pulled out to the last position, a circuit is established to the switch terminals for the headlamps, instrument lights and tail lights. The current for the instrument lights passes through a rheostat which is regulated by the light switch knob. By turning the switch knob clockwise, the instrument lights can be dimmed or turned off completely. The dome lamp may be turned on while retaining use of the instrument panel lights by rotating the lighting switch knob to the extreme counterclockwise position (through the detent or "click").

The mechanically operated stop light switch, located behind the instrument panel and above the steering column, is attached to the underside of the brace supporting the brake pedal (fig. 11). When the switch is "off" in the pedal released position, the switch plunger is held into the switch by the pedal arm. When the pedal arm is moved forward to apply the brakes, a spring within the switch causes the switch arm to follow the pedal, closing the circuit within the switch and lighting the stop lights.

The dimmer switch is located on the toe pan to the left of the clutch pedal. When the headlights are on, this switch is used to change the lights from high to low beam or from low beam to high beam. Each time the switch button is depressed and released the lights are switched from one beam to another.

When the upper beams are in use a red indicator light will be visible through a small opening in the speedometer dial.

All models are equipped with "Sealed Beam" headlight units in which the light source, reflector, lens and gasket are all assembled in a securely sealed unit. Fig. 9 T-3 Safety Aimer Type A shows the component parts of the light. This sealed construction prevents tarnished reflectors and keeps dirt and moisture from the reflector and the inside of the lens. This assures maximum lighting efficiency throughout the life of the unit.

The "Sealed Beam" units have two separate filaments located in the unit in positions to produce an upper (country) light beam and a lower (traffic) light beam. The upper beam is designed to illuminate the road evenly and should only be used on the open highway when no other vehicles are approaching. The lower beam is adjusted to illuminate the right side of the road a reasonable distance ahead but to prevent glare in the eyes of oncoming drivers.

Parking lamps are mounted below each headlamp opening. A 3 candlepower single filament bulb is used on models without direction signal and a double filament 4-32 candlepower bulb is used on models equipped with direction signal.

All models use combination stop and tail lamps with double filament bulbs having 4 candlepower for the tail light and 32 candlepower for the stop light. Where equipped with direction signal, the stop light filament is used as a turn indicator light. The stop light is controlled by the stop light switch and the tail light by the lighting switch.

The rear license plate is mounted to the rear deck lid and is illuminated by a 3 candlepower lamp in each bumper guard.

The instruments are indirectly illuminated by three small bulbs installed through the back of the cluster. Each individual light socket is fitted with a snap-in feature so that it can be retained to or easily removed from its opening in the cluster. These sockets are also equipped with an electrical quick-disconnect feature which permits detachment of leads from the cluster without removal of the sockets. These lights are turned on by the regular light switch and can be dimmed by the light switch rheostat. Generator, oil pressure, direction signal and hi-beam telltale lamps are installed to the cluster in the same manner. See Specifications for bulb information on instrument and indicator lamps.

Service Operations

In general, care of the lighting units and wiring system consists of an occasional check to see that all wiring connections are tight, that the lighting units are tightly mounted to provide a good ground and that the wiring is not pinched or worn to a point that a short circuit might result. All lights should be checked regularly to see that they light correctly and do not flicker.

Sealed Beam Unit Replacement

Remove three headlamp door retaining screws and remove door with gasket.
Unhook the spring from the retaining ring ( fig. 5 ) and, holding sealed beam unit, retaining ring and mounting ring as an assembly, remove mounting ring from adjustingscrews. DO NOT disturb the adjusting screws.
Pull assembly forward, disconnect connector plug from sealed beam unit (fig. 6 ) and remove assembly.
While holding mounting ring, lift retaining ring as shown in (fig. 7) to release the retaining ring from the retaining ring tab opposite spring attachment of assembly. Remove retaining ring and sealed beam unit from mounting ring. While lifting retaining ring caution should be exercised to avoid damage to molded lugs on face of T-3 headlamp unit.
Place new sealed beam unit in mounting ring and position unit so that "top" inscription on front of glass is to the right of retaining ring tab. Then index molded lugs on back of sealed beam unit to indentations in mounting ring.
Index hole in retaining ring over tab on mounting ring (fig. 7).
Connect plug to sealed beam unit (fig. 6 ) and engage mounting ring adjusting screw tabs with vertical and horizontal adjusting screws.
Engage spring of lamp body in spring slot in retaining ring.
Install headlamp door with gasket and install retaining screws

Parking Lamp Service

To remove bulb, remove the three lens retaining screws and remove bulb.
To remove housing:
- Disconnect parking lamp wire from connector at fender skirt in area of regulator and horn relay.
- Free wiring from spring clips and pull through grommets in fender skirt baffle and fender skirt.
- From behind fender remove the three retaining nuts, then remove lamp assembly by pulling forward out of opening.
To assemble, reverse the above procedure and test operation of the parking lamp.

Tail and Stoplight Service

To replace bulb, remove two lens attaching screws, remove lens and replace bulb.
To remove right tail and stop lamp assebly, remove center screw at bottom of lamp housing, then remove four screws in center section of lamp holding lamp housing to fender. Disconnect haress plug from lamp and remove lamp.
To assemble, reverse the above procedure and test operation of the lamp.
To remove left tail and stop lamp assembly, remove center screw at bottom of door. Unlatch filler housing and remove four screws exposed by opening filler housing. Disconnect harness plug from lamp and remove lamp.
To assemble reverse procedure and test operation of lamp.

Sealed Beam Unit Replacement

Remove three headlamp door retaining screws and remove door with gasket.
Unhook the spring from the retaining ring (fig. 5 ) and, holding sealed beam unit, retaining ring and mounting ring as an assembly, remove mounting ring from adjustingscrews. DO NOT disturb the adjusting screws.
Pull assembly forward, disconnect connector plug from sealed beam unit (fig. 6 ) and remove assembly.
While holding mounting ring, lift retaining ring as shown in (fig. 7 ) to release the retaining ring from the retaining ring tab opposite spring attachment of assembly. Remove retaining ring and sealed beam unit from mounting ring. While lifting retaining ring caution should be exercised to avoid damage to molded lugs on face of T-3 headlamp unit ( fig. 8).
Place new sealed beam unit in mounting ring and position unit so that "top" inscription on front of glass is to the right of retaining ring tab. Then index molded lugs on back of sealed beam unit to indentations in mounting ring.
Index hole in retaining ring over tab on mounting ring (fig. 7 )
Connect plug to sealed beam unit (fig. 6 ) and engage mounting ring adjusting screw tabs with vertical and horizontal adjusting screws.
Engage spring of lamp body in spring slot in retaining ring.
Install headlamp door with gasket and install retaining screws
SEALED BEAM HEADLAMP OVERHAUL

Refer to (fig 9.)

1. Remove headlamp door retaining ring and sealed beam unit as instructed above. Note position of rubber gasket in groove of door, then remove gasket from headlamp door.

2. Unhook the coil spring from mounting ring and remove mounting ring.

3. Disconnect upper and lower beam lead wires from connector clipped to fender skirt near regulator and horn relay. Also remove screw and lockwasher securing headlamp ground wire to fender skirt baffle. Free headlamp wiring from spring clip.

4. Remove the four screws attaching lamp body to fender then pull harness through grommets in fender skirt baffle and fender skirt and remove lamp body and gasket. Remove adjusting screws from lamp body.

5. Work headlamp harness through grommet in body and remove socket and wire assembly. Replace all damaged parts.

6. To reassemble, work headlamp wires through grommet in body and install socket and wire assembly to body.

7. Work headlamp wires through grommets in fender skirt and baffle, then place gasket and lamp body in position and install the four attaching screws.

8. Install headlamp wires to connector and secure ground wire to baffle with screw and lockwasher. Install wiring to spring clip.

9. Install the two adjusting screws in body, then place mounting ring in position and install the spring on the lamp body to the mounting ring.

10. Connect sealed beam unit to lamp socket and install sealed beam unit, retaining ring and attaching screws.

11. Adjust the headlamp beam as previously described under Headlamp Beam Adjustment.

12. Install rubber gasket to headlamp door groove, then install door and tighten the two attaching screws.

LIGHTING SWITCH REPLACEMENT

1. Disconnect all wires from the lighting switch and either tag them for correct installation or note the relation of each wire to its proper terminal (fig. 3).

2. Reach under the instrument panel and depress the switch shaft retainer (fig. 10) and remove knob and shaft.

3. Remove the retaining ferrule. Also remove the escutcheon used on Bel-Air models only and remove switch assembly.

4. Place a new switch in position and install the escutcheon (Bel-Air models) and the ferrule.

5. Push the switch knob and shaft assembly into position so that the retainer seats in the groove.

6. Connect all wires to the proper lighting switch terminals (fig. 3) and check operation of lights.

STOPLIGHT SWITCH REPLACEMENT

Refer to Figure 11

1. From under the instrument panel adjacent to the brake pedal, disconnect the two switch wires from the connectors.

2. Remove the lock nut from plunger end of switch and remove switch. Early production switches are threaded into the brace while later run switches are retained by a second lock nut in lieu of threads.

3. Locate new switch in same approximate position on brace as switch previously removed.

4. Adjust switch position for proper operation of switch. Electrical contact should be made when brake pedal is depressed 5/8" from fully released position.

5. After proper adjustment, install lock nut, then install the two switch wires to the connector and check operation of switch.

DIMMER SWITCH REPLACEMENT

1. Lift interior floor mat and remove the two switch attaching screws.

2. From underneath car at switch location on left side of floor toe pan, lower dimmer switch and wiring to lowest point and remove wiring connectors from switch terminals.

3. Connect the wires to the proper terminals of a new switch (fig. 12) and place switch in position.

4. Install the attaching screws securely and test operation of switch.

Instruments and Gauges

Description and Operation

The generator telltale light, oil pressure indicator, fuel gauge, temperature indicator and speedometer come under the classification of instruments and gauges.

GENERATOR TELLTALE LIGHT

The generator telltale light, located in the instrument cluster, is connected in a circuit with the battery and generator through the ignition switch (see fig. 1-Section 6y). When the generator begins to charge, the voltage build up from the generator in the circuit opposes the battery voltage to the indicator light. As the differential between the generator and battery voltage in the circuit continues to decrease, the light will go out. It is emphasized that the telltale light functions primarily as an indicator of generator output, not necessarily of current flow to or from the battery as in the case of the ammeter.

OIL PRESSURE INDICATOR

The oil pressure indicator light is located at the right side of the instrument cluster. The light is connected in a circuit with the ignition switch and a pressure operated switch which is located in the main oil gallery on the left side of the engine. The switch is calibrated to close when oil pressure is 4-6 psi.

FUEL GAUGE

The fuel gauge consists of a dash or indicating unit and a tank unit plus the necessary wiring (fig. 13). The circuit is connected to the dead side of the ignition switch; therefore, it registers only when the ignition switch is on. The dash unit operates on the electromagnetic principle counteracted by a counterweight on the indicator hand. When the current is OFF the counterweight returns the hand to the "empty" position. When the switch is ON, current flows from the ignition switch to the fuel gauge dash unit, through the choke or limiting coil to the common connection between the two coils. From this point current can travel through the operating coil of the dash unit to ground and also over the wire to the tank unit and through this unit to ground. When the tank is empty the float is near the bottom of the tank and the contact finger will cut out the resistance of the tank unit. Most of the current will then pass to the tank unit and directly to ground while very little

TEMPERATURE INDICATOR

The temperature indicator consists of an indicating unit in the instrument cluster, an ether filled bulb in the cylinder head and a small tube forming an air tight connection between the two units. As the water in the cylinder head warms up, the ether expands causing the hand on the dash unit to move toward the right indicating the water temperature.

SPEEDOMETER

The speedometer and odometer is centrally located in the instrument cluster. The driven gear and shaft are of one piece nylon construction. The transmission end of the speedometer shaft is slipped into the transmission case with the driven gear coming to rest on the underside of the drive gear. The assembly is retained at the transmission by a key which is bolted to the transmission.

Service Operations

INSTRUMENT CLUSTER

All component parts of instruments and gauges which are installed to the instrument panel are contained within the cluster. The cluster may be conveniently removed from the opening above the steering column, after which any desired instrument or gauge may be detached from the cluster. AR indicator or cluster illuminating lamp bulb sockets are clip retained and may be quickly snapped in or out of position. Also, electrical quick disconnects are provided at each socket. Fuel, temperature and speedometer gauges are installed to the cluster with screws. Figure 14 shows all connections to the back of the cluster.

Removal

1. Slide rubber seal up along mast jacket a short distance, then remove screw and lockwasher from steering gear jacket lower cover and remove lower and upper covers (fig. 15).

2. Remove two screws retaining bottom of cluster. On Powerglide models, disconnect indicator rod from lever.

3. Remove two screws under lip of instrument panel cluster hood.

4. Pull cluster carefully toward rear of car out of panel opening. Disconnect leads to instruments and gauges at connectors as required.

Installation

1. With all instruments and gauges connected, place cluster in instrument panel opening.

2. Install four retaining screws, two to lower edge of cluster and two under instrument cluster hood lip. On Powerglide models install indicator rod to lever.

3. Place upper cover in position then install lower cover with attaching screw and washer. Slide rubber seal to position against covers.

GENERATOR TELLTALE

Ignition on, engine not running and telltale light off-

1. Indicator bulb burned out, replace bulb.

2. Open circuit or loose connection in the telltale light circuit.

Telltale light stays on after engine is started-

1. If light stays on at idle only, check for low idle speed.

2. Check generator output.

OIL PRESSURE INDICATOR

If the light does not come on when ignition switch is turned on, or if light stays on after engine is started, either the oil pressure is low or the wiring or a unit in this circuit is defective. Telltale light on, engine running-

1. Oil pressure low. First check oil level, then if pressure is still low, remove pressure switch and check oil pressure with a reliable pressure gauge.

2. Electric circuit grounded between telltale light and pressure switch.

3. Oil pressure switch not functioning properly. Replace switch.

4. Oil pressure switch calibration wrong. Increase engine speed slightly and check to see if light goes out.

Ignition on, engine not running and telltale light off-

1. Telltale light burned out, replace bulb.

2. Open circuit between light and ignition switch or between light and pressure switch.

3. Pressure switch stuck. Replace switch.

4. Pressure switch not grounded. Check threads of switch for foreign material.

FUEL GAUGE

The most common cause of fuel gauge trouble is high resistance in the circuit. Make sure all connections are tight and free from dirt, paint or corrosion.

Since the fuel gauge consists of two remotely located units and the connecting wires, it is sometimes difficult to determine which unit is at fault. A gas gauge tester No. KMO-204 (fig. 16) is available for testing the unit. If a unit is proved to be faulty it should be replaced.

If KMO-204 gas gauge tester is not available a tester may readily be made by obtaining a tank unit from parts stock. Attach a spring terminal clip to a 5-foot piece of insulated colored wire and connect the other end of this wire to the binding part of the tank unit. Next, attach two spring terminal clips to a similar piece of black insulated wire and the tester is ready for use.

Testing

A-

1. With ignition switch "OFF," disconnect the brown tank unit wire from back of gauge on instrument cluster.

2. Attach the colored wire of the tester to this terminal and black wire to a good ground.

3. Turn ignition switch ON-move tester arm up and down, dash unit should register FULL and EMPTY if it is OK. If so, turn ignition switch OFF and reconnect tank wire.

4. If dash unit does not register at all on above test, make certain before replacing it that it is getting current from the ignition switch. This can be quickly tested by connecting a 12-volt lamp from ignition terminal (pink wire terminal on back of dash unit) to ground.

B- If dash unit is OK, next check the wiring between dash and tank units as follows:

1. Disconnect tank unit wire at the tank unit.

2. Attach the colored wire of the tester to the end of the wire removed from the tank unit and clip the black wire to ground.

3. If on this test dash unit reads EMPTY at all times or the reading is noticeably lower than during the check at the dash unit, look for shorts or leaks in the wiring between dash and tank. Leaks are most likely to occur at terminal junctions. If dash unit reads above FULL at all times or if it reads higher at EMPTY and FULL than readings obtained when checking at the dash, look for poor connection or break in the wiring. Be sure all contacts are clean.

C-

1. If dash unit and wiring check OK, remove tank unit (see Section 8) from tank. Clean away all dirt that has collected around tank unit terminal as road dirt, particularly calcium chloride, causes an electrical leak that will cause an error in reading.

2. After cleaning thoroughly, connect tank unit to the wire leading to dash, grounding the tank unit with a short piece of wire from the outer edge to any part of the car. Turn ignition switch ON and move the float arm up and down. If this unit is OK, the dash unit will give corresponding EMPTY and FULL readings.

3. If tank unit is OK, reinstall in the tank if not, replace with a new tank unit but first repeat above test before installing in the tank.

NOTE: Always check tank units for freedom of movement of the float arm by raising it to various positions and observing that it will fall to EMPTY position in every instance.

Temperature Indicator

TEMPERATURE INDICATOR

The temperature indicator requires very little attention other than avoiding damage to the line between the indicator and the bulb. If this unit fails to register at all, look for leaks or restricted line. If the hand stays in the "hot" range it likely was caused by an overheated engine. The only remedy is replacement of the entire unit.

Replacement

1. Drain the cooling system until the coolant level is below the top of cylinder head. Remove fitting nut from left rear corner of cylinder head and remove indicator bulb.

2. Remove grommet from dash so that bulb can be pulled through. Remove instrument cluster and remove dash unit from cluster.

3. Work bulb of new unit through opening in dash being careful not to bend pipe excessively. Connect indicator securely to instrument cluster, install cluster and work the surplus pipe through dash. Install grommet, make necessary coil or long bends in pipe to take up slack, place bulb in fitting on cylinder head and tighten securely. Refill cooling system.

SPEEDOMETER SERVICE

The speedometer head requires comparatively little service, and as special equipment is required to render this service, most automobile dealers send the speedometer to an authorized AC speedometer service station.

Cable Replacement or Lubrication

1. Disconnect the speedometer cable from the speedometer head and fitting at the transmission. Remove the old cable by pulling it out from speedometer end of conduit.

NOTE: If old cable is broken it may be necessary to remove lower piece from transmission end of conduit.

2. Lubricate the lower 3/4 of cable with AC speedometer cable lubricant and push the cable into the conduit. Connect lower end to fitting on transmission and upper end to speedometer head. Road test vehicle for speedometer operation.

Head Replacement

1. Detach and pull instrument cluster assembly from instrument panel opening.

2. Disconnect speedometer cable from head.

3. Remove generator and oil pressure telltale and high beam indicator lamps from back of speedometer. Also remove main wiring harness from clip on back of speedometer.

4. Remove the five speedometer assembly to cluster retaining screws and remove speedometer and plate assembly.

5. Remove 2 screws, rubber pad, 2 rubber grommets and 2 grommet retainers connecting speedometer head plate to speedometer head and remove speedometer head.

6. Place rubber pad inside head plate over cable spindle, then install speedometer head in head plate. Push two rubber grommets and retainers through from the outside. Install 2 screws and tighten.

7. Position speedometer and plate assembly on cluster, install five attaching screws and tighten.

8. Connect speedometer cable to head, install lamp bulbs to back of speedometer and assemble cluster to instrument panel opening.

Horns and Horn Relay

Description and Operation

The two horns, which are located on the fender skirt baffles on all 1955 passenger car models, are designed to give a blended tone when operated together. Each of these horns uses a magnetically actuated diaphragm to develop a resonating air column in the horn projector.

The power plant of these horns ( fig. 17) consists of a field coil, a laminated pole piece, an armature linked to a diaphragm and a set of contacts in series with the field coil. Current flows from the battery through the field coil and contact points to ground whenever the horn circuit is completed through the relay. Current flow through the windings produces a magnetic field around the pole piece which attracts the armature toward the pole. Since the diaphragm is connected to the armature, movement of the armature causes similar movement of the diaphragm. The normally closed contacts are opened as the armature moves toward the pole, thus interrupting the current flow and collapsing the magnetic field around the pole piece. The "spring-back" of the diaphragm then causes the armature to return to its original position and the contacts re-close. This cycle is repeated many times per second resulting in a rapid vibration of the diaphragm. A resistance is connected across the con

The horn relay is a magnetically operated switch, remotely controlled by the horn push button switch. This relay completes the circuit directly to the horn through the relay contacts.

When the horn button completes the circuit, a small amount of current flows from the battery through the relay windings. The armature is attracted to the magnetic field, causing the contact points to close. Current from the battery then flows directly across the contact points in the relay to the horn. The relay is used to provide a higher voltage to the horn by avoiding a voltage drop in the long circuit to the horn button.

Service Operations

QUICK CHECKS FOR HORN TROUBLE

When analyzing faulty horn operation, any of the following three basic conditions may be found:

1. Horn will not operate.

2. Horn operates, but has poor tone.

3. Horn operates intermittently.

Horn Will Not Operate

If the battery checks GOOD, then this condition may be caused from:

1. Defective relay, horn button, or wiring.

2. Defects within the horn (See Horn Bench Checks).

To locate the trouble, connect a jumper lead to the "H" and "B" terminals of the relay (see Figure 21 for terminal location). If the horn blows, the trouble is in the relay, horn button, or wiring.

To determine whether the relay, horn button, or wiring is at fault, ground the "S" terminal of the relay (see Figure 21 for terminal location). If the horn blows, the horn button or wiring is at fault. If the horn does not blow and the wiring between the battery and relay is not defective, connect a voltmeter between horn terminal and the horn mounting nut. Again connect the jumper lead to the "H" and "B" terminals of the relay and note the voltmeter reading.

If no voltmeter reading is obtained, the wiring between the relay and horn is open or the horn is not grounded. If the voltmeter reading is less than 7.0 volts, the trouble is due to high resistance connections in the wiring or a faulty horn. If the reading is above 7.0 volts, the trouble is due to a faulty horn which should be removed for a bench check (see Horn Bench Check).

Horn Operates But Has Poor Tone

This condition may result from:

1. Low available voltage at the horn.

2. Defects within the horn (see Horn Bench Check).

Although the horn should blow at any voltage above 7.0 volts, a weak or poor tone may occur at operating voltages below 11.25 volts. If the horn has a weak or poor tone at an operating voltage of 11.25 volts or higher, remove the horn for a bench check (see Horn Bench Check).

Horn Operates Intermittently

This condition may result from:

1. Loose or intermittent connections in the horn relay or horn circuit.

2. Defective horn switch.

3. Defective horn relay (see Horn Relay Adjustments).

4. Defects within the horn (see Horn Bench Check).

HORN REPLACEMENT

1. Raise hood and disconnect wire from horn.

2. Remove nuts and lock washers retaining horn to mounting bracket and remove horn.

3. Install new horn and fasten securely to mounting bracket.

4. Replace wires to horn connections and test for operation. Adjust if necessary and lower hood (see Horn Bench Check).

HORN BENCH CHECK

Inspection

Remove the back shell and carefully check the horn for loose or broken leads and connections. Inspect the air gap to make certain it is free of foreign material such as burrs, steel shavings, etc. If no cause for failure is found, check the current adjustment.

NOTE: Horns usually have a rasping sound when vital parts are broken or loose. A loose backshell may affect tone. Tighten all collar screws, mounting nuts and studs, and replace all damaged parts. Excessive arcing at contact points or fused contact points may be caused by improper current adjustments.

Adjustment

To check the current adjustment connect an ammeter into horn circuit at horn terminal and measure the current draw of the horn while the horn is operating. The low note horn (identified by an "L" on the edge of the casting) should have a current draw of 8.5-10.5 amperes at 11.5 volts. The high note horn (identified by an "H" on the edge of the casting) should have a current draw of 7.5-9.5 amperes at 11.5 volts. To change the current adjustment, disconnect the battery, loosen the lock nut on the armature stud at the center of the horn (fig. 18) and rotate the current adjusting nut counterclockwise to increase the current or clockwise to decrease the current. This adjustment is very sensitive and the adjusting nut should be moved only 1/10 of a turn at a time and locked in position before operating the horn to recheck the current adjustment. Adjust the current to the specified value.

NOTE: The horn will not function properly if field windings within horn are open circuited, short circuited or grounded,. If there is no indication of current flowing when contact points are closed, windings are open circuited. The ammeter will indicate an excessive flow of current if windings are short circuited or grounded.

An alternate method of adjusting the horn is as follows: Insert a .007 inch feeler gage (not more than 1/4 inch wide) between the adjusting nut and the contact blade insulator (fig. 18). Do not allow the gage to touch the contact points. Connect the horn to a 12-volt battery and turn the adjusting nut to a position where the horn will just operate. Lock the adjusting nut in position and check the horn performance with feeler gage removed.

HORN RELAY ADJUSTMENTS

Three checks and adjustments are required on the horn relay; air gap, point opening and closing voltage. The air gap and contact point opening checks and adjustments should be made with the battery disconnected.

Air Gap-The air gap should not normally require adjustment unless the relay has been misadjusted. Check the air gap with the points barely touching and adjust if necessary by bending the lower point support (fig. 19). Air gap should be .014".

Point Opening-Check the contact point opening and adjust by bending the upper armature stop (fig. 20). Point opening should be .027".

Closing Voltage- To check the relay closing voltage, connect a variable resistance or potentiometer of sufficient value (not less than 10 ohms) in series with the "B" terminal, connect a voltmeter to the "S" and "B" terminals, and ground the "S" terminal as shown in Figure 21. Slowly decrease the amount of resistance in order to check the relay closing voltage. Closing voltage should be 5.0-9.5 volts. Adjust the closing voltage by bending the armature spring post (fig. 22). Bend down to increase the closing voltage and bend up to decrease the closing voltage.

Direction Signal

DESCRIPTION AND OPERATION

A six wire crossover circuit, which includes a direction signal switch and flasher unit, is used on the 1955 Chevrolet Direction Signal. This circuit utilizes the parking light for indication at the front and the stop light at the rear. Telltale indicators are also provided in the instrument cluster. Current is directed through the flasher unit to the switch, from where it is directed to the desired indicator lamps. A cancelling feature is provided in the mechanism to return the switch to the "off" position after completion of a turn. Overload protection is provided within the flasher unit.

Figure 2 shows the direction signal circuit diagram while Figure 23 shows a simplified 9 schematic of the installation together with the operation of the signal switch. Figure 24 shows an exploded view of the direction signal housing.

SERVICE OPERATIONS

ADJUSTMENTS

The direction signal switch requires no adjustments due to its simplicity of design. However, if any malfunction of this mechanism should occur, the steering wheel may be removed and the 2. mechanism checked for defective parts. Any defective parts should be replaced.

TROUBLE DIAGNOSIS

Presented below are a few of the more common direction signal troubles and their probable causes:

1. If indicator light in cluster burns steady when lever is placed in a turn position-

a. Check for burned out bulb in park or stop lamp.

b. Check for faulty flasher if park and stop lamp bulbs are both on.

2. If indicator light in cluster does not bum when lever is placed in a turn position-

a. Check for burned out bulb.

b. Check for a faulty flasher.

3. If switch fails to cancel after completion of turn, worn or broken parts within the mechanism in the housing is indicated. The steering wheel should be removed and the mechanism checked for defective parts.

Troubles and Remedies

Symptom and Probable Cause
Probable Remedy

HEADLAMP AND CIRCUIT

Headlights Dim (engine idling or shut off)

Partly discharged battery
Charge battery

Defective cells in battery
Replace battery

High resistance in light circuit
Check headlight circuit including ground connection. Make necessary repairs

Faulty bulbs
Replace bulbs or sealed beam units

Headlights Dim (engine running above idle)

High resistance in lighting circuit
Check lighting circuit including ground connection. Make necessary repairs

Faulty bulbs or reflectors
Replace sealed beam units

Faulty voltage control unit
Test voltage control and generator. Make necessary repairs

Lights Flicker

Loose connections or damaged wires in lighting circuit
Tighten connections and check for damaged wiring

Light wiring insulation damaged producing momentary short
Check light wiring and replace or tape damaged wires

Lights Burn Out Frequently

High voltage regulator setting
Adjust voltage regulator

Loose connections in lighting circuit
Check circuit for loose connections

Lights Will Not Light

Discharged battery
Recharge battery and correct cause

Loose connections in lighting circuit
Tighten connections

Burned out bulbs
Replace bulbs or sealed beam unit

Open or corroded contacts in lighting switch
Replace lighting switch

Open or corroded contacts in dimmer switch
Replace dimmer switch

Thermal Circuit Breaker Causing Current Interruption

Short in wiring
Check wiring of circuits in use for short circuits and make necessary repairs

Short within some light or instrument in use
Check lights or instruments for short. Headlamps and parking lamps are on separate circuit breaker from remainder of lighting units.

Gasoline Gauge

Gauge Shows Empty at All Times

Tank unit shorted Replace unit
Replace unit

Wire from dash unit to tank unit shorted
Replace wire or repair short

Float stuck in empty position
Replace tank unit

Dash unit improperly grounded on instrument panel.
Properly ground dash unit

Gauge Shows Full at All Times

Tank unit burned out
Replace tank unit

Wire between units disconnected or broken
Connect or replace wire

High resistance in wire between units
Clean-connections and terminals

Float stuck in full position
Replace tank unit

Gauge Does Not Register Accurately (within normal limits)

Bent hand on dash unit
Replace unit or straighten hand

High resistance in circuit
Check and correct circuit

Partial short in circuit
Correct cause of short

Loose electrical connections
Tighten connections at dash unit and tank unit

STOPLIGHT AND CIRCUIT

Will Not Light

Switch faulty
Replace switch

Wires broken, disconnected or loose
Make necessary repairs

Bulb burned out
Replace bulb

Loose connection or poorly grounded lamp body
Tighten loose connection or properly ground lamp body

Will Not Blow

Loose connections or broken wire
Tighten loose connection or replace broken wire

Horn button not making contact
Adjust horn button contact

Horn improperly adjusted or faulty
Adjust or replace horn

Defective horn relay
Adjust or replace relay

Horn Tone Poor

Horn improperly adjusted
Adjust horn

Horn Operates Intermittently

Loose connections or intermittent connections in horn relay or horn circuit
Correct loose or intermittent connections condition or replace horn relay

Defective horn switch
Adjust horn button contact

Defective horn relay
Adjust or replace relay

Defects within the horn
Adjust or replace horn

GENERATOR TELLTALE LIGHT

Ignition on, Engine Not Running, Telltale Off

Indicator bulb burned out
Replace bulb

Open circuit or loose connection in telltale circuit
Locate open circuit or loose connection and correct

Telltale Light Stays On After Engine is Started

If on at idle only, improper idle speed
Adjust idle speed

Low generator output
Check generator output

OIL PRESSURE INDICATOR

Telltale Light On, Engine Running

Circuit grounded between telltale light and pressure switch
Locate and correct grounded condition

Oil pressure switch not functioning properly
Replace switch

Oil pressure switch calibration wrong
Replace switch

Oil pressure low
Correct as necessary

Ignition On, Engine Not Running, Telltale Off

Telltale light burned out
Replace bulb

Open circuit between light and ignition switch or light and pressure switch
Find and correct open circuit

Pressure switch stuck
Replace switch

Pressure switch not grounded
Check threads of switch for foreign material

Specifications
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Bulb Specifications
 
Name                              Candlepower    Number

Headlamp Unit - Upper..........   50 W       Sealed Beam
              - Lower..........   40 W       Sealed Beam
Parking Lamp
  Without Direction Signal.....     3               67
  With Direction Signal........  4-32             1034
Tail and Stop Lamp.............  4-32             1034
Direction Signal Telltale Lamp.     1               53
License Plate Lamp.............     3               67
Ignition Switch Lamp...........     1               53
High Beam Indicator Lamp.......     1               53
Instrument Cluster Lamp........     2               57
Clock Lamp.....................     3               67
Glove Compartment Lamp.........     2               57
Dome Lamp......................    15             1004
Rear Quarter Lamp-Sport Coupe..     6               90
Courtesy Lamp-Convertible.......    6               89
Generator Indicator Lamp........    2               57
Oil Pressure Indicator Lamp.....    2               57
Powerglide Quadrant Lamp........    2               57
Radio Dial Lamp.................    2               57
Heater Control Panel Lamp.......    2               57
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Horn Relay

Air Gap...................................  .014"
Point Opening.............................  .027"
Closing Voltage........................5.0-9.5 volts

Horns


Current Draw
  Low ("L") Horn.........8.5-10.5 amperes @ 11.5 volts
  High ("H") Horn........7.5- 9.5 amperes @ 11.5 volts
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Assembly Manual

Electrical
Wiring
Instrument Cluster
Instrument Cluster Wiring - LD
Instrument Cluster Wiring - RD
Ignition Lock
Light Switch
Cigarette Lighter
Clock and Bezel
Glove Box Light
Stop Lamp Switch and Signal Flasher
Battery Support and Cable
Dimmer Switch
Resistor, Oil Pressure Switch and Thermo Gauge
Starter Motor Wiring
Coil and Spark Plug Wires
Voltage Regulator and Horn Relay
Horn
Generator Wiring
Headlamp
Headlamp Adjusting
Parking Lamp Mounting Panel
Parking and Signal Lamp
Tail and Stop Lamp
License Plate Lamp
License Plate Bracket
Speedometer Shaft
Clip and Grommet Location - LD
Clip and Grommet Location - RD
Signal Switch
Wiring Diagram - LD
Wiring Diagram - RD

SECTION 12

CHASSIS ELECTRICAL

CONTENTS OF THIS SECTION

Description and Operation

DESCRIPTION AND OPERATION

SERVICE OPERATIONS