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Electrical Demons Tail/Stop Lighting Systems

Stop & Tail lights are a Combination type lamp in most cases, having a Double-Contact,
Double-Filament bulb. The tail light filament is about 4 candlepower, & the Stop
light filament is about 32 candlepower. the replacement bulb is the #1157 or #198.
Check to be sure you have the right light bulb for you application & that the bulb
is in good working order. Replace any bad or burned out bulbs at this time.

The number one problem I have found in rear lighting systems is Corrosion &/or Bad Grounds.
Be sure your lamp housings are clean & have no rust or corrosion. Be sure the lamps are
grounded to the truck body & that the truck body is grounded to the frame. Test your ground
by using a long jump wire, hook one end of it to the ground post of the battery, & the
other end to the lamp housing. If your light work as they should, repair the grounds.

Fire Wall Connecter

The Firewall connector Used 1962-1966 is just above the throttle Paddle.
For a Look at the 1960-61 Set up, Check David R Leifheit Site.

The fire wall connecter should like something like the drawing above. It is located just above the
Throttle Peddle and you must get to it from under the dash in side the cab. The stop lights are powered
through the Dark Green & Yellow wires. The rear tail lights get power from the Brown wire. At
this wiring connecter you can test all switches & in cab Wiring. With the light switch pulled out
& brakes applied, all three wires at the fire wall connector will have power & it should read 12
volts. If you have power on all three wires, check wiring from there to the tail lamps. Check the
plugs at the fire wall that feeds the wires to the rear of your truck & at the rear that feed to
the lamps. You want a tight fitting plug, with no corrosion. Now trace the wires to the rear &
check for broken or chaffed wires. Repair any bad spots in the wiring harness. Your lamps should be
clean & have no rust or corrosion. Be sure the lamps are grounded.

Testing Your Stop lights. The Stop lights get power from the the fuse box, stop light fuse.
check the Stop light fuse to see if it is good. Now on the pedal mount you should find
the stop light switch, it has two wires. Is there any power at the Stop light switch?
One wire should have power, if not trace the wire back to the fuse box & repair as needed.
If you have power, apply the brake & check for power on both sides of the switch. Next it
goes up the steering column. The Stop light problem can be in the steering column, best way
to check is it at the fire wall connection. You should have power there on TWO of the wires
(side by side in the connector) with the brake applied, if so, than it's OK, if not you'll
need to repair the wiring in the column. You most likely have a bad turn single switch,
as Stop light power is canceled by the turning switch.

Testing Your Tail lights. The tail light get power from the headlight switch, whick get power from
the fuse box. Be Sure to use the Switch & Wire Diagram for your year truck in all testing.

[1960-63 Light Switch] [1964-66 Light Switch]

[1960-63 Light Diagram] [1964-66 Light Diagram]

If the fuses are good check for incoming power at the light switch, it should be 12 volts. Next
Check for power on the tail lamps post with the switch in the on position. If no power, you
have a fulty headlight switch, replace with known good switch & retest. If you have power
on the tail light post of the switch but not the tail light wire at the fire wall connector,
trace wiring from switch to fire wal and repair as needed.

Turn Lights are the same as the stop lights, if they do not flash, replace the flasher can. If
one side or the other does not work, you'll need to check the turn switch and wiring in the
steering column. All these chacks can also apply to the front park/turn lights. For more
about the front Park/Turn lights, see the Park Lamps Page.

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Lighting & Electrical Projects
&/or Demons in Your GMC

Installing Fog Lights

Installing Cornering Lamps

Rear Fog Light Project

GMC Trucks Cargo Lights

HeadLight Trouble Shooting

Park/Tail/Stop Light Trouble Shooting

Custom Instrument Panel

Turn Signal Systems

Install the Delco-Remy 12SI Alternator

Trans Temp Gauge

Wiring Diagrams

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