This question is about my new truck project that I just got. It doesn't really have a slant 6, but I've always gotten great advice on this forum, so I thought I'd ask.

I just noticed that the left rear tail light on my '73 D-100 truck project stays on all the time as long as a live battery is installed. Doesn't matter if the light switch is on or not. Also, with the switch on, there are no headlights! I do get the front running lights with the switch on, and the turn signals also work with the switch on, except the right rear which is always dead (probably a bad socket or dead bulb)!

So basically I have front running lights with the switch on, and some turn signal response, no headlights ever, no brake lights, and a left rear that stays on always unless I disconnect the battery.

Does this all sound like a bad light switch?
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vapor genie

I would look at the headlights before condemning the switch. Don't forget the dimmer switch either. I'd look at the back of the truck for the tail light problem. Some people really screw up wiring when they try and rig up trailer lights. If everything out back looks decent then move forward.

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Joshua

Both headlights show continuity across the bulb spades, so the filiments are good, they just arn't getting any voltage. The back wiring may be bad, but the left rear light wouldn't be on with the switch off unless voltage was traveling from up front some other way, so I doubt the problem is in the back. According to the wiring diagram, the rear running lights get power directly from the 'R' terminal on the light switch, so as I see it, the only way that light can be on with the switch off is a problem with the switch.

Anyone agree?
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medical marijuana doctor

Bad switches can do a variety of things. Various other things can cause the switch to come under fire.

Here is my .02

"R" terminal has 1 wire output and runs both parklights in the rear in parallel. In this case, either the wire going from said illuminated parklight to the other side is bad/bad connection, or there is a ground issue on that socket (as you stated as a possibility) which is why the other isn't on.

Sounds like a short to battery power (either in the switch, or under the dash) for this issue.

Headlights don't work, so I am going to guess either bad wire/connection from the switch to the high/low beam switch, bad high/low beam switch, bad main switch, or bad wire/connection from low/high beam to headlights. I am unsure of whether the high/low beam switch is before or after the main switch. I don't have a diagram infront of me to check.

The brake lights don't work , but that has nothing at all to do with the function of running lights. That is most likely related to the switch on the pedal, it either has no power, the switch itself is bad, the turn signal cam is bad in the column, or the wire running into the turn signal cam is bad/has bad connection. Since you have turn signals that kinda work, that verifies that the output wire from the turn signal cam is at least functional.

Far as the right rear that doesn't work, that should be an easy fix, and is not causing the issues you have otherwise.



~THOR~

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1984 D100 Shorty Custom
Certified Auto Appraiser - RevItUp Classic Appraisals
President - Cherry City Bombers CC
Part of Tyrde-Browne Racing

Thanks THOR and Joshie. Thor, good stuff to know. I also think the switch is somehow passing B+ even while off. I'll check these things and get back here.
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R129

If you want to eliminate the headlight switch as the power source for the tail light just unplug it. I was suggesting that someone mistakenly got trailer auxiliary power connected to the one light.

It's not at all difficult for the headlight switch to go bad and not power the headlights, but all the headlight power goes from the dash switch then through the dimmer switch on the floor.

As Thor mentioned the brake light circuit goes from the fuse box to the brake light switch and through the turn signal switch to the brake lights. The turn signal switch interrupts the circuit to the brake lights to flash them.

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Joshua

On several occasions I've seen the plastic multipin connector for the headlight switch melt due to heat from corroded connections - you might want to take a careful look at that too.

I changed out the headlight switch (and did a little repair of the rear wiring) and now have all rear lights working correctly, but still no headlights. I read voltage at the dimmer switch at both outlets and got good voltage, so it must be some wiring problem between the dimmer and the headlights. I'm checking further.
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R1100RT

Given you have no high or low beam lights (if I read you correctly), a faulty ground comes up as a prime suspect. The sheetmetal (body) grounds for the headlamps were cheap and easy for the factory to implement, but not electrically very good when new, and they get much worse with age. Put your voltmeter or test light across the headlamp socket + slots and battery negative (or alternator housing) and see if you show line voltage that way. Left vertical slot high beam feed, top horizontal slot low beam feed, right vertical slot common ground. If you find this to be the case, or you discover faults in the feed wiring, you may want to rework the wiring this way to fix the problem all the way fixed.

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