Headlight switch is grounding 12v to body

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Uncle Ben

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Hey guys im new to the forum, ive read many threads on headlight issues and dimmer switch problems and i have checked those components already.

The car is a 1971 Demon sizzler with a 340 swapped in, 4 speed as well.

Ive been putting together a rallye dash and ive purchased new harnesses from year one already. The only harness I haven’t changed was the tail light harness. Figured it should be plug and play but alas nothing ever is. All the lights were working a couple months ago but now i have no dash, headlights, marker lights, or tail lights. When i use my power probe on the car with the key off and headlight switch on, the body of the car is reading 12.5v! Also ive noticed with the headlight harness plugged in with the key in acc. or run it also reads 12.5v. Ive pulled the entire harness back out of the car and cant see anything wrong with it. Any advice?
 
Where and how are you measuring this?

I’ve probed multiple locations on the body that should give me good ground readings. Including one of the screws holding the dimmer to the floor. The results are the same regardless of location. Im using a “Power Probe” tool. Very nifty little device. Allows you to supply power or ground to any circuit to check wiring or bulbs, like the dash lights for example i can add voltage there to illuminate the whole cluster. Passively you can touch the probe to a wire and it will tell you if its hot or ground like a multimeter.
 
You are WAY better of to actually learn what you are measuring with a conventional multimeter. Somebody is missing something here, I know not what. In this case even a simple test lamp might be better to find the problem

"Voltage" is always in reference to "something else." You can NOT have "voltage on the body" unless the battery is ungrounded, and even then, it will be in reference to the battery NEG post, or the POS post whatever.

For all I know, you don't have a ground wire from the block or the battery to the body
 
Keep in mind that a function of the light switch is the dome light, for this to work the switch connects to ground, it becomes the door switch.

I have never followed this ground through the switch to see how and where it is but it is there.


Alan
 
You are WAY better of to actually learn what you are measuring with a conventional multimeter. Somebody is missing something here, I know not what. In this case even a simple test lamp might be better to find the problem

The power probe is both a multimeter and a testlight in one. Regardless i think you might be on to something with the body not being grounded. I have a strap to the engine but nothing direct to the body. Ill try that too. The car hasnt been on the road since 2012 from what i can tell, the original owner had really butcher’d the poor thing. So im not sure whats been removed. Like i said brand new wire harnesses. Also should i attach a ground to the rallye cluster?
 
I’ve probed multiple locations on the body that should give me good ground readings. Including one of the screws holding the dimmer to the floor. The results are the same regardless of location. Im using a “Power Probe” tool


If you are getting 12v between the body and the NEGITIVE battery terminal you are missing a ground between the block and the body AND you have a path to the positive terminal of the battery thru any number of things, lights, motors etc.

If you are hooking the positive side of your test light to battery positive it will light up when touched to any ground on the car.

This is why I am asking for specifics on where and how you are testing.

As 67dart273 said. A multimeter will diagnose things better than a test probe light.

Always go from battery negative to your voltage source.
 
UPDATE: i went to the store and purchased a ground strap to connect the battery to car body. Wont have time until Sunday to see if it helps with the issues. Thanks everyone for the input, helps straighten out my head...
 
UPDATE: installed the ground cable and... what’do ya know! EVERYTHING WORKS! I also added a ground cable to the back of the rallye cluster and i can say that also helped the dash lights out, definitely not flickering like they had before.

Thanks for everyone’s help! You guys really are the best knowledge base around!

Such a simple thing to overlook!

THANKS AGAIN EVERYONE!
 
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