Headlight circuit problem

MoparMuscleMan440

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It seems that I'm suddenly having an issue with an apparent short in the headlamp circuit of my '87 Fifth Ave. When the low beam lights are on, I'm seeing a slight voltage being chanelled to the high beam lamps, very faint, but noticeable all the same. Also, when you disconnect the left high lamp, the right high lamp comes on very bright, all the while being on the low beam selector position. Very perplexing. I checked the dimmer switch, and it apparently, according to the manual, tests OK. Anyone with any insight as to why this situation exists, or how I might resolve it, would be greatly appreciated. Thanks.
 

69-

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As high and low beam have the same ground, I would check on those for all lamps.
Further, if one of those dual light lamps goes bad inside, some strange things can happen. Had that with a partly bad stop/parklight bulb.
To test, just remove the connector of the headlights one by one.
 

Mikes5thAve

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2nd the ground.
There's one ground to the inner fender at the front that does strange things if it's bad. On mine the headlights were on very dim all the time.
 

Aspen500

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Like they said. A poor ground will make the voltage find ground however it can, like through a different bulb filament.

There should be a ground wire on each side on the inner fender, unless they changed things between '79 and '87 that is. That's were I'd start anyways and then go from there.
 

MoparMuscleMan440

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First of all, thanks for the input from those who gave me the advice concerning the ground connections to the headlight system. I took that advice and ran with it, unfortunately, even after cleaning up all ground connections, the same problem existed. This problem still exists, by the way, with brand new lamps in the system. So, I've narrowed it down to a possible faulty headlight switch, which I've ordered, brand new, and I'm going out on a limb, hoping that solves it. Got a great price on a NOS item, so based on that, I'm willing to take a chance. Like I said in an earlier post, I've checked the dimmer switch, and it tests ok, so, onward and upward! So, barring a short in the wiring somewhere, we should be good to go. I hope, lol.
 

Sub03

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Did you check the ground connection at the lamp too?

I had the same problem on a Chevy once. They get converted to H4 halogen bulbs here, but same electric laws apply.
The ground connection at the bulb was corroded, so like Aspen500 said, the current through the low beam filament finds ground through the high beam, making them both glow because they now is like to bulbs connected in series.
Cleaning the connector with som abrasive and a dash of contact cleaner fixed the problem.

I doubt the headlight switch is the culprit since it's just a on/off switch feeding the dimmer switch, but maybe I'm wrong.
Let us know what you find out.
 

Oldiron440

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The dimmer grounds at the mounting screws, are they louse?
 

69-

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The dimmer has no ground, just mounting screws.
There is just one cable in (power from light switch) and two out. One to high and one to low.
What happens if you switch from low to high?
 

Camtron

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I had headlight issues when I first bought my 5th Ave. was regularly burning out the drivers side low beam. The back of the headlight switch and harness connector had melted from a previous owner trying to splice something in at the headlight switch. Almost looked like tachometer wires buy the looks of them.
Cleaned up the wiring and connectors, got a new switch from a member on here and haven’t had any issues since.
 

Aspen500

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Something we say at work about electricity is, it is not an inanimate thing, it is a living, breathing, conniving, cynical son of a gun (edited for TV version). Even worse when you get into circuits that operated on millivolts. The slightest little thing, even fretting corrosion (corrosion so thin you can't even see it) will mess things up. It always takes a methodical approach, starting with the most likely cause of the problem, and the easiest to check, then go on down the line. Having a wiring diagram is important because you can then study it and form a plan of attack, or even eliminate areas that can't be the cause. Voltage drop tests work wonders with high amp circuits, using jumper wires to bypass a switch or relay is another way to verify or eliminate a part as the cause. There are times though when no cause is found and everything tests perfect, then it's back to the old "install known good part and retest". Note I said known good, not new. I had an auto instructor, way back when, that said "new just means new, it doesn't necessarily mean good", and I'll be darned if he wasn't right.

Doubt any of that random rambling will help you find the headlight problem though. :confused:
 
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MoparMuscleMan440

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The dimmer has no ground, just mounting screws.
There is just one cable in (power from light switch) and two out. One to high and one to low.
What happens if you switch from low to high?
Well, I've eliminated the dimmer switch, as being the problem. I tested it with an ohm meter across both circuits, and found no shorting occurring. To answer your question, however, when the high beams are activated, all the bulbs light up, but the low beams are noticeably dimmer than the highs are. On the low side, the highs are coming on faintly, as if there is a low voltage leaking to them. I have a NOS switch arriving today. We'll see what happens with that. If that doesn't correct it, the search and mystery will continue.
 

AJ/FormS

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If you have indeed proved the grounds from the headlights are seeing battery negative, then I would;
First prove the Fusible Link
then, if the Link is Not damaged;
I would bypass the dimmer switch, one circuit atta time. You said it tested good for no shorts; but in fact, it is a shorting device. It is supposed to short power from the headlight switch to ONE of the other two circuits; that is how it works. So when you test it, one side should test infinite resistance while the other side should show near zero resistance; and vice-versa.
However, I suppose power could be leaking between the two circuits, at the bulkhead connectors or as mentioned; in the sockets, or right in the bulbs themselves; both of which you have already proved. This only leaves, AFAIK, the bulkhead and/or the dimmer; but NOT the headlight switch, unless the connector to it is failing. At the dimmer you can pull some amps thru the headlight switch, and watch to see if the supply-voltage drops, which it should but Not by much.. If it does, well then , maybe the fault is in the headlamp switch, or in the wiring between it and the dimmer, or go back to the fusible link, or to any of the power connections between the headlight sw and the battery.
 
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