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What the heck? Headlight issue..

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farna View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: Jan/24/2018 at 6:42am
Ya made me pull my 68 TSM and put my reading glasses on! I can still see fine to read, but fine print and lines drawn close together is easier with a little help...

Okay, if you have voltage on the headlight plug with the dimmer switch disconnected you have a serious wiring problem somewhere. There are only two wires going from the dimmer to the headlights, a white (bright) and green (dim).  Should be two white wires on the same connector, the other one goes to the instrument panel connector (bright indicator light on dash).

At the headlight connector there is a third black wire, which goes to ground. Doesn't matter where that ground wire connects as long as it's a good ground. Both headlights may ground at the same point. Wiring diagram indicates that both headlights and side marker lights are spliced, with another wire going between that splice and the body ground. Body ground point has a wire going to a terminal on the headlight switch and a wire to the blower motor.

The only way you can get feedback on the ground wire is if one of the other two wires are grounded or shorted to a wire carrying power. Or the headlight switch isn't wired right. The only wire at the dimmer that should have power when the headlight switch is on is a brown wire.

I don't recall if the headlight switch has a single one position connector or individual wires. If it has a single connector it could be bad and shorting internally. Check all those connections against a wiring diagram. If those are good you need to check the wires themselves. Just unplug the headlights (both!) and the dimmer. Connect a test light to the + post of the battery. Touch to the wires in the connector -- if you get a light the wire is shorted to ground somewhere. The only one that should light up is the black in the headlight connector, none of the wires on the dimmer plug should get a light with the switch off (brown is power, so don't touch it with switch on!).
Frank Swygert
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Post Options Post Options   Thanks (0) Thanks(0)   Quote UNFORD Quote  Post ReplyReply Direct Link To This Post Posted: Mar/25/2018 at 9:58pm
I finally had a chance to do some more trouble shooting on my headlight issue. Since I trailer the car having the headlights working is not high priority right now.

Today I unplugged the harness from both headlights and metered the plugs to see if I have a good ground to the headlights. I metered directly from the battery post to each of the headlight plugs' ground wires, they have separate ground wires, not shared, and got around 4.9 ohms on each plug.  Does that sound about right for the amount of resistance in 8-10 feet of wire from the plug back to the battery? If it is then I'd say there is no grounding problem.

Next I turned on the headlight switch to see how much voltage I was getting at the headlight plugs. I could only measure one side of the dimmer switch output because the car was in the trailer and I couldn't activate the dimmer switch below to check the other side. What I did get was about 2.9V on both headlight plugs on the one wire I could check voltage on. Don't know if it's high or low beams, as explained previously.

Although two months ago when I took my left kick panel off and checked the dimmer switch, I only metered the resistance from the center lug to each side lug and it seemed to be okay to me. At the time I didn't think it was necessary to measure the voltage through the dimmer switch since the resistance check seemed okay. I did have 12V at the dimmer switch harness center plug.

Each of the headlight plugs has it's own ground wire, not shared with each other. Another thing I found was that the left headlight plug has one of its spade connectors inside the plug looking messed up. It might not be making a good connection on the left headlight, but that shouldn't stop the other side of the bulb (high/low) from working, nor should it stop the right headlight from working either. I think I should be able to replace the plug by carefully removing the connectors inside first.

So before today I was thinking grounding issue. Right now I'm leaning towards bad dimmer switch again. Ideas or comments?
Mark Melvin
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Gelalthedamned Quote  Post ReplyReply Direct Link To This Post Posted: Mar/26/2018 at 3:38am
I deleted my dimmer switch after it came apart in 5 pieces. The dims are straight wired to the hot feed now.

You can bypass that dimmer with a jumper to see if the lights fire, that’ll rule in or out a bad dimmer.

What kinda headlights you got? I had some fancy blue ones once that’d go out on random occasions... cause the amperage draw was too much for the tiny alternator.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: Mar/26/2018 at 6:14am
Sounds to me like either a bad dimmer or bad wiring between dimmer and headlights. You need to check voltage coming OUT of the dimmer.... might have to skin a bit off a wire...
Frank Swygert
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bigbad69 Quote  Post ReplyReply Direct Link To This Post Posted: Mar/26/2018 at 8:46am
Originally posted by UNFORD UNFORD wrote:

... and got around 4.9 ohms on each plug..
When measuring low resistances, you should baseline the reading by shorting the two probes together. Ideally, you get a reading of .1 or .2 ohms. You may have to twist the probes back and forth at the meter to break the oxidation. Once you establish your "0 ohm" baseline, subtract that amount from whatever reading you  get. That 4.9 ohms you are reading may be mostly in your measuring device.

Having said that, I agree with Farna, and think you may have a problem with the dimmer switch.  Bypassing it at the harness connector will tell you for sure.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Lyle Quote  Post ReplyReply Direct Link To This Post Posted: Mar/26/2018 at 9:35am
Try shorting the meter terminals like stated above and then measuring from the battery ground to the engine block or good bare spot on the chassis. If you still get 4.9 ohms then your battery ground cable or it's connection point is bad. Fix this first before trouble shooting anything else.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote UNFORD Quote  Post ReplyReply Direct Link To This Post Posted: Mar/26/2018 at 11:23am
Jumping the plug is such a simple idea I didn't even think of it. Ha! That's funny for me because I work on elevators and almost on a daily basis have to use jumpers on control circuits to find out what's going on and get'em running. So ya, next Saturday I'll have a jumper ready to go and give that a try.

Headlights are stock run of the mill..

Originally posted by Gelalthedamned Gelalthedamned wrote:



You can bypass that dimmer with a jumper to see if the lights fire, that’ll rule in or out a bad dimmer.

What kinda headlights you got? I had some fancy blue ones once that’d go out on random occasions... cause the amperage draw was too much for the tiny alternator.
Mark Melvin
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Post Options Post Options   Thanks (1) Thanks(1)   Quote UNFORD Quote  Post ReplyReply Direct Link To This Post Posted: Apr/09/2018 at 2:30am
I wanted to give one final update for everybody. The headlight issue has been resolved.

Today I took my car over to John Siciliano's house. He was preparing to drop a newly rebuilt engine into his wife's 69 AMX and Allen Tyler was coming over to help. Both would be available to help look at my car afterwards.

Last weekend on my own I did some additional troubleshooting. I measured the voltage at the headlight plug and was getting 12V, on all three connections including ground. Okay, the problem is not on the hot side of the circuit. It's on the ground side, right? It makes sense now when someone previously commented that the 12V measured on the ground might be finding a path through the filament of the bulbs.

John had a new toy to help us today, a tool called a "Power Probe." It gives a red light for a positive voltage detected and a green light if ground is detected. It also has a LCD screen to show the actual voltage. Fairly handy.

Soon we had the problem narrowed down to the exact point. Can you believe it, a poor ground? That's never happened before to anyone. The stud above the blower motor evidently was a poor ground for the two black wires that are bolted to it, which one or both appears to be the ground wire for the headlights. The headlights had been working fine for the first two years after the car's restoration but recently had decided to become a problem.

With the headlights fixed we turned our attention to the dash lights that were no longer working. After going around in circles for a while and taking the dash apart we found that the fuse to the dash light circuit was open. With a new 9A fuse in place everything electrical was back in working order.

Good times..
Mark Melvin
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Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: Apr/09/2018 at 7:43am
I had problems several times with glass fuses that looked good but were open on the end inside on my motorcycle. Sort of understandable since most motorcycle engines are hard bolted to the frame and there is more vibration. After it left me stranded the second time I yanked the little fuse panel out and wired in a modern fuse panel with the blade type fuses. Only took a little 6 fuse panel. I fed the panel with a relay from the switch as well, so switch just activated relay. That last time I was stranded I DID look at the fuses, but you can't tell if one is broke in the very end! Should have stuck the spare in the main fuse location anyway, could have rode it home...
Frank Swygert
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 6PakBee Quote  Post ReplyReply Direct Link To This Post Posted: Apr/09/2018 at 8:43am
That Power Probe looks like an amazing trouble shooting tool.  And it doesn't cost a gazillion $ either.
Roger Gazur
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All project cars.

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