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What the heck? Headlight issue..

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UNFORD View Drop Down
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    Posted: Jan/21/2018 at 7:28pm
I spent about 2 hours trouble shooting an electrical issue on my headlights on both Saturday and Sunday, total of about 4 hours. Should have stayed on it longer today but wanted to watch the first playoff game.

On my 68 AMX the headlights stopped working. I discovered that if I turned the headlight switch all the way counter clock wise to the click position to turn on the courtesy lights that the headlights would come on. I figured that the headlight switch was bad so I ordered a new one and installed it, but still no headlights.

After swapping out the switch if I turn the switch fully counter clock wise for the courtesy lights, the headlights don't come on where as they used to before I swapped out the switch.

So now I realized I had to kick the trouble shooting up a notch and rethink everything. Instead of replacing parts for the sake of replacing parts, I wanted to find and fix the actual problem.

After looking at a schematic I decided to take a look at the dimmer switch. A few friends I talked to on the phone about the problem agreed, ya, ya, the dimmer switch.

So I pulled the carpet back to get at the dimmer switch, used a multimeter to test the switch and determine it was okay. I moved on in the trouble shooting. The AMC manuals say there is a breaker inside the headlight switch. I believe the voltage goes from the battery, to the headlight switch, through the dimmer switch, out to the headlights, bypassing the fuse panel.

I checked the voltage at the dimmer switch plug. There was 12v on all three terminals. didn't seem right but what do I know. So put the plug back on the dimmer switch and moved to the front to check the headlight plug itself. I found 12v on all three terminals there as well. 

Wait, the black is ground, the white and green wires are the high and low wires. How can I have 12v on the ground wire? I pulled the plug off the dimmer switch and had same results. 12v on all three terminals at the headlight plug.

I decided to wrap it up for the day and go watch the game and consult the wiring prints at home and make a few more phone calls.

The same friend I called earlier said that with the new information about 12v on the headlight ground he thinks the turn signal is the next place to look. I told him, ya, the turn signal lever was a little bit floppy and loose.

So if the turn signal switch is the culprit, how is the voltage feeding back into the headlight ground?
Mark Melvin
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bigbad69 Quote  Post ReplyReply Direct Link To This Post Posted: Jan/21/2018 at 7:46pm
Headlights and turn signals are not interconnected.

There is a separate ground connection for the headlights. If memory serves, it is on the rad saddle (maybe on the firewall, I forget). Make sure it is a good connection - measure it with your meter.

I would re-check the dimmer switch. The meter you used to check it does not draw a lot of current and may hide problems. Pull the plug off the switch and insert a jumper wire on the harness to bypass the switch. If the headlights work, it's the dimmer switch. If they don't, then suspect your replacement headlight switch
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 70amxvegas Quote  Post ReplyReply Direct Link To This Post Posted: Jan/21/2018 at 9:12pm
Ground them that is the issue
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tomj Quote  Post ReplyReply Direct Link To This Post Posted: Jan/21/2018 at 9:20pm
car harnesses often have multiple grounds, often a black wire that exits the wrapped bundle in some innocuous (and hidden) place. don't AMX/Javs have one on the firewall somewhere related to lighting?

grounds have to be solid and clean like any other connection. bright shiny metal at all points including inside the crimped ring terminal, the metal it's bolted to, and the screw itself.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote billd Quote  Post ReplyReply Direct Link To This Post Posted: Jan/21/2018 at 9:55pm
Headlights will often go back and seek ground backwards - and you can't tell by looking if a connection is good. (I mean like the others are suggesting, if they can't ground you often find that because the high and low interconnect as far as GROUND, if the ground isn't good you'll have ~12v where there wouldn't otherwise be)
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Post Options Post Options   Thanks (0) Thanks(0)   Quote george w Quote  Post ReplyReply Direct Link To This Post Posted: Jan/22/2018 at 7:26pm
It sounds as if you lost the headlight ground somewhere between the headlight socket and the body.
There's only one ground for both headlights and it's on the passenger side of the body. The headlight grounds attach in the area of the radiator support panel or inner fender area. First off you should have 12 volts going into the floor dimmer switch and only on one output terminal at a time. You're better off simply unplugging the connector at the dimmer switch and metering for continuity between the center input terminal and both output terminals to insure only the high or the low beam terminal have continuity at the same time.

Next remove BOTH headlights to insure that you don't have shorted together filaments in one of the lamps. Then check to make sure you have a good ground connection between the ground terminal of each headlight plug and the body. If you have an open ground you'll see 12 volts on the ground side which can be misleading. You have to have a good ground to show a voltage drop across a lamp bulb. Without a ground path on one side you'll see the supply voltage on both sides of the lamp filament.

One more thing. The OEM headlight switch should have a grounding terminal riveted to the metal at the back of the switch. There's a single ground wire that goes to this terminal to provide the ground path for the interior lights when the headlight switch is rotated fully CCW. This extra ground terminal is provided because the plastic dash does not provide the ground path that a metal dash would. Make sure your new switch has this ground if you want your interior lights to work when the switch is rotated fully CCW past the detent. This terminal also provides the ground path for the electric wipers assuming: A, that you have electric wipers and B, that the copper ground strap is still there behind the dash that ties the headlight switch body to the wiper switch body.

Edited by george w - Jan/22/2018 at 7:35pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: Jan/23/2018 at 6:59am
I was wondering about that copper ground strap and extra wire. Not sure the 68 AMX has them, but check! I suspect the dimmer switch is bad, but you need to unplug it and check voltages at the plug. Should be 12V on only one of the three wires...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote UNFORD Quote  Post ReplyReply Direct Link To This Post Posted: Jan/23/2018 at 11:32am
Originally posted by farna farna wrote:

I was wondering about that copper ground strap and extra wire. Not sure the 68 AMX has them, but check! I suspect the dimmer switch is bad, but you need to unplug it and check voltages at the plug. Should be 12V on only one of the three wires...

The darn thing about trouble shooting is almost always it's never the first thing you look at. One always has to go additional steps that eventually lead to the source. That's where I'm at. I replaced the switch. that didn't do it so now I'm off down the path digging into this more.

I agree, there should be 12v on only 1 wire. Voltage is feeding in from somewhere else. The dimmer switch is not the problem. There's voltage on the headlight plug when the with the wiring disconnected from the dimmer switch. Plus I checked the dimmer switch with a multimeter.
Mark Melvin
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Post Options Post Options   Thanks (0) Thanks(0)   Quote UNFORD Quote  Post ReplyReply Direct Link To This Post Posted: Jan/23/2018 at 11:51am
George W, thank you for the detailed reply. It makes sense that a ground is not making up somewhere and the voltage is feeding back through the filaments. I'll take a copy of your reply with me tomorrow when I next take a look at the car and the headlight problem. I'm interested in the location of the ground location you mention near the radiator support.

If not solved quickly tomorrow morning I will have to shelve the issue for a short while. I'm on short time tomorrow as I have to put the car back together and get it shined up so I can get it moved over to the LA County Fairgrounds where it's going to be displayed at the Grand National Roadster Show over the weekend. At least the car runs and I only have to move it between my trailer and the building.

About your comments below. Yes there's a ground wire going to the back of the headlight switch, and there is a copper sheet behind the dash that provides a ground to the wiper switch. 

Originally posted by george w george w wrote:

It sounds as if you lost the headlight ground somewhere between the headlight socket and the body.
There's only one ground for both headlights and it's on the passenger side of the body. The headlight grounds attach in the area of the radiator support panel or inner fender area. First off you should have 12 volts going into the floor dimmer switch and only on one output terminal at a time. You're better off simply unplugging the connector at the dimmer switch and metering for continuity between the center input terminal and both output terminals to insure only the high or the low beam terminal have continuity at the same time.

Next remove BOTH headlights to insure that you don't have shorted together filaments in one of the lamps. Then check to make sure you have a good ground connection between the ground terminal of each headlight plug and the body. If you have an open ground you'll see 12 volts on the ground side which can be misleading. You have to have a good ground to show a voltage drop across a lamp bulb. Without a ground path on one side you'll see the supply voltage on both sides of the lamp filament.

One more thing. The OEM headlight switch should have a grounding terminal riveted to the metal at the back of the switch. There's a single ground wire that goes to this terminal to provide the ground path for the interior lights when the headlight switch is rotated fully CCW. This extra ground terminal is provided because the plastic dash does not provide the ground path that a metal dash would. Make sure your new switch has this ground if you want your interior lights to work when the switch is rotated fully CCW past the detent. This terminal also provides the ground path for the electric wipers assuming: A, that you have electric wipers and B, that the copper ground strap is still there behind the dash that ties the headlight switch body to the wiper switch body.
Mark Melvin
AMO #8858
socalamx.net
playmateamx.com

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Post Options Post Options   Thanks (0) Thanks(0)   Quote bigbad69 Quote  Post ReplyReply Direct Link To This Post Posted: Jan/23/2018 at 6:40pm
Originally posted by UNFORD UNFORD wrote:

There's voltage on the headlight plug when the with the wiring disconnected from the dimmer switch.
Unplugging the dimmer switch should completely disconnect power from the headlights. The centre pin of the harness side of the connector should be powered only with the headlight switch on. The side pins go to the low beams and high beams. If there is power on either of the side pins with the connector unplugged, 12V is getting in from somewhere else i.e. a short.
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