Headlight voltage issue

[silver356]

I am reassembling a 70T after painting, and its all coming together pretty well. It was a glass out job, but the car was not fully dismantled, just the exterior stuff. I have one problem with the headlights, however, that I don't understand. Wiring and electricity are definitely not my strong suits! The left headlight works fine(these are sealed beams with sugar scoops), but the right one is very dim, barely on. I got a new sealed beam, same issue, and it works fine when plugged into the working left side. So I pulled both headlights to read the voltages. I have roughly 11 volts between ground and the yellow wire, and the same between the yellow and the white wire on both headlight plugs. On the working left plug, I have 0 voltage between the brown and the white wire, but on the non working right side, I have about .18 volts.

I am assuming that that's my problem, but I cant figure out where that bleeding voltage would be coming from. The wires all seem to go directly to the fuse panel. The fuse panel looks pretty good, and the fuses are all good. Does anyone have any ideas what would introduce voltage?

Thanks for any ideas,
Cheers,
David

[911 Rod]

I would go back to when it worked and think about what has changed since.

[timmy2]

Check grounds are all solid and well grounded. Also check the 6 pin plug is well seated as it goes through the fender. You may even want to do voltage checks at the plug male terminals to see f the failure is in the front wiring harness or the headlamp harness.

[silver356]

Thanks for the ideas, but that's my point. Its not that I am not getting voltage, its that I'm getting small voltage where I shouldn't be-its bleeding in from somewhere. I purchased this car, and it immediately went to my body guy, because the body and paint was a mess. Unfortunately, it took a year before it was finally painted, so there is really no really "worked "place to go back to. I pulled all of the trim and stuff off just before painting, but didn't have a chance to really use the car. I checked and reseated the plug through the fender, and the small voltage is too small to show up on an ohms reading. Other thoughts?

[timmy2]

Ignore the 0.18 volts on the high beam wire white to ground.
You should only have voltage to ground on it when the high beams are selected. You should never have voltage between yellow and white. Brown is ground.

Your problem is likely a high resistance to ground after the voltage passes through the light filament. If you have 12 (11) volts and the light works on one side and you have 12 volts on the other side but the lamp is dim it is because there is a high resistance in the path limiting the amount of power the lamp has to use.
Try connecting a ground wire jumper from the brown wire on the lamp connector to a good clean body ground by the battery negative and connect the lamp. I bet the lamp gets brighter.

On these cars the ground for lighting depends on body grounds making the path back to the negative terminal through the fenders, not a problem on factory paint jobs as they would scratch through the paint in areas mounting the fenders to ensure good electrical contact. If your fenders were painted off the car and then bolted back on, there may not be a proper metal to metal contact point scratched in.
Pull off a nut and bolt and scratch the paint on both sides to remedy. Or run a ground wire back to a solid clean chassis ground

[timmy2]

Another possibility is a high resistance connection in the front headlamp harness. The multiple ground wires are crimped together by the factory inside the harness and can become corroded or loose over time.
Check the resistance of the brown wire at the lamp plug to the brown wires elsewhere in the harness.
Should be zero ohms or close to it. If higher you have found the problem.

[silver356]

Thanks , timmy2, that may well be my problem. The right fender, once we got into the body work, turned out to be fiberglass, and was replaced by a Dansk reproduction fender. It was primed and painted black when we got it, and then was painted off the car and bolted on. The car just went back to my body guy for a couple of body fixes, but when I get it back, I will definitely try your idea of running a jumper wire to a fresh new ground, and see if that solves the problem. I suspect it will. Then I will try and form a good ground by scratching the joint between the fender and the car. The ground from the headlight/turn indicator box goes to a freshly cleaned terminal inside the fender, but if the fender is not getting good contact with the rest of the car, then that would cause the light to be dim if I understand you correctly.
I'll follow up with you all once I get the car back, in a week or two.
Cheers,
David

[David E. Clark]

Fluke: Diagnosing Voltage Drop - Electrical Automotive Troubleshooting