Headlight Voltage

[76912E]

I'm investigating adding headlight relays to my '76 912E. I was measuring voltages at the headlight and dimmer switches and at the before and after fuses for the headlights. I noticed, that when the low beams are on, there was approximately 2 to 5 volts at the high beam fuse or dimmer white wire. When I physically remove the headlights, there is 0V at the HB fuse when the low beams are on and 0V at the LB fuse when the high beams are on. If I plug one headlight in, the stray voltage at the HB fuse is about 5V and if I plug both headlights in the stray voltage drops to around 2.5V. I've swapped out different kinds of headlights (sealed and H4) to see if this is the issue, but got the same results. I've checked both the headlight and dimmer switched and back voltage seams to be coming from when the headlights are plugged in. I haven't tested to see if a relay will energize on the low backfed voltage.

My questions are: Do plugged in headlights result in backfeeding voltage to one circuit or the other? Is there some other mechanism causing the voltage backfeeding? Should I be concerned or is this normal?

[pete3799]

Check your grounds at the headlights.

[thetorch]

i have seen stray voltage from the high beam indicator light when parking lights are on (or low beams) -- enough to trigger a relay on the high beams. Have not yet investigated if if it's a short at the tach indicator ground or something else. Very annoying. Check each input wire individually at the HB fuse -- you might find it's only the blue indicator light causing it, and can terminate that until you sort it out, and live without a high beam indicator.

[76912E]

I spent the last couple of days chasing down this electrical issue. I removed the dimmer switch to measure voltage and resistance. I also removed the headlight switch after finding out the previous owner decided to epoxy the knob on. I neglected to mention that the car was stripped down to the shell for a complete repaint. In order to take the front fenders off, the headlight socket must be dewired to get the wires out of the fenders. Of course I took lots of pictures so I could put it back together correctly. With my luck, the phone died with hundreds of pictures on it. So, I went online to find a wiring diagram for an H4 headlight plug and hooked them up. It turns out the diagram was for the bulb side and not the plug end. I finally figured out the ground and high beam wires were swapped. I was grounding the low beam through the high beams, hence the 5V back feed. Once I swapped the wires, the issue went away! Thanks for the help!