71 exmod headlight problem

[siii8873]

I am trying to get my headlights working on my 71 exmod.
I have done some testing and will start with the high beams circuit.
I have power to the front Blue/white source wire. I reconnected and with a test light have power at the headlight connector, test lamp connected from connector to a ground lights well. (BTW it has Hella upgraded lights). White running lights are also lit. I then plug in the bulb and nothing, running lights go out and headlight bulb does not light (tried a few bulbs, not the problem). I'm thinking a ground problem because the difference between the test light and headlight is that I am directly connecting the test light to a ground. What confuses me is why do the running lights go out (think they are called side markers, white ones)
Any ideas, really want to avoid doing any major rewiring.

[siii8873]

did some more testing and become more confused. Is there something odd about Hella H4 bulbs. I hooked up my test light between the ground tab on the bulb and a good ground. I then connected a hot lead to each of the other tabs directly from the battery. The light did not light but the test lamp did light. For the test light to light current had to pass through the filaments????????? I then tested the bulbs with an ohmmeter and they test good.

[VTBKRDS]

Had the same issue on my Defender. Took the headlight bucket out, found the bullet connectors corroded and loose fitting. Cleaned them up, pinched them together to make them fit the bullet connectors tighter. Then took the ground wires off the inner fender where they attach and cleaned the surfaces back to clean metal. Put some grease to the parts, put it all back together and the problem went away. Also try tightening the female spade connectors at the light connector. Sometimes they don't fit quite tight enough to make proper contact.

[KevinNY]

A weak ground will replicate your problems. Recheck the ground to the breakfast and make sure it it is clean and solid. I've found that when things get weird with voltage, always suspect your grounds.

[siii8873]

Lights are back working. Ended up there were two issues. i removed some hella fog lights and after much head scratching I checked the relay that was used and it was DPDT and set up to open the low beams when the fogs were on, thus I interuppted my source. Was that ever fun to trace down. The second problem was a ground. I reestablished a good ground and all is good.
I still do not understand why the H4 bulbs would not light at all when I had my test light hooked up as a ground and connected the hot to the battery. I think it tells you how good the ground has to be. The light must cause a voltage drop just like a bad ground.
Now I can get this thing inspected and start driving it so I can find out what else needs attention ha ha.

[Duckman]

Welcome to the joy & pain of owning a 30+ year old ex-MOD vehicle. A few weeks ago I decided to install a license plate light to make my nocturnal travels somewhat more legal - simple right? Three hours later after after multiple head scratching, replacing of corroded wires & bad connectors I was finished.

For all the grief it's still worth it for the looks you get wheile driving and the "What the heck is that?" questions you get from on-lookers. :D