The Stovebolt Site Forums The Shop Area The Electrical Bay headlight mystery

Following up on my query from a week ago: 1950 GMC 150, trying to track down an electrical gremlin. I replaced the original headlight switch with a new remanufactured one, but when I engage the last stop on the switch--which should turn on the headlights--the engine cuts out. The other lights--stop, parking, turn signals--work fine. So the new switch has fixed most issues, but I am at a loss about the headlight thing. Could something be wrong with the ammeter?

Do some testing

something is wrong with the way it's wired

Bill

The ignition is generally separate from the headlight switch. As Bill says, you have something wired wrong. There should be no connection between ignition and headlight switch. Or coil and headlight switch. Get a 1950 wiring diagram and follow every wire from/to the headlight switch. If that's OK. Report.

Also inspect the horn relay circuits and ignition lamp wiring.

Somehow you are disconnecting the coil power. Something very goofy. Something being tuned off.

If something is being shorted out, it's not fuse protected.

I've read your other post and am not sure of the present situation.

1. Does motor start and run fine with light off?
2. Does motor only die on full headlight out position? Runs on half out position?
3. Does ammeter seem normal reading while running?
4. Any other problems?
5. Battery voltage reads per spec at rest and at high idle?
6. The present issue is just that motor dies when H/L sw is full out?

Check to see that your wires are secure on the ammeter posts. I loose wire there will cause your symptoms. Ask me how I know. The current owner of my '50 failed to tighten the nuts back there...
That loose wire was still making enough contact to keep the coil happy, but when I load that weak connection by turning on the headlights, it tripped the breaker in the headlight switch while at the same time robbed the meager amount of juice going to the coil, causing the engine to die.
That is what I am guessing is what happened. It needs to be made clear that I am not that convinced that electricity is real at all. The only evidence that I have ever seem to give an indication that electricity does actually exist is when I let the smoke out of a wire.
In any case, once I tightened the nut on the ammeter, the issue went away.
Carl

I always look at the oldcarmanualproject wiring diagrams. I can never read them much. I will save Tim's link for a clear diagram. Thanks Tim.
You can see that the ignition switch power feed is chained in with other power, including ignition switch. Something is crossed somewhere. You need to trace all associated wires using Tim's diagram.

I'm guessing this toasted piece of 10 gauge between the ammeter and switch had something to do with my problems. I have replaced every bit of the original wiring with new stuff, according to the diagram. Many thanks to all of you who posted your helpful replies, you got me on the right track. Now I've got entirely functioning lights, and the battery even seems to be charging better with new wiring.

When you have an infinite number of monkeys and an infinite number of typewriters, an infinite number of monkeys will type the novel, War and Peace, in a very short amount of time.
This site is full of useful monkeys who have had everything possible go wrong with these trucks, so of course you were led in the right direction.
Carl