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55 1st 6 volt new chevy duty wire harness all the lights work turn,tail,running,headlights,dash. No problems till you turn headlights on then all lights go out after a few minutes, then wait for a few they come back on. seem like some thing is heating up then cooling down? Any help would be great

*Brian,

I believe there is a thermal overload built into your light switch. This would act just like you describe.

This thermal is there to protect the light circuit from excess current draw. It appears yours is "overloaded" and is tripping the breaker.

This circuit should only react to headlights and taillights. Is everything wired correctly...the dimmer switch included.

Is there a wire slightly pinched up near the cowl or where it goes from left side to right side. Maybe a wire mashed and partially grounding in the loom to the rear of the truck. Is there a light that doesn't work, or is dim?

The other option would be a defective aftermarket light switch. Maybe the switch came from China and they had to use an old razor blade for the thermal contact.

This is protection, and it's doing its job! You have something drawing too much current and will have to find it.

Stuart

I hope you find the problem before the smoke detectors go off, but I'd hate to be you looking for it in a new harness. The good part is it should stick out like a sore thumb just waiting to be found. Good luck.

The boys are 100% spot on Brian. I’m a tad unclear about your one line Stu, but all of the lights in the truck are fed thru the thermal cut out including the dash lights and dimmer. Unless they are an accessory that is connected to the battery terminal or power buss at the back of the switch.

Brian, this is a common problem with the original GM light switches of the first half of the 50’s. There is a bi-metal strip that the contacts are mounted on. When the circuit draws more than around 40 amps it should heat up enough to open the circuit.
A lot of back yard mechanics mistakenly bend the mounting tab for the thermal strip thinking that if a little tension is good then a bunch more is better. This only throws out the calibration and allows a higher current to pass, thus over loading the entire circuit, your new wiring included. This is NOT a good solution. Also the phenolic contact board is made with a paper substrate and with years of exposure to moisture it swells and warps. Most of them that I have found are warped this way.

There is a high resistance somewhere in your circuit possibly at one of the connections. If you simply changed the wiring loom without thoroughly cleaning the oxidation from all of the mating metal points then there may still be a high resistance load at one of the new connections.

The other place to consider could be inside the light switch it self. The old grease migrates onto the internal contacts and dries out preventing them from making full contact. The copper contacts oxidize which again introduces a high resistance into the circuit and sometimes they become intermittent because of these problems.

I have been working diligently over the winter to develop parts and tooling to for new internal parts with the intention of rebuilding the original light switches. This would most likely only be of interest to those that are interested in keeping everything absolutely factory original. So far there really doesn’t seem to be much interest in this alternative since I have only been able to find a handful of cores to work with that have not been tampered with. This rebuild is very labor intensive and although it is possible, if you are very careful, to open the factory sealed head light section up, you will most likely find that the internal parts are not only dirty from dried grease but that the contacts and board have a groove worn in them or are burned or oxidized. And as mentioned above, that the contact board is distorted also and needs to be replaced.
The vendors sell new after market switches for around $20 and since it is behind the dash and out of sight no one will ever know if it is the original switch or not, so you can see why not many people are willing to go through the effort of restoring a light switch.
Well good luck with your problem, and if you do end up choosing to replace the switch, keep me in mind for the old cadaver.
Denny Graham
Sandwich, IL
If you care to see some pics of the switches internals here are a few: http://community.webshots.com/album/555161678EhIoCb?start=288

I would check the headlight wiring first for a short. You say park and tail and dashlights etc. are not causing a problem. But when you turn on the headlights it overloads the thermal switch.
Cleon

Brian;
Another thought or two from the peanut gallery in addition to all the good info you've already been given:

1. Are the headlights and taillights bulbs the correct ones??
2. Do the parking lights go OFF when the headlights are on?
(if not, could have a wiring cross or internal switch problem)

Nice work on those AD switch restorations Denny!

edit: Did you try turning your dash lights control off (full CCW) while headlights are on? Could be someone put the wrong bulbs in there too and the problem doesn't show up until the headlights are on (?) Hey, anything's possible.

Denny Graham,

Thanks for the info. Not being a 'dissector of switches' I didn't really know if the dash lights went through the thermal or not.

Because all the lights are on the load side of the overload, and the overload is so gradual, it going to be a tough find!

Stuart

A little gray area goin on here Brian. In your first post are you saying that you can run the "tail, parking and dash lights" only and it doesn't trip? And that it only trips when you add the headlights?
If that's the case you sure have narrowed it down to one problem area, as Cleon noted. Should be easy to find in that case Stu.
Denny Graham
Sandwich, IL

Denny Graham,

How so! If the total load on the load side of the thermal is more than the rating of the thermal...which light is the culprit?

Pulling on the headlight might be innocent if the dash lights are pulling tons of current..or the taillights are going nuts. It merely means the headlight put the draw over the top and tripped the overload. It doesn't mean it's the headlights that are the cause.

Is this thinking defective!

Stuart

I think your thinken' is right on. Could be anything. It could be that something (maybe incorrect bulbs?) is pulling too much current but not enough to trip the "breaker" until the headlights are turned on. OR - it still could be something with the headlights too. Confusin' aint it?

Nope Stu, think ya might have somethin there. Assuming that the cutout was still in calibration, (which is pretty unlikely) if the load was 38 amps and ya added the headlights, that would put ya over the top. The parking lights are taken out of the ckt when ya pull the head lights on so ya might be able to eliminate them.
If it were me I would simply start pullin the wires off the light switch and pokin' an ammeter in there and see what I was actually drawing.
Might be a moot point though, taint heard from the patient since Friday night.
Denny Graham
Sandwich, IL

easy way to check switch unplug headlights and pull switch into on position and see what happens.could be internal short of the switch.