55.2-59 Tail stop/turn lights go out when headlights are turned on.

[dyno]

I noticed last night that my rear tail stop/turn lights quit working when the headlight switch is turned on. Front turn signals work fine. All new wiring that I installed a couple years ago when I restored the truck. I checked all the connections on the headlight switch, they seem fine. Could it be a bad headlight switch? 1958 GMC 150

[kazoocruiser]

Maybe you could take the switch apart and make sure the contacts aren't corroded to the point that they aren't making good contact anymore. 60 years on a switch seems like a possible culprit.

[_Ogre]

stop/turn lights are not connected to anything headlight switch related other than the grounds of the tail lights, i'd start there
you probably lost the grounds in back and are grounding the stop/turn thru the headlight switch wiring
when the headlight switch is turned on, you lose that ground path
power to the stop/turn circuit is from flashers or brake switch

[dyno]

Originally Posted by _Ogre

stop/turn lights are not connected to anything headlight switch related other than the grounds of the tail lights, i'd start there
you probably lost the grounds in back and are grounding the stop/turn thru the headlight switch wiring
when the headlight switch is turned on, you lose that ground path
power to the stop/turn circuit is from flashers or brake switch

Checking the grounds now. I noticed that if I open the lower contact points on the switch, that the flasher/turn signals work with headlights on.

[_Ogre]

that sort of proves my point

the contacts are auto reset breakers for the lights, in a basic truck this was your only fuses
there are 2 "contact points" on the headlight switch, 1 side for headlights, i side for tail lights
if you rewired your truck the brake lights should be on their own fuse
when you opened the points, either your headlights or tail lights went out

check your grounds

[dyno]

Problem solved. Never had a ground strap from the frame to the bed! Thanks.

[MiraclePieCo]

I've seen bad ground cause all sorts of weird inexplicable problems.

[mr48chev]

Originally Posted by dyno

Problem solved. Never had a ground strap from the frame to the bed! Thanks.

Good that you found it that quick. It's those simple things that usually drive us nuts on these trucks.

I'd have to say that with all the coats of paint, rubber insulators and what not that we tend to use we sometimes compromise the original ground setups on these trucks too. Not a bad thing, we just have to compensate for it with extra ground straps and making sure we have good clean connections.

[dsraven]

also ensure you have a good ground strap to the rad support and cab. your lights may be brighter if the system has been looking for a ground through another circuit. grounds need to be as large as the entire load would be for that particular part. eg: the rad support may be the ground for the headlights, turn signals, marker lights and horn or anything else that is mounted there and needs a ground. if all of those components were on at the same time you need to be able to handle that load through the single ground strap and back through the frame to the battery. grounds should be to a bare steel connection with a star washer then, once tightened down, they should be covered in some sort of anticorrosion coating. check the ground from the battery as well. if it is grounded on the engine only, with a nice big battery cable, then the engine needs a similar sized cable that goes to the frame, which is the common ground for everything else.
good grounding is something that a lot of people neglect. they run awesome looking circuits but then a flimsy little ground wire to complete the circuits.

[_Ogre]

glad it was an easy fix.
oem had no ground strap to the bed, the rear sill is bolted to the frame
oem barely had any battery ground, just that short braided cable to the cab
i know truk has no ground strap on the bed

[DransportGarage]

Originally Posted by dyno

Problem solved. Never had a ground strap from the frame to the bed! Thanks.

You nailed it, Ogre !!