Head Lights On No Tail Lights

[sundin]

I Have A Tail Light And License Plate Light Problem. They Work Fine Except When The Head Lights Are On.all Other Lights Work Fine At All Times. The Headlight Switch Is New And So Is The Harness For The Rear. The Tail Lights And License Plate Light Work When The Parking Lights Are On. Could The Headlight Switch Be Wired Wrong?
Thanks Jim

[cdowns]

do you have the ground wire installed from the box to the frame like the factory installed it?

[sundin]

If All The Other Lights Work Could It Still Be The Ground?

Thanks Jim

[68gmsee]

Yep, it could still be a weak ground. Enough to let some lights work but not the others. Also, check your connections in the wiring harness to make sure they're not corroded or dirty. Check connections behind the housing to make sure they are clean, etc.

I'm in the process of going through something similar but it has to do with one of the brake lights. Bulb is good, have voltage with no load going to it, but if I step on the brakes with the bulb in it, it doesn't work??

My next step is to follow the cabling back to check all connections and grounds.

[cdowns]

having a good positive ground connection from the box to the frame was important enough for general motors to install one for the lights to work properly when the truck came from the factory

[Luvlegs]

Originally Posted by cdowns

having a good positive ground connection from the box to the frame was important enough for general motors to install one for the lights to work properly when the truck came from the factory

Where would this be located? Was it a bolt/box to frame type or flexible strap(s)?

[DennyB]

Originally Posted by sundin

If All The Other Lights Work Could It Still Be The Ground?

Thanks Jim

Yes, I had the same problem. Tail tights worked till I turned on the headlights. You may also find that when you hit the brakes the rear lights go out.
It happened when I had my exhaust replaced. Apparently the old exhaust was making the ground.
I put in new grounds from cab to frame and bed to frame. Problem solved.

To test pull off a tailight lense and ground a wire to the frame or exhaust and to the housing. Make sure you have good connection. If lights come on you know its your problem.

[cdowns]

Originally Posted by Luvlegs

Where would this be located? Was it a bolt/box to frame type or flexible strap(s)?

there was a cable from a crosssill of the bed to the frame crossmember similar to cables from the firewall to valve covers

[Luvlegs]

Originally Posted by cdowns

there was a cable from a crosssill of the bed to the frame crossmember similar to cables from the firewall to valve covers

Not that I'm even coming close to a full restoration but for those who might be (and because I'm curious) can anyone I.D. the correct locations?

[68gmsee]

I didn't see one on my stock 68 gmc when I removed the bed some years ago. I think, in my case at least, it was the bolts to the frame that provided the ground.

I painted the bottom of the bed and part of the frame where it sits. I suspect that's where my lack of ground is. The ground strap is a good idea since a lot of these trucks get rust, dirt and grime under the bed.

[sundin]

Thanks For All The Info. I Will Check The Grounds This Weekend.

Thanks Again