The Stovebolt Site Forums The Shop Area The Electrical Bay Tail lights won't work when headlights on??

Howdy Stovebolters.
I'm having a problem that I can't figure out...When my light switch is in the parking light position, my tail lights work fine, but when I switch to head lights, the headlights work fine but no tail lights. Brake lights and turn signals are fine anytime.
I believe the switch may be bad. I can get the tail lights to work if I put a jumper wire from the middle side terminal to the tail light terminal, but I'm thinking this is the wrong thing to do. Also, are the lights beneath the head lights supposed to be on as parking lights, or should they only come on as blinkers? Should they be two-filament lamps?
Thanks,
Jason

You don't say what year your truck is, so it's hard to answer some of your questions. Yes, your headlight switch is probably bad.

On older trucks there were no turn signals at all. If yours had turn signals from the factory, then the parking lights may or may not have done double duty as turn signals as well.

On my truck, the front running lights go on in the "parking light" position, and off if you turn the headlights on. Maybe your front running lights and the taillight wires are reversed.

Sorry Guys,
I've got a '53 gmc 1/2 ton on a '76 chevy frame. I have found part of my problem, and I think you've helped me with the other! One of my taillight wires had been cut off inside the wiring harness. I believe the other problem is that the taillight wire is in the wrong position as you pointed out. I'm going to try fixing it in a moment and post the results. I sure do appreciate the help. Thank goodness for the internet! I was able to type in a question about taking the light switch out and found out about that button on the switch that lets the switch rod/button slide out. Good grief! I would have never thought of such a thing.
Thanks for your help.
BTW, if I can quiz you further...
I've got a slow battery drain. I've pulled all the fuses and looked over every wire except two small ones on the distributor. Everything checks out o.k. as far as pulling fuses had no effect on the battery drain---it was still there.
I'm wondering if there's any possibility that there could be some kind of problem in the starter or starter solenoid that could be causing this. The alternator is totally disconnected and the drain is still there. I took the two power wires loose from the firewall block and the voltage went down, but I still had 4 volts from the neg. side of the battery to the ground clamp even after doing that. If anyone has an idea I would be very appreciative.
Thanks again,
Jason

If you have 4 volts between the negative post of the battery and ground, then you have a grounding issue. Make sure you clean the battery posts and the connectors and do the same where the battery's ground cable attaches to the frame. You should have ZERO volts between ground and the negative terminal of the battery. Also, there should be a ground wire between the frame and the engine. If that's missing, go buy one and install it. That ground wire should be a braided wire mounted to the frame and a bolt on the starter or the flywheel.

I don't think I explained my problem well enough....
I do have a braided ground from the frame to the battery. My problem is that my battery is draining over a couple of days because of a slow drain. Something is going to ground somewhere. What I'm doing to see if I can fix the problem is taking all the fuses out and disconnecting each circuit and measuring the voltage between the neg. battery post and ground, trying to find the circuit that is shorting to ground. Right now I'm getting 12.38 volts (the battery is low right now or it would probably be more) from the neg. battery post to the frame across my cheapo meter. I've checked every fuse and connection I can find other than the starter and solenoid. The only one that has any affect on the voltage that I can find is if I take loose the 2 positive red wires that are going to the firewall terminal. But, even then it only drops down to about 4 volts instead of zero volts like I would expect if that was eliminating the short.
I'm totally baffled by this problem. I just can't seem to find the short. I'm wondering if it's possible that somehow the starter solenoid or starter itself is letting something go to ground. Is that possible?
Thanks for your patience----mine is wearing thin with this problem.
Jason

Let's make sure we're talking about the same thing. I'm understanding you to say that you put the positive lead of your meter on the negative post of the battery and the negative lead of the meter on the frame and you're measuring 4 volts. While you did this, the battery cable was connected both to the negative post of the battery and to the frame.

Now, if what you're saying is that you disconnected the negative battery cable, then put the positive lead of your meter to the negative post of the battery and the negative lead of the meter to ground, then the 4 volts you're measuring makes no sense. You should be reading battery voltage in that case.

Regardless of any current drain on the system, the voltage from the negative terminal to ground, with the battery cable disconnected, should be almost exactly the same as if you measured from the positive post to ground. The internal resistance of most batteries can be measured in the tenths of an ohm (or effectively zero for most purposes.)

Since this happened recently, can you think of any changes you made to your electrical system within the same time frame?

I had the same problem on my '65 c10... and at various times, these problems came up which I had to fix:


headlight switch

grounding issues at the rear lights

a short(broken wire inside the insulation) in various places in the wires... in my case, the wires toward the rear of the truck... and later I found a melted wire behind the fuse panel on the inside firewall.

a bad glass tube fuse(the original old style) which sometimes allows current across... mine was not black, but broken.... I replaced it and everything was fine.

Eventually, I did have to replace my switch even after I took it apart trying to clean it up... something was just worn out in there.

Anyway, some crazy things for you to check.

-W

Do you have an aftermarket radio installed? If so it has a memory wire connected to the battery at al times and can cause a draw large enough to drain a battery in a week or so.
Did switching the wires on the headlamp switch repair the park lamp issue? They were origionaly hooked up so that when the headlamps were on the front park lamps were off, but when in the park lamp position all the park lamps were on.

GOt if fixed! I appreciate your response. I had a tachometer with a short in it, and the lights were wired wrong on the switch.

Thanks,
Jason