Truck started and Running BUT

will.rynerson

will.rynerson

Senior Member
Hey you guys,

I’m trying to figure out why my headlights and my tail lights have power to them when the battery is hooked up but the ignition is not on. That will eventually kill my battery. Anybody have any ideas? I’ve tried retracing wires and swapping the wires but nothing resolves it. Also, I’ve been noticing my brake lights. Don’t turn on when I push the brakes.

Anybody run into this type of problem before and solve it?
 
If you’re following the stock wiring diagram, the headlights, tail lights, and brake lights are not wired through the ignition switch. They will have power when the ignition switch is off but it won’t drain your battery.

Congrats on getting it running!
 
That's how all old lights are wired up unless you mean the lights are on all of the time when you hookup the battery regardless of the headlight switch position. The ignition switch has nothing to do with the lights. Even a little more modern era car such as an early Mustang has an ignition switch that turns on the ignition and some accessories like the radio but not the lights. They have their own separate switch and circuit. The current would be too much for the ignition switch to handle all of that stuff and it would burn the contacts in the switch. You can install a relay that the ignition switch turns on and then the relay sends the power from the battery to the headlight switch. That way the relay carries the main current for the headlights, the ignition switch just uses a little current to turn on the relay.

If it does have a headlight switch and you remove the wires from the switch and the headlights still stay on they are either wired wrong or you have a short to power in the wire from the swtich to the headlights.

You need to check for power at the brake light switch and if it's there jump the two wires together. If the brake lights work the switch is most likely bad. I worked on a truck one time that that the brake light fuse had no power on either side of it. Normally that would mean a bad wire between the battery and the fuse. This one was wired from the factory so that the power went to the brake light switch first and then to the fuse. I could not check for power at the fuse unless the brakes were depressed. Normally you would not even look at the switch until you found out why the fuse did not have power to it. It turned out to be a bad switch.
 
Thank you, The headlights turn off with the headlight switch. But tail lights stay on right when the battery is connected.
 
Thank you, I appreciate that. Yeah I can see how that headlights being turned off at the light switch won’t drain the battery, but my tail light stay on even with the light switch turned off. And I’m not getting brake light so hard to check out the other recommendation.
 
Well, thank you so much for all that information. The headlights do turn off with the light switch. The running lights are now turning signals, and the tail lights have the turn signal and brake light as well as the running light. However, the running lights stay on whether the light switch is on or off.

I like your idea of putting a relay for the headlights. I’ll have to look at a relay set up for that. Any recommendations would be wonderful as well.

I will check out the sending unit power for the brake lights. I thought I put a brand new one in there. I’m not sure where the fuse is for the brake lights. Is that the circuit breaker?
 
Do I remember you writing somewhere you were using a fuse box? Ford used circuit breakers on these cars, instead of fuses. For stock wiring, the circuit breaker would feed your electric devices. The brake lights run from the circuit breaker to the brake light switch on the master cylinder.

You’ll have to trace your running light wire back to see where it is connected to power. It’s getting power somewhere from a non-switched source if the lights are constantly on.

And for brake lights, try really pressing hard on the pedal. These pressure switches seem to only activate when you nearly stand on the pedal.
 
cmbrucew said:
Will
Check and make sure the brake light and tail light wires are not switched.


Bruce
Click to expand...

I was thinking the same thing about the switched wires between the taillights and brake lights. Maybe the brake light switch is stuck on. Those hydraulic brake light switches fail all the time. Ford used them until 1964, they are on the very early Mustangs. The electric pedal mounted switches that replaced them in 1965 were not any better. I've had 15 early Mustangs. I would walk out in the garage and find the brake lights on.

There is a low pressure and a high pressure switch available. The master cylinder has a residual brake valve that holds a little pressure in the system at all times. This keeps the wheel cylinder cups pushed out against the cylinders, so they don't leak when the vehicle is sitting. That low residual pressure can also activate the low-pressure brake light switch if you have a switch that is too sensitive, the brake lights stay on all of the time. I've had a couple of them fail on the same vehicle. I switched to the high-pressure switch and then I never had the brake lights stay on again.
 
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