Having trouble identifying electrical problem - Help!

My brake lights only work when the engine is OFF but the key is in the ON position. If I start the car so that engine is ON, the brake lights immediately go OFF. I have looked the wiring over and everything looks ok, but clearly it isn't. Is this a ground issue? How can I trouble shoot?


thanks so much,

Bantam

 

Look in your charging system wiring. Just as the GEN light will go out once alternator starts charging, it gets the ground in the circuit. Did this just start acting up, or have you just finished wiring?

 

not a ground issue....seems to me they wouldnt work at all if that were the case.check for power at the switch on your master cyl. then check for power at the socket of the tail lite. chances are the issue is somwhere between the ignition switch to the switch on the master if thats how its hooked up. it could vibrating the wire's on the the master switch when its running.....

 

yep that could be it as well....are you running 12volt or 6 volt?

 

you could tell us a lot more about the car...for starters....

 

Yea it would be very helpful to no if this is new wiring and when the problem started, if it was after some changes etc...Alot of light issues are ground related. Start by checking your feeds startin at the source and if ok (runnin and not runnin) start checking grounds(light circuit, body, chassis, and engine. Use a good test light and make up along jumper wire that you can bypass grounds all the way to the battery to see if that changes anything. Also to eliminate the alternater problem disconnect the small wires on the alternator and start it up and see if the problem persist. A volt meter would be better then a test light for voltage drops etc. Good luck! TomCat 1

 

what kind of vehicle are you having problems with? better check the tailight connection first then the ......

 

It's a 49 ford. Converted to 12v and an alternator. Hadn't started car in a while and noticed the problem when I went to take it out of the garage. However, I guess I could have had the problem before I stored it for a few months, because I am not sure how quickly I would have noticed I didn't have tail lights.

Thank for the suggestions and any others you all might have. I will do some more checking tomorrow after work.

 

Do you have a power brake booster installed? Could be a difference in (brake) hydraulic pressure when engine is on or off.