Expert: Kevin Pilotte - 9/20/2006

Question
I just rebuilt the motor (258) in a 1977 Jeep CJ7 and cleaned up the underhood wiring (clue?).  Now it runs great.  Dash voltmeter reads 12V at ignition on, and at least 14V running.

The issue is when I turn the lights on (running lights or headlights).  The voltmeter bottoms out though the lights seem as bright as usual.

I know that there should be some drop, but this seems drastic as if there is a short or bad ground.  I tried running a wire from the headlight ground screws back to the battery negative, but that made no difference.

Thanks for any assistance you can give me!

Answer
Matt,
I hope you have a Voltmeter and know how to use it.
You need to do a "Voltage Drop".
With the engine running, lights on, measure the voltage between the positive battery post and the battery terminal at the alternator. It should read less than 1 Volt, yes I said 1 Volt. Place a Voltmeter lead on the battery negative post, the other lead to the engine block. It should be less than 1/2 Volt (.5V) If it's more, there is excessive resistance it the wiring.i.e corrosion
Measure the Voltage at the battery with the lights on, engine running. It should be 12+ Volts.
Take these readings and let me know what you find.
Kevin