Battery Light On With Key Off

[jmungo1]

I need some advice ... My battery light and radio stay on after I turn the ignition key off on my 1982 Spider. The light goes off when I start the car.

I checked the archives and found several threads associated with this problem. The problem most often mentioned is a bad diode in the alternator. Starting with this premise, and using previous suggestions, this is what I have tried so far.

First, I pulled the wire from the spade terminal on the back of the alternator. This turned the battery light off but the radio stayed on. I guess this is a good thing that suggests the battery light isn't being wrongly powered by some other short circuit?

Second, with the car running, I measured the voltage off the back of the alternator and measured 12.2 volts. I guess with a bad diode I would expect to lose 1/3 of the voltage or read about 8 volts.

So, does this indicate a bad alternator?

Or, should I be looking elsewhere like the ignition? Or somewhere else?

Thanks,

Joe

[azruss]

Your volt reading when the car is on is coming from the battery. It just means the alternator is not puting out enough juice to charge the system and the battery. Does sound like a bad diode.

[jmungo1]

Okay, just to be sure before I go out an purchase a new alternator, how best can I test for a bad diode. Is there a test I can perform in my garage? Say a volt meter test or something else?

Another question, regarding the radio powered on with the ignition off. Is this also related to a bad alternator? I ask because I checked the wiring diagram and it appears that the radio is directly powered by the INT position on ignition switch. Or does this indicate a short circuit somewhere that is allowing the radio to get power?

[azruss]

if you have an alternator/auto electric shop in your area, they can test the alternator and just replace diode if required. If it is a brush issue, they can refurb the entire unit for much cheaper than you can buy a new one.
On the radio, my guess is the PO wired the radio from a non-keyed power source.

[jmungo1]

Quick question regarding getting the alternator tested at a local shop, generally, do I need to remove it and bring it in or can it be tested while still in the car?

[azruss]

I've always removed it from the car myself. You might want to call them and see if they can run a diagnostics or replace the diode while it is in the car. If it needs work, it will need to be removed. You can pay them to do it or do it yourself.