I have a '72 98 and was driving tonight when I turned on the rear window defroster and noticed the GEN light come on. Not brilliantly or immediately, but certainly illuminated a bit. I know the rear defroster pulls a good bit of amps ...just wondering if there's anything I can do to not have the GEN light come on. NOTE: I do have a brand new alternator in the car. It's from Autozone and may not be top of the line....

Thanks for any help.

Jay

 

Grounds grounds grounds...
Something has a high resistance likely due to corrosion. Its 50 years old.
Trace the systems power & grounds. Clean all connections.
Is this a blower type or wire grid type?
The blower type may use the blower housing as a ground if no wire is present. Pull the housing clean the mounting area and fasteners.
Always follow up cleaned grounds with clear battery post stray protector

You should probably go through the whole car and clean as many grounds as you can locate.
Your charging system will be much happier. A wire diagram will be your road map.

Did the old alternator test bad?
Did you clean both ends of both battery cables??

 

Solid advice on the grounds. Question: I recall a particular thread or posts pertaining to a certain ALT + configuration (72 88/98 w/ AC + rear defroster?) where it was recommended to employ an additional ground strap on the ALT NEG (-) post? Was it this ALT combination or am I mistaken?

 

Ref defrost is wire type, in the glass. Will definitely go "ground" hunting and clean all - then use dielectric grease. Re old alternator, I don't think it did - I think it was "okay" but I had already bought and installed new one ...only to realize it was a bad ground on the firewall. Both ends of cable cleaned.

 

I'll check it out. Thank you!

 

Hopefully you hung on to your USA made alternator and didn't turn it in for a core on the chineeeeese unit.
Dont rule out the defroster switch. The contacts could be shot adding a ton of resistance.
I recommend doing a load drop test on the system.

 

Alt is gone. Will try the switch, and the grounds. Thank you

 

Get your alternator retested just to make sure. A few years ago, I bought an Autozone alternator. It turned out to be bad. Went back to the store and had them to test it to verify. They had 3 others in stock. I had them test them all, and all of them were bad too.

 

I love old alternator and starter stores for rebuilding.

 

Clean all the connections, make sure they are all tight.

Next, get a voltmeter. Start the engine, let it idle. Turn on everything electrical, tge high beams, fan on high, rear defrost, etc, then see what the charging voltage is. If there is a significant drop you may just need an alternator with more capacity. Cars equipped with defrost and other power hungry option came factory with higher capacity alternators, you may have installed an alternator that can’t keep up.

 

All good posts, just thinking about what hasn’t been mentioned. At the risk of insult, alternator belt tight? Idle speed ample? Quick, simple checks.

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I've had this problem as well. I replaced the bridge rectifier in the alternator. It turns out that one diode in the bridge was bad. The voltage measured OK even with the bad diode, so you can't go by that.