1980 Chevy 1/2 ton truck Alternator amps problem

Hi! I have a 1980 Chevy 1/2 ton truck(350ci 4 barrell).

It is only holding a charge for about 2 hours of driving. It will only hold a charge for about 30 minutes if I have the AC or headlights on. So I have to hook it up every night to my battery charger.

I replaced the alternator twice.

I have replaced the battery twice.

My mechanic looked it over VERY WELL for a draw or short. He said that there is no draw or short. He said that the alternator was charging right(13.3 volts, or something like that.) He said that the battery was in great shape.

When this problem first occured(6 months ago), the first thing I did was replace the alternator. When I went to buy it, I did not have the old one w/ me. They(Auto Zone) had the three different alternators for that truck. I bought the cheapest one.

Last night a spent 20 minutes on the phone w/ a guy at AutoZone and found out that my truck was sold that year w/ two different alternators(not sure why): One puts out 63 amps, and the other puts out 100 amps.

So here is my theory: My truck was made to have the 100-amp alternator. When it went bad, I replaced it w/ the 63-amp alternator. AutoZone confirmed last night on the phone that I did buy the 63-amp unit. My wife accidentaly tossed the original alternator in the dumpster, so I cant confirm what the original amp-out-put was on it. So my theory is that if I upgrade to the 100-amp alternator, that might fix the problem.

Any thoughts? Does that make sense?

Jason M.
Moore, OK. Both alternators should be able to charge your battery. The problem is the discharge after shutoff. I'm certain you have a drain somewhere, somehow. Prove it by disconnecting the battery some evening and showing the car will start the next morning. If this works, check back here for more tests. You will need a multimeter. Mistake one is buying an alternator from autozone...I buy them from advanced or orielly. I also think that you have a good sized drain, but that would not make any sense about being able to run for an hour or so and then haveing to put it on the charger. Check for a good draw or a belt slipping.....good luck. Hi Gentlemen! Thank you for helping me!!!

stevefI wrote:
"The problem is the discharge after shutoff."

I am not 100% sure what you mean by that. I will say that it only seems to lose it's charge when it is running. I can charge it up real well, let it site for a week, and it will still have a good charge. Is that what you were suggesting, that it's even draining when it's not running??

stevefI also wrote:
"I'm certain you have a drain somewhere, somehow."

I want to agree w/ you on that, but my mechanic looked it over for 3 days and found nothing. Yes, he is a human and can make mistakes.

stevefI also wrote:
"Prove it by disconnecting the battery some evening and showing the car will start the next morning. If this works, check back here for more tests."


I did this last night. Like I said, it only losses it's charge while I'm driving it.

What now....??? I'm certainly no mechanic, but the first place I would look is a belt slipping..... Sounds to me like your battery isn't charging--which means your alternator isn't working properly... but if the alternator is new..... I would say it's a belt slipping.. Also have someone check to see if you are getting 14 volts to the battery it self, with a meter on the battery load up the system with headlights and blower and at 1500 rpm it should be between 13-14 volts.

Well, the belt does not appear to be slipping. Does that mean it's not slipping? It doesnt feel as tight as it could be, but I cant seem to get it any tighter. My original diagnosis was not correct but your problem description misled me. So it appears the battery is not being charged by the alternator? Do what other posters have recommended. Put a voltmeter across the battery posts. Measure the voltage when the car is not running and get about 12.7 volts. Then measure it while running. There must be a notable increase, typically around 14 volts. So what will that mean? If you get about 14 volts on the posts (not the terminals attached to them) it means your alternator is doing what it is supposed to do. The battery would then be suspect. A lesser voltage would indict the alternator. BTW, I'm still not absolutely clear from your descriptions when the battery holds a charge and when it doesn't.