Good evening Corvette forum,

After going through several pages of google results I was unable to find an answer to my question. I’ll give you a timeline of what has happened so far.

1. About a month ago I found the car wouldn’t start unless jumped. Read 11.8 volts at the battery, assumed it was bad, replaced it, then all was well
2. A while later the same thing happens again. I noticed that the battery gauge would never go into the positive, so I bought a 63 amp 12SI unit
3. While taking the old alternator out, I noticed the rear brush was completely worn down and gone. I’m guessing due to a belt misalignment, which I corrected by shimming behind the alt mount bracket
4. New alternator goes in. While running the battery reads 14 point something (I forget) and 12.5 when off, seems like problem solved. I drove around a bit to charge the battery up observing the battery gauge being in the positive the whole time
5. I go for another drive. The battery gauge is in the positive after revving up the engine, goes all the way to +40, then nothing. It hasn’t been in the positive since this.
6. Putting a meter on the battery now shows 12.5 on or off, which I would imagine would go down over time using electronics on the car
7. All fusible links appear to be good, all connections are tight and clean, the two prong connector is installed correctly (red wire on top with alt clocked to 10 o clockish)
8. Putting a screwdriver on the back of the alternator with the car on and revved up does not give me a magnetic feel

I’m currently stumped, but would like to be able to drive my car without worrying if it’s going to start or not. What are your recommendations?

 

Your new replacement alternator is toast. More than once I found newly rebuilt alternators to be defective

 

Not certain brand or where you purchased. If you purchased top of the line. Or cheapest.
And none of that matters now.
Take it off and take it back.

 

That wouldn’t surprise me given the quality of parts available today. I’ll take it off and get a parts store to test it tonight.

 

I would replace the alternator with a good solid unit built by a reputable manufacturer. New parts that are bad are happening way too often.

Whenever a battery is very low and discharged the best thing to do is to disconnect the battery from the car and charge it up with a 10-15 amp alternator until it gets up to 15 volts at which time the battery should start to gas off a bit. This does two important things, first it replaces the current that was removed from the battery and second it will de-stratify the electrolyte which is critical in a battery that has sat for any time period over a few weeks.

Whenever I have any kind of issue that involves the battery I will first disconnect the battery and charge it completely. Then once you have a good fully charged battery you can start doing the troubleshooting that is required.

I just last week had my alternator replaced in my Buick DD and it was over $800...... I wish it was as easy to replace as the Corvette alternators are on the C3 models. The Buick alternator was charging batteries way over the voltage limit of 14.30 and I would see over 16+ volts on the display. Thankfully I use Flooded Lead Acid batteries and they simply use more water from the electrolyte which is easily replaced.

During the summer months I have some 20 watt solar panels that I use to keep my vehicle batteries floating along. This way they can and will last longer in my C3.

 

I'd take the alternator to AutoZone, or wherever, and have them test it.

Is there a chance that the output stud of the alternator is no longer connected to the battery? Often folks will upsize this wire, and delete the old one. Perhaps yours deleted itself?

 

Good call. I also suspect a failed alternator.
If I was you I'd buy a 94-amp 12si..... should cost about the same and has more capability. When I had a 63-amp it was just barely adequate at night with just the lights, the heater fan, and the radio on so I put a 94-amp 12si in..... nowadays I have all kinds of other stuff so have different alternator but from a quick glance I suspect you do not so the 94-amp would probably work great for you.

 

I would rebuild if original alternator.
bearings and brushes..at min

My experience with remanufactured chinesium is not good

 

Totally agree with Interpon. I've rebuilt many old alternators. EZPZ, and you get to keep your original! It's not hard to replace brushes, bearings, etc. Buy the parts at any of the large parts places and they'll test it for you when you're done. Clean up the housing while it's apart and give it a light coat of high-temp satin clear and it will look good for years.

 

Check inside the white plastic harness clip for grunge build-up and maybe even a severed wire inside.
Clean the terminals with Electrical Contact Cleaner. (red spray can at Walmart)

The GM ALT pigtail can be found on Ebay if needed.

 

The voltage regulator is most likely bad. Try taping the Alternator case with a hammer. It may start working again, but it still needs repair.

 

Unlike other here I've had pretty good luck with remanufactured alternators & starters. I have gotten mine from O'reilly(was Kragen) or Autozone or NAPA..... So if I wasn't comfortable rebuilding my own I'd just go get a rebuilt unit.

 

Thanks for the responses!

I brought the new alternator to the store today.



They didn’t have a V-belt for their alternator tester, so I wasn’t able to find out if it was bad. However, they gave me a new one under warranty anyways.

So the new new alternator goes in. Still nothing. I’m getting 11.8 volts at the battery regardless if the car is on or off, and the battery gauge isn’t going into the positive. However, I am getting a magnetic feel at the rear of the alt when the car is on.

At this point I don’t believe it’s the alternator itself, but something else. Like I said before, all connections are clean and tight. I disassembled the two prong connector to be sure. Any recommendations as to what it could be?

And yes, I do agree working on your car yourself using high quality materials is the way to go, but at this point I’m not blaming it on the parts store alternator for the issue.

 

Are you 100% sure the connections in the two prong connector are correct (and not just at the alternator end)?

One of them is needed to excite the alternator. If something got cooked, the feedback loop might not be completed. It could be as simple (or ridiculous) as the "alt" bulb in your dash. Or the ammeter gauge. You don't have a voltmeter, correct?

It never hurts to post photos!

 

I didn't know that tampon Tim had seat belt laws for alternators.

 

Piersonpie
Find another parts store with a tester. ..... or you could go to the boneyard and get a generic gm alternator and swap on a grooved pulled.

What you describe really sounds like the alternator..... unless, as mentioned above, you have a bad connection somewhere. Does you car have an ALT, BATT (or whatever) indicator light?

 

My car is a ‘76. With an ammeter and no alternator or battery light. I’ll double check all my connections tonight.

Another test I can do is pull the alternator out of my other ‘76, which is one that I know works, and test it in this car. I’ll post back with the results.

 

Yeah. that's a good idea...... to swap it out and see what happens. That'll be a good test of the wiring setup.
PS- In my mind I thought they had eliminated the ammeter before '76..... but I was just guessing.

 

Well I took the one from the maroon car. Has a different style fan and the clocking is wrong, but none of that affects functionality.



And… it works!



A bit annoying that I had two bad apples from the get go, but I’ll see if I can get a refund for it. For a more permanent solution I’ll be searching the junkyards and rebuilding a unit myself. The car has LED bulbs throughout, no defroster, no electric fans, and the entire HVAC system has been deleted. If anything my car draws less amps than a stock one would, so an 63 amp unit should be fine until my LS plans in the future. Any specific models from a vehicle I should look for? And who has the best rebuild kits?

 

Tried it in the autozone alternator tester. Failed obviously, but they gave me a full refund as well as the price of the core I brought them. Can’t complain about that.



As of now the white car works, and the maroon one can be put on the back burner for a bit. I’m glad I was able to figure this out without having to tear deeper into the wiring. Thanks for the advice everyone.