odd alternator issue

Reply Subscribe

Thread Tools

Search this Thread

Jun 20, 2019 | 07:07 AM

Tiger Joe

Thread Starter

Drifting

Joined: Nov 2016

Posts: 1,960

Likes: 579

From: Pittsburgh PA

odd alternator issue

when i bought my 68, the owner gave me the original numbers matching alternator in a bag- said he had it rebuilt but it didn't work.

so I decided I need to clean up my garage, and get this on the car and off my floor

so I dropped the #s matching alternator off at a local shop Tuesday- it tests out fine. putting out 47 amps, which is 10 more than its rated at.

last night swap it on the car, no charge. swap the old alternator back on, car charges just fine.

how can it test ok on the bench and not work on the car? also, considering the car charges fine with the current alternator, that sorta eliminates anything else on the car doesnt it?

Jun 20, 2019 | 08:57 AM

bradleyb66

Burning Brakes

Joined: May 2013

Posts: 789

Likes: 129

From: Los Banos CA

I'm sure someone with a little more alternator knowledge will respond soon - but how did you verify 'charging'? Voltmeter or ammeter, and what RPM was the engine at? If I remember correctly, the older style alternators need to reach a certain rpm before they start charging?

Jun 20, 2019 | 09:11 AM

Tiger Joe

Thread Starter

Drifting

Joined: Nov 2016

Posts: 1,960

Likes: 579

From: Pittsburgh PA

checked with the amp gauge in the car and a multimeter.

good alternator on car
1- amp meter immediately jumps up to the positive side on startup
2- battery reading 13 something at idle, jumps over 14 with revs, reading 12.6 with car off

numbers matching on car:
1- amp gauge doesn't move on startup. pull out headlights it drops way down in the negative
2- battery reading drops all the time- 12.6 car off, 12 with car running. revving has no effect

Last edited by Tiger Joe; Jun 20, 2019 at 09:11 AM.

Jun 20, 2019 | 09:21 AM

bradleyb66

Burning Brakes

Joined: May 2013

Posts: 789

Likes: 129

From: Los Banos CA

I agree - that is odd. Are you taking your voltage measurements at the battery or the alternator? Since it worked on the bench, I'd verify voltage between the terminal on the alternator and the case of the alternator (basically using your car as the 'test bench'). Maybe there is something about the numbers matching alternator that makes it not ground the same, or the positive connector isn't making contact the same way?

Jun 20, 2019 | 11:33 AM

69L46vert

Pro

Joined: Jul 2006

Posts: 505

Likes: 25

From: Buxton Maine

Can we assume that the working alt. is for a 68? Externally regulated?

Jun 20, 2019 | 12:52 PM

7T1vette

Team Owner

Joined: Jan 2006

Posts: 37,637

Likes: 3,118

From: Crossville TN

The alternator for 1968 did not have any voltage regulator inside the case. The voltage regulator was on the firewall inside the engine compartment. If your car got upgraded to a later model alternator (with internal regulator), the regulator on your car may have been removed. For the original unit to function properly, you will need to have the proper voltage regulator and wiring in working order.

It's nice to have the original alt available. But your best option to get your car up and running again would be to buy a replacement alternator for a '69-72 model Corvette (42 amp or higher) if that is what failed.

Last edited by 7T1vette; Jun 20, 2019 at 12:55 PM.

Jun 20, 2019 | 02:36 PM

Tiger Joe

Thread Starter

Drifting

Joined: Nov 2016

Posts: 1,960

Likes: 579

From: Pittsburgh PA

Problem solved.

Whoever rebuilt the original alternator years ago forgot to put any of the nuts on inside the case. So while the alternator was making voltage, it was not getting voltage at the terminal to the regulator.