69SSC5

ok so i put the volt meter on my battery with the car off and i get 12.5volts, then i turn the car on and test @ the battery and i get less than 12.5 volts. this tells me the altinator is not working. so i took the altinator off and got it tested at two different auto parts stores and it works like new... so why does my battery not charge? my voltage gauge on the dash is always discharging never charging. is there a fuse or a fusible link that i need to change to get power form the altinator to the battery?

1981 corvette 4 speed w/383 stroker

cm2dinger

Some parts stores can only test the output and not the diodes. Although I am not sure of the set up in an 81.

69SSC5

im not sure how the two parts stores tested the altinator but they put it in a machine of some sort

ReyRey76

How old is this said alternator?

I would double check the battery with the car off to be sure you aren't getting a false read. (I've had it happen after attempting to jump a car. Battery showed full charge from the jump but had a bad cell and wasn't working. Didn't figure that out until after the tow.) Then check the cords for any wear or breaks from the battery. If everything looks good then bite the bullet and replace the alternator.

Good luck.

DRIVESHAFT

On an 81 the charge wire off of the alternator should go directly back to the battery.
If you look in the battery compartment there will be a wire coming off of the positive terminal that goes to a junction block.
Check that wire for a blown fusible link, or a bad connection at the block.

Hef

To verify the alternator is ok, with the engine running, carefully connect the volt meter from ground to the output terminal of the alternator - that should be around 13-14 Volts.

Tim H

The alt should put out 14.2 volts.
Either the battery has a dry cell or the alt is about to give up.
1: start the car and take a screw driver and touch the back bearing with the tip of the screwdriver and if its magnetized the alt is charging.
2: while running unhook the negative battery cable from the battery post, if car stays running the alt is good.
3: hook up your meter to the battery and start the car and while its cranking the meter goes down to around 9 volts the battery has a dry cell.
4: replace alt and batt and be done with it.

...Roger...

Sounds like you might have a problem in the exciter circuit.
Try this very simple test,connect a standard 12v test light like the picture.
If this makes the alternator start charging , your exciter circuit is open.

69SSC5

Is the charge wire the red wire

'75

Yes. It should have battery voltage.

Scott1980

I had the same situation. With mine (1980), there are two wires going under the battery box/compartment. If you look under the car there is an aluminum shield which these wires rest. Somehow my wire was skinned and arching? I put 2 new alternators on which continued to show discharging before I found the chaffed wire. Fixed the wire and it seems to be charging now.

MotorHead

Put a voltmeter across the battery terminals when it is running and if it reads 14-14.5V go out and buy a new battery they only last 4-5 years nowadays, if your lucky. The Chineseium in them breaks down faster than a few years ago

69SSC5

thanks for the tips, i will try these things this week and let you guys know if it helped me narrow down my charging problem... thanks for the help, gotta love this forum

69SSC5

hahaha now thats funny

bluthundr

R6n350GT

Just in case this helps anyone else out, i changed my globes out for SMDs and a week later my car was struggling to start. Apparently Field windings according to the wiring diagram are charging through the globe so the SMD/LED in the battery / Alt light was stopping the batter from being charged !!! who would of thought.. Took SMD bulb out and put old style bulb in and i now have 14.35volts back at the alternator not 12v with the SMD led ...