Alternator over-charging

Aaron · Post by **Aaron** » Sun Dec 14, 2008 4:25 pm

What could cause the alternator to be charging 18v at idle?

The battery is good, and has 12.58v when the car is off. But when I start the car, it jumps up to 18v. The alternator is wired in correctly.

cactus bastard · Post by **cactus bastard** » Sun Dec 14, 2008 4:32 pm

The voltage regulator?
I had this exact same problem last week, my voltage gauge suddenly jumped to 18 and the light came on. I bought a new voltage regulator for it, then realized I was going to have to remove the alternator to replace the regulator anyway (it's inside the alt). So I replaced the whole alternator, didn't want to fuck around with it anymore than I had to. I'm on call this week, and if the phone rings and my car's not running, then I take a taxi (so much for a paycheque).

When I replaced the alternator, I noticed the main power lead was very badly corroded, and one of the wires going to the alternator (the S pin) was completely broken. So that was probably my whole problem from the beginning. Oh well, now I have a spare alt at least...

Um, my story didn't help you did it. I still think it's probably the voltage regulator. You can also get shorts inside the stator that can cause it to overcharge. This will kill your battery obviously, and has the potential to kill other electronic stuff (car stereo, maybe even the PCM).

Aaron · Post by **Aaron** » Sun Dec 14, 2008 4:37 pm

No it did help, I appreciate the advice.

The power feed to the power dist block is good, and the only wires to the alt are good as well (There's only 2 on the DOHC).

I will pull off the alternator and have the auto parts store check it. Because if the voltage reg is bad, it will be bad on the test bench as well.

cactus bastard · Post by **cactus bastard** » Sun Dec 14, 2008 4:38 pm

Here we go, testing procedures to find out what's wrong with your alternator:
http://www.alternatorparts.com/cs130_sbpage1.htm

If you've got a CS that is, which you probably should.

MstangsBware · Post by **MstangsBware** » Sun Dec 14, 2008 5:01 pm

What are you using to test the voltage coming from the Alt?

Aaron · Post by **Aaron** » Sun Dec 14, 2008 5:30 pm

A volt meter, what else?

Aaron · Post by **Aaron** » Wed Dec 17, 2008 1:03 pm

Alternator checked out good, putting out 14v perfect.

This leads me to believe that it's a ground issue, but the grounds are good, and setup just like factory. Big strap to the block, small cable to the frame, and I cleaned the connections well.

Aaron · Post by **Aaron** » Wed Dec 17, 2008 1:45 pm

I checked the negative to frame ground, and it was good, with no resistance.

I checked the negative to engine block, and got some really weird readings. resistance jumping around and such. I pulled off the ground cable, cleaned the engine-side terminal, cleaned the boss in the block, and wire-brushed the bolt. Put it all back together, started the car, and getting a perfect 14.75-14.76 volts. Very nice. It also seemed to run a bit smoother than before, although the idle is still a bit high. I think that will tone down once I get it driving, and into closed loop.

Unfortunately, his car does no have cruise, so I don't have a clutch switch to tie into to get the ECU its 12v from that. So his idle may be a bit weird, and he will get a SES light every now and again. But I don't expect too many problems, I'm running my 94-95 PCM, which doesn't have a clutch switch input, and its idle is perfect.

His clutch system has a problem somewhere. I'm going to bleed that, bleed the brakes, and see if I can't take it for a drive now that the roads are clear.