Re: HELP! 86'Firebird 2.8 won't charge more than 12V

I hate stuff like this because its all really a guess unless you have the right trouble shooting manuals from the factory. Here are a few things off the top of my head.... First off, although it sounds weird it could be possible for an alt to check out fine on the bench but when it actually is in the car it wont charge. Who bench checked it and how did they do the bench check? Did they open it up and check everything inside with an ohm meter? I don't know which cars had this problem (I cant remember but I think it was a GM) but here it is anyway. As far as i can remember certain GM engines had a bad or insufficient ground going from the alternator itself to the engine. Even though the alt is mounted to a metal bracket that is mounted to the engine it still couldnt get a good enough ground and the alt wouldnt charge. It seems impossible but i've seen it happen. More than likely this isn't your problem but at least its something to think about. You could always run a wire from the alt mounting bolt to a really clean engine ground to see if it helps. These cars do have internal regulators right?

The plug that plugs into the alternator:

One terminal is for the light/gauge, and one terminal excites the field. When the battery is fully charged, turn on the key with the white plug removed from the alternator to see if you are receiving current to the terminals on the white plug.

When it was bench tested, they put twelve volts to the field terminal in the alternator. If you aren't receiving voltage to the white plug when you turn on the key, the field isn't getting the current to cause the magnetism, hence. No charge.

First look at the fuses, possibly there will be a charge, alt, batt, gauge fuse. it might be as simple as that. If not, you're in for some diagnostics.

I think the web site is

Refinish King

As someone else mentioned, make sure the alternator is grounded. I have seen the brackets not ground a lot of times over the years on GM's. You can check that by taking a booster cable and running from the battery negative to the alternator case to see if it turns on.

My 86 Jeep runs the same GM alternator and it gets it's excite power from the coil power wire just before the ballast wire (on the 12 volt side).

I also have seen this same alternator have the wires switched on the small white plug on the alternator. Some versions have the A and B switched for some reason on different years of Jeeps anyway.

Maybe your harness is for the other version of the alternator and those two wires are in the wrong places?

Mike

Gil wrote:

Close King, it's

Thank you kindly for the correction:

I like to help, not give out misinformation!

Sincerely,

Refinish King

Ok, latest senerio: Ran a good ground (jumper cable) from neg batt term to nice clean part on alternator new alternator. no luck still right at 11.8 volts when running. Checked small plug at alternator- key on = 12volts on brown wire. Checked fusable link in red wire behind passenger side headlight (under air box) = OK by piercing wire casing with pins either side of the link & ohm test.

Took alternator back to Autozone for another bench test = OK

now what??? Hmmmmmmmm

Still Gil

snipped-for-privacy@cin.net

Ok.

Now look on the back of the alternator case for the plug numbers on the white plug.

The small brown wire needs to be on pin 1 on the case side and the red wire on pin 2.

I have seen the pin numbers on the alternator be opposite on some so the brown and red wires need to be swapped in the plug. They come out of the plug by pushing on a tab with a mini screw driver if needed.

The brown wire is on top on mine, my old tenant had his on the bottom.

Also the place to check for output is right on the alternator bolt for the big red wire and on the alternator case. That gives a reading off the alternator not off any wiring that might be bad.

Mike

alternator

Mike,

That was it! now charging @13.5v!!! hoohoooooooo!

Ya think somebody or some instuction sheet could could have mentioned that when the Alt. was bought?

Car now F/S not to bad for an 86 f'n bird.

Thanks all for your help!

Gil

Your alternator

You are welcome.

It is a strange one though isn't it?

I figured this out by helping folks with Jeep wiring. Jeep uses the GM alternator for a mess of years and with all the mixing and matching folks do on old vehicles...

Mike

Your alternator

Well, the only thing I'm wondering now is, the volts are not droping back down to around 12.5,only for a r short time after the first start. After that its staying up at 13.5+, even with all acc. off. Hmmm. maybe be overcharging now? getting a draw somewhere? or we weakened the battery by draining it down to many times? So far its running & the kid has not called home to come & get him.

Thanks Again!

Gil

Dropping down to 12 would mean a problem. 13.5 volts is about right in most cases. Usually should be between 13 & 14 volts from the alternator I think.

That sounds normal. Depending on the state of the battery, my alternator varies on the gauge between 13 and 14.5 volts.

Mike

Great catch on that Mike!

I couldn't have done that without having the parts in my hand in plain sight!

Congratulations for being a long distance diagnostician!

Refinish King

You Da Man!

get? Your alternator

still going....

Actually...it better NOT drop back down to under 13 when running...or you have an issue again..

It sounds like its fine....