Hello everyone,

I have a charging issue on a 94 camaro Z28.
2nd New alternator (bench test working),
3 month old battery (lots of charging with an automatic battery charger).

I'm getting about 12.5 volts with car off.
With car on I am getting voltage dropping from 12.5v to dead.

Tested positive wire from alternator to battery 0 ohms

So, of course I think the alternator is bad, but it’s brand new and bench test working.

I have also tested the single wire from the instrument cluster to the alternator and am reading about 1 volt less than the battery

Also the ignition voltage (from pcm link) with key on/run is approx. 1volt lower than the battery voltage.

Could this be a grounding issue? a short? bad wiring? With such a simple system i don't know why I'm having such a problem locating the cause.

I am also throwing a SES light, but unable to pull the codes(I don't have software to pull all possible fault codes from the pcm, and its a 94 so I haveing trouble finding a OBDI handheld scanner that actually works for the car)

Any advice or personal experience on the matter would be helpful. Thanks

 

What do you mean by the bench test? What is the output voltage of the alternator? Does it hold that voltage at the rated current of the alternator?

It's still possible that the battery could be an issue. I've seen them so bad at times, that it can't keep the car running but for a short time....alternator could not keep up with the battery's severe current pull.

Have you had the battery load tested?

 

Thanks for the reply

The alternator was tested at an autoparts store. A computerized "bench test" machine was used. output was 14volts and held at 120amps

I was thinking your senerio as well but,
When I have the battery charger at 10ah on the running vehicle the voltage drops, but not at 50ah.

I'm getting the battery load tested today and will report my findings.

 

Well, I got the battery tested, and its good.

I decided to replace it anyways as I have a 3 year replacement warranty on it.

got the new battery and first off had it tested. No problems

I would also like to point out that I do not have the ground cable running to the passenger motor mount. It is running directly to the back of the alternator. Could this be messing up the alternator?

I ran through the tests on the car again,

Vehicle off - 12.70V
Vehicle on - 12.22V - pretty steady over 15 mins (seemed to increase slightly to 12.28V

So, Its not the alternator, the battery, or the +cable from the alternator to the battery.

The wire from the instrument cluster is still reading 1 volt less than battery voltage.

Is it possible I'm not getting a high enough voltage from the instrument cluster to turn on the alternator?

Any ideas please let me know.

 

Where on the back of the alternator is the ground running? Why is it not where it is supposed to go?

 

I agree. Put that wire where it should go. A bad ground can make an entire electrical system go nuts.

 

Thank you for your input,

The ground was relocated to the back of the alternator(on the bolt connecting the rear alternator bracket to the intake) because when swaping the motor i did not put the correct bolt into the engine mount where the ground should have gone, as displayed below. I will relocate this to the proper location and post my results tomorrow.

 

Today I went through and ran a new ground to its proper location. I also replaced the positive wire from the alternator to the battery, the positive wire to the junction box on the passenger side engine compartment, the negative wire to the junction on the passenger side.

Vehicle Off-12.70v
Vehicle On-12.22v

I'm still getting a lower voltage at the wire connecting the instrument panel to the alternator, approx. .5volt less. (More accurate than previous posts)

I would also like to point out that I have a few items unplugged, the air pump and the cooling fans (I'm still working on these). I wouldn't think these would cause the system not to charge though.

Is there anything else that would not cause the alternator to turn on? What signal is actually being sent to the alternator to activate it? I'm getting an approx 12v signal with key on/running.

I'm at a loss here. Anyone?

 

Do not follow my bad advice in this post!!! Read on down this thread!!!

Start the car and pull the positive off the battery. Check your voltage then(I'm hoping the alternator can keep the car running).

Your voltage with the car running(and the battery attached) should be in the range of 13.5-14 volts. You may have to keep going the route you are now....disconnecting circuits until you nail the problem.

 

I've always been under the impression that it is not a good idea to pull the battery cable while the car is running, couldn't this cause a surge in the electrical system. Blown fuses/wires/PCM?

I'll be back tracing the line from the instrument panel to the alternator today and checking resistance and quadruple checking fuses.

 

Well, I went out to do some tests on the car today, and figured I would start it up to get another baseline reading on the charging voltage.

Started the car and read 14.44v dropping to 13.97v (stopped car before overheat .fans not installed)

So I've got a "gremlin" in my electrical, I believe this must be an incrementally open circuit in the wiring from the instrument cluster to the alternator. I'll be ordering a pigtail for replacement, and post my results when I get it installed.

Thanks everyone for your help and wish me luck.

 

My logic was to determine if something with the battery was pulling down the voltage. I didn't think it will blow anything, but in doing some research I've found varying answers as to whether the test will work.

There is one point to where someone mentioned that the output voltage could be erratic due to the stator excitation current not being supplied by the battery. This very well "could" be the case. With this, you were right not to follow my advice. Some suggest that the output voltage could spike. I don't think this would happen...I think the car would just die rather than anything serious happening, but it would be better to err on the side of caution. I do apologize for throwing that advice out there without doing my homework. You certainly don't need anymore problems(especially caused by my bad advice!!!).

I'll try to get a more definitive answer on this subject.

 

In doing some more reading, the alternator's regulator output may be a ripple voltage. The battery may be smoothing out this ripple voltage. That may be enough to give the PCM, radio, and other sensitive components a hard time if you disconnect the battery while the car is running. I'll do some digging in my texts to see I can find some more.....

 

This is pretty good here.....

http://jaguar.professional.org/Alternator.php
http://www.misterfixit.com/alterntr.htm

I think one of the big concerns is if you really do have a bad alternator. If you have a bad alternator(with one of the phases out), then your voltage will swing low, high, low, high if disconnecting the battery. I think that is the main issue with disconnecting the battery.

After doing my homework, it is best not to disconnect the battery while the car is running!!!!

 

Thanks for the research Ace. I'm afraid that many people believe that disconnecting the battery is the best way to check for the alternator. I believe this stems from "Veteran mechanics". On older vehicles, pre-PCM's, it really isn't an issue.