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Gary, Pontiac Technician
Category: Pontiac
Satisfied Customers: 17034
Experience:  15 years GM master tech/15 years ASE master
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Pontiac Bonneville SSEi: I have a 1998 Pontiac Bonneville.

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I have a 1998 Pontiac Bonneville. I recently noticed that my batter gauge is dragging down when I try to use my defroster or a/c. I had AAA come out and check the electrical - battery, starter and alternator - all showed normal pull when all systems were being used.

I was told it could be a general electrical problem because the AAA tech used his battery cables connected to his truck and put them on my battery, had me start the car and it started right up. He took the battery cables connected to the tester and put them on my battery, had me start the car and it started in 2-3 seconds, more engine cranking involved.

I was told it could be the a/c compressor is going out, but it works fine when used and is cold, except for the battery dragging down.

I was told it could be a resistor regarding the a/c, don't know where that is.

Can you please give me your advice to get checked. I can't use the defroster now for the Fall and the a/c this Summer. Get back with me and thanks!

Welcome to Justanswer ! my name is XXXXX XXXXX i will do my best to assist you with this problem

so your battery will go dead ?

how low will the charge read with those things on ?

Customer: replied 4 years ago.

Well I hope your answers get better as we go. NO my battery did not go dead. They just tested the battery and alternator for a minute to check that they were charging properly with the a/c, lights on.


The gauge started going down while he was testing and moved back to normal when I would shut the a/c off. Does this help you.


I did not want the gauge to go into the 'danger' area which would take longer than a minute or two.

as you can see it was not an answer I gave you but a few questions to gain more insight on your issue

how long have you owned the car ?

will it only do this at idle ?

while driving with the A/C on or the rear defroster on ?

Customer: replied 4 years ago.

Okay...I have owned the car for 13 years and have had no problems like this in the past and I keep everything!


The gauge goes down while setting at a stop sign but will continue to go down slower while I am driving, but still goes down.


I ONLY have the problem when I am trying to use the a/c or defroster.

ok thank you for the information

the A/C and the defroster are the biggest draw on this charging system when on

when the engine is running the alternator should be able to keep up with any electrical draws or demands asked of the electrical system at all times

since it will do this when driving also we can rule out a basic low engine idle so to say that will not increase the idle at a stop when the engine is on and those things are turned on

driving at speed and it does this would narrow this problem down to an internal alternator issue possible battery cable connections

that alternator either can now physically keep up with the demands due to internally failing OR the alternator is not being told to full charge when you demand more when turning those things on

the PCM = engine computer monitors the demands and will tell the alternator how much to charge

a shop needs to get this to act up and unplug the alternator and connect the alternator test tool and see if the PCM is telling it to charge if the PCM is telling it to charge the alternator would need to be changed

if it is not telling it to charge then the PCM would more than likely be bad and would need to be replaced


Gary and 7 other Pontiac Specialists are ready to help you
Customer: replied 4 years ago.

If I understand what you have written, it is either a computer malfunction, meaning the PCM is not giving the instructions to the alternator, or if it is, then the alternator is not functioning properly.


Is that what you are trying to tell me? If so, this gives me something to give to my shop owner when taking it to him on my next visit.

yes that is what I am telling you