97 Pontiac Grand Prix V6 3800 motor

Tiny
ANONYMOUS
  • MEMBER
  • 1997 PONTIAC GRAND PRIX
  • 160,000 MILES
97 pontiac grand prix v6 3800 motor. The car battery will not charge. Battery is new and had altenator tested at auto store, tested as good. The car will jump start just fine. But then battery looses charge after awhile. Battery light also comes on then goes off. I had car running, disconnected battery and car did run off altenator for about 40 seconds then died. Is there an altenator sensor telling the car not to charge? Help.
Monday, February 25th, 2013 AT 10:53 PM

1 Reply

Tiny
CARADIODOC
  • EXPERT
AAAAGGGGHHH! DO NOT REMOVE THE BATTERY CABLE WHILE THE ENGINE IS RUNNING!

That was a trick done many years ago by uninformed mechanics who didn't understand how these simple circuits work or how to test them. The battery is the key component in helping the voltage regulator maintain a safe system voltage. On older cars all that would happen is you'd burn out any light bulbs that were turned on and possibly destroy the radio. You have dozens of computers that can be destroyed in an instant, especially if you raise engine speed above idle. I did this every year as a demonstration for my students to show what will happen. Your generator can easily reach an output voltage of over 30 volts. That will at the very least destroy the internal diodes and voltage regulator.

Your test method didn't prove anything anyway. Since the warning light turns on and off, you have an intermittent problem that at that moment wasn't acting up. Any testing has to be done while the problem is occurring.

Now that I'm done slapping your hand, use an inexpensive digital voltmeter to measure the battery voltage while the problem is occurring. You must find between 13.75 and 14.75 volts. If it is high, the voltage regulator is shorted. It can be replaced separately but your generator is very hard to disassemble. It's easier to just replace the entire unit. If the voltage is low, that can be caused by a bad regulator too, but since it's intermittent, a more likely cause is worn brushes. Again, disassembly is real hard. Before you condemn the generator, measure the voltage on the output terminal. If you should find it is a lot higher than the battery voltage, there is a break in that wire going back to the battery. The most common cause of that is loose nuts on a large bolted-in fuse in the under-hood fuse box.
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Tuesday, February 26th, 2013 AT 12:19 AM

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