How should I check the alternator on my '99 Chevy S-10?

Tiny
CSEMRADGO
  • MEMBER
  • 1999 CHEVROLET S-10
  • 145,000 MILES
My battery light came on this morning, so I had Auto Zone test and recharge my battery. Everything is fine there. They said it is probably a problem with my alternator, and I should do some more testing before I have it bench tested. My question is this: what tests should I do on it? Also, my fuel gauge is now stuck 180 degrees from empty, and it's stuck there. What should I do about that?
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Wednesday, September 28th, 2011 AT 9:15 PM

5 Replies

Tiny
CARADIODOC
  • EXPERT
Use an inexpensive digital voltmeter to measure the battery voltage while the engine is running. It must be between 13.75 and 14.75 volts. If it's low, suspect the generator. It is real common to go through four to six of them on GM products, after the '86 models, in the live of the vehicle. To reduce the number of repeat failures, replace the battery at the same time as you replace the generator. As they age, they lose their ability to dampen and absorb the huge voltage spikes the generators produce due to their design. Those spikes can destroy the internal diodes and voltage regulator leading to the repeat failures, and they can cause running problems due to interference with computer sensor signals.

Here is a guide to help

https://www.2carpros.com/articles/how-to-check-a-car-alternator

The fuel gauge should find its way to the proper position when you fill the tank. It looks for the shortest way to the desired setting and that might currently be clockwise against the stop.
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Wednesday, September 28th, 2011 AT 9:26 PM
Tiny
CSEMRADGO
  • MEMBER
After testing the battery with a voltmeter, I can tell the battery is not charging because it has roughly 11.6 volts when the engine is running. Next test?
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Wednesday, September 28th, 2011 AT 9:34 PM
Tiny
CARADIODOC
  • EXPERT
That was a test done many years ago by people who didn't understand how these simple systems work, and it was done on cars with no computers. Many cars today with perfectly good generators will stall when the battery cable is removed. Also, with one shorted or open diode inside the generator, the engine will still run but you will lose two thirds of the generator's rated capacity. You need a load test and a ripple test to find that. The battery is the key component in helping the voltage regulator control system voltage. Every year I did a demonstration for my students on what can happen with no battery in the system. It is REAL easy for system voltage to exceed 30 volts. That will destroy the internal diodes in the generator, any light bulbs that are turned on, and a number of computers. Use the voltmeter. You can find perfectly good ones at Harbor Freight Tools for $4.99.
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Wednesday, September 28th, 2011 AT 9:34 PM
Tiny
CSEMRADGO
  • MEMBER
So I checked the voltage. 12.8 engine off, 11.6 engine running. What now?
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Wednesday, September 28th, 2011 AT 9:45 PM
Tiny
KEN
  • ADMIN
It sounds like the alternator has gone bad I would replace it with a rebuilt unit. Here is a guide that will give you an idea of how to do the job.

https://www.2carpros.com/articles/how-to-replace-an-alternator

Use 2CarPros anytime, we are here to help. Please tell a friend.

Cheers, Ken
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Wednesday, January 17th, 2018 AT 2:47 PM

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