1996 Chevrolet Silverado battery light on

Tiny
JOE1953
  • MEMBER
  • 1996 CHEVROLET SILVERADO
  • 5.7L
  • RWD
  • AUTOMATIC
  • 224,000 MILES
My battry light is on and when I turn the headlights on they are dim but the engine is still cranking the battry voltage when the engine is running is below 12 voltage what is causing the battry light to be on when the engine is running?
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Wednesday, September 23rd, 2015 AT 8:03 PM

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Tiny
CARADIODOC
  • EXPERT
The "Battery" light hasn't changed since the 1960s. It means the generator isn't recharging the battery while you're driving. What HAS changed is GM redesigned their generators for the '87 model year and it's a very poor design. They develop huge voltage spikes that can damage the internal diodes and voltage regulator, and can interfere with computer sensor signals. It is real common to go through four to six replacement generators in the life of the vehicle. To reduce the number of repeat failures, replace the battery at the same time unless it's less than about two years old. As they age, the lead flakes off the plates and they lose their ability to absorb those voltage spikes.

You already noticed the charging voltage is too low. Measured at the battery with the engine running, the voltage must be between 13.75 to 14.75 volts. If it is low, suspect the generator. If the voltage is okay, you need a professional load tester for the second part of the test. That will test for maximum output current and "ripple" voltage. If one of the six diodes has been damaged, you'll only be able to get exactly one third of the generator's rated maximum current, and ripple voltage will be very high.
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Wednesday, September 23rd, 2015 AT 11:17 PM
Tiny
JOE1953
  • MEMBER
I replace the alternater June 11th. 2013 I replace the battery June 12th. 2013. So I didn't replace both the same time.
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Thursday, September 24th, 2015 AT 7:34 AM
Tiny
JOE1953
  • MEMBER
I replace the alternater on today September 24 but I didn't replace the battery the engine is still cranking good, after I replace the alternater what do I do next drive the veichel?
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Thursday, September 24th, 2015 AT 5:07 PM
Tiny
CARADIODOC
  • EXPERT
You didn't list any symptoms now after the generator was replaced. The battery is likely still okay, so leave that alone for now. The old generator from 2013 worked okay for two years, so replacing the battery the next day was good enough to avoid another generator failure. The failure now was just "the nature of the beast". Every GM owner replaces their generator two or three times if they keep the vehicle long enough.

My recommendation now is to have the charging system professionally tested by a mechanic, not at an auto parts store. In particular, what I want to see is "regulation voltage" between 13.75 and 14.75 volts, full-load charging current very close to the generator's rated value, and "ripple voltage" should not be excessively high.

The common generators for your truck were a 105 amp, a 124 amp, and a 140 amp unit. Full-load current will be very close to one of those values, or, ... If there's a defective diode inside the generator, you'll get exactly one third of that value. A generator is physically incapable of developing more current than it's designed for.

If ripple voltage is high, that is caused by the generator developing harmful voltage spikes, but it's the job of the battery to dampen and absorb them. Consider replacing the battery if ripple voltage is high/
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Friday, September 25th, 2015 AT 9:24 PM

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