1996 Chevy Blazer Alternators are frying

1996 CHEVROLET BLAZER
120,000 MILES • 6 CYL • 4WD • AUTOMATIC
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My son's 1995 Chevy Blazer is frying alternators. This started when the transmission needed to be rebuilt (another story :) ). Put a new alternator in and it fries shortly there after. Have a Haynes manual but cannot figure out what is going wrong. This connections from the alternator are two wires of the plug that go to the dash charging gauge. These seem to test out okay and when the alternator is charging, it shows about 14v. Off the main terminal on the alternator, one wire goes back to the positive on the battery. There are two other wires off the same terminal that go into the main wiring harness never to be seen again.

The Haynes manual does not show these two. I have a feeling it is somewhere in these two that the problem lies. I had the boy make sure the ground from the battery to the engine block is good (cleaned block area with sand paper), but I'm at a loss to figure out where the problem could be.

The local Chevy dealer says the problem is that the wiring harness was being shorted between the firewall and the transmission block. So the transmission guy rerouted the wiring harness and found a bent connector but last night a new alternator in and poof, toast.

It would be really nice to be able to test this out before toasting another alternator. Advanced Auto is getting a little upset with the returns :)

Any help will be greatly appreciated.
May 7, 2008 at 10:40 AM
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LEGITIMATE007
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I just have a quick question, does your son happen to have a stereo system with an amplfier or something pulling a lot of current?
May 7, 2008 at 11:13 AM
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[quote:9d6fa8eebe="legitimate007"]I just have a quick question, does your son happen to have a stereo system with an amplfier or something pulling a lot of current?[/quote:9d6fa8eebe]

Nope, no amp, no abnormal draw. Yesterday he put in a new one after the transmission guy looked at the wiring harness. Within a few moments, the alternator stopped working as he went down the road.

I'm figuring there is some type of "open" or short but I'll be darned if I can figure it out.

Thanks for taking the time to look however.
May 7, 2008 at 11:29 AM
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LEGITIMATE007
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TEST ALL OF YOUR HIGH CURRENT RELAYS UNDER THE HOOD, LOOK FOR ANY FAULTY RELAYS/FUSES
May 7, 2008 at 12:44 PM
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CAMERSON72000
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is it the fuse
Mar 12, 2012 at 7:38 PM
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CARADIODOC
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Explain how a fuse can be blown but a new generator works at first? The fuse in question is the "gauges" fuse. If it blows, a lot of things on the dash won't work.

This post is four years old. I suspect the problem is solved by now.

In fact, there is nothing on a vehicle that can destroy a generator from over-current as they are all self-regulating in that respect. You simply can't get more than what they're designed for.

What GM IS famous for is repeat failures of their generators. Since the new design was introduced in the '87 models, they went from the world's second best AC generator to by far the worst pile ever dumped on the buying public, and bbergquist has the proof. Because the voltage regulator switches on and off 400 times per second, the field winding acts exactly like an ignition coil primary winding and creates voltage spikes. Those spikes destroy the generator's internal diodes and voltage regulator. They can also radiate into adjacent wires and "induce" voltage spikes in them. When that happens to a sensor wire that computer can cause running problems when those spikes confuse it. Many running problems are identified as being caused by the generator by unplugging it while the engine is running. When the problem clears up, you know it's the generator.

The cure for those voltage spikes and the repeat generator failures is to replace the perfectly good battery at the same time. As they age, they lose their ability to dampen and absorb those spikes. That can be a problem already when it is just two years old. That old battery will work fine in '86 and older vehicles.
Mar 12, 2012 at 11:25 PM