1999 Mitsubishi Montero Alternator issues

  • 3.0L
  • 6 CYL
  • 2WD
  • 200,000 MILES
I've had the alternator replaced twice within the last 2 months. I bought a brand new battery and replaced the starter too all within 2 months. Today. The battery light and the break light lit up in the dash and from prior experience, I know it's the alternator again. I have unplugged the aftermarket stereo from the vehicle and unplugged the ground battery post to reset it but the lights still light up. What could be affecting the alternator so much in such a short period of time?
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Tuesday, September 29th, 2015 AT 11:22 PM

1 Reply

You'd have to autopsy the failed generators to figure out why they failed. The charging system should be tested with the generator on the engine, and voltages should be checked on the wires going to it. One test that only a professional load tester can do is measure "ripple" voltage. If that is high, voltage spikes can damage the internal voltage regulator, and that will stop the generator from working. Wiring problems are more common when there are what seems to be repeat failures, although the clue is the system will generally start working on its own again intermittently.

Also look for any misrouted or added wires near those related to the generator. The biggest offender is spark plug wires near the wire that feeds the field circuit. Those can magnetically induce huge voltage spikes that will damage the voltage regulator or a diode. There's always at least six diodes, and if one fails, you'll lose two thirds of the generator's output capacity. That will usually not turn on the warning light.

Some people will tell you a certain brand of rebuilt generator is of poor quality, but I don't know if that is accurate. It's more of what parts did they reuse and which ones did they replace. This is where knowing how it failed would be helpful, but the only way I know to find that out is in an Automotive shop classroom. I had my students test parts only to understand how these worked. Mechanics never do this kind of testing on the job.
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Wednesday, September 30th, 2015 AT 12:04 AM

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