Question about 1996 Isuzu Trooper

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Alternator output fluxuates

At cruise speed, the output of my '96 Trooper's alternator will drop from about 14 volts down to 10-12 volts, then it will jump back to normal without apparent reason. This sometimes happens during acceleration, as well. I'll check connections, but does this problem sound like a diode failure?

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  • brianwmack Feb 11, 2009

    Oops! That should read "fluctuates."



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It sounds like your alternator could be on its way out

Posted on Feb 11, 2009

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It could be a number of problems, pretty much all of which would go in the alternator category.

Voltage regulator, diodes, etc. It's a warning sign, so get it taken care of soon. As long as your BATTERY itself doesn't get less than 12 volts, then you'll still run OK. But you don't want the alternator dying at nighttime with you stuck trying to get home while burning all your headlights, heater motor, etc.

Once it gets so dead, it'll eventually not have enough power to keep the engine going, then you be dead.

You can have the alternator taken off and tested, but it sounds like you should just get a new one.

The parts store will often offer a "new" alternator, or a "new rebuilt" alternator. The prices difference is usually drastic, so it just depends on your experience with rebuilt parts. They'll both have warranties. If you chose a rebuilt one, they'll need your old one for a core charge.

You could also try Ebay. Sometimes they'll have a really good deal on a new or rebuilt one that's just sitting on someone's shelf.

Depending on your engine, it's not too hard of a job to do. Just make sure you disconnect your battery first and get the "+" and "-" in the right spots on the alternator. You'll probably have a serpentine belt to remove, then a few bolts for the alternator, then connections going to your alternator. If you're lucky, your alternator may be sitting on the topside of your engine rather than being tucked down in a hard to reach area.

Posted on Feb 11, 2009


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1 Answer

What would cause the battery gauge on a 93 jeep grand Cherokee Laredo to bounce between 14v and 9v. this is also running the battery dead and not charging the battery back up.

Several things need to be checked before an accurate diagnosis can be made, however in general it is most likely your alternator going out OR a bad connection in your alternator/charging circuit wiring.

It's also important to test/verify that you have a good fully charged battery (with no dead cells) before any tests of the charging system. An alternator needs ample battery power into it before it can produce good charging voltage and amps out of it and back into the battery.

If you test the voltage directly out of the alternator's output post/connector (with the output wire disconnected), you will be reading direct alternator voltage output, which should be approx 13.5 to 14 volts, steady. If the voltage is at ~14 volts, then suddenly cycles down to 9 volts (or anything less than 12 volts), then you have a defective alternator.
If the alternator, while isolated, tests out at a constant 13.5 to 14 volts, then the problem is most likely a bad wire or connection in the charging circuit wiring.

There is also the possibility of the battery (how old is it?) shorting out internally, causing the voltage fluctuation. That's why I previously said you need to verify each battery cell is fully charged and good. You can usually test this with a battery hydrometer (if it's not a "sealed" maintenance free battery).

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Not sure I follow your problem.
The alternator puts out 13-14 volts at idle but very few amps. If you turn on the headlights at idle the voltage may drop to 12 until you raise engine speed. The amperage goes up with engine speed.
Is the battery voltage at least 14 when the engine speed is 1500RPM or more ?

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