Edmunds Answers



  • zaken1 03/06/12 2:45 am PST

    Here's a simple test you can do yourself, in the comfort and privacy of your garage or driveway. Walk around the vehicle at night, and make sure that no lights are visible inside or outside the car. Include the glove compartment and brake lights in your inspection. If no lights are on; disconnect the battery ground cable, wait one full minute, and briefly touch the ground cable clamp back to the negative battery post from which it was removed. If there is a spark when the cable clamp touches the battery post; do not connect the cable at this time. Then disconnect the heavy power cable from the alternator. Wrap the metal terminal on the end of the cable in a rag or cover it with tape, to prevent it from touching a grounded object. Then touch the ground cable clamp to the negative battery post, and see if there is a spark. If there is still a spark; have the vehicle's electrical system checked for shorts to ground; or to do this yourself; disconnect all the fuses and circuit breakers from the fuse block one by one; and check for a spark at the battery negative terminal after each one. When the sparks stop; the last fuse or breaker that was disconnected is for the circuit that has the short.

    If there is no spark when testing with the alternator power cable disconnected; your "new" alternator is shorted internally; probably due to a bad diode. I regularly warn people on this site about the epedemic of sloppily repaired, poorly tested or untested alternators which are "repaired" by unskilled laborers in Mexico and sold by discount parts stores; and have said again and again that NAPA parts stores are the only reliable local source for alternators which have been properly repaired and thoroughly tested. The employees in discount parts stores which offer free alternator "testing" also do not seem to know how to test alternators for shorted diodes.

  • arislan3 12/18/12 6:34 pm PST

    I don't have another answer to the question, but I want to say that the answer from Zaken1 helped me locate the bad circuit in my 2001 Mitsubishi Montero Sport. Zaken's instructions were exactly what I was looking for to point me in the right direction. I haven't been able to locate the exact location of the short, but if I pull the engine compartment fuse for the dome lights, (top, left if looking from the drivers side over the fuse box), the faulty circuit is interrupted. For now, I'll just drive the car without interior dome lights so the battery stays charged. A minor inconvenience compared to a dead battery every other day.


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