Edmunds Answers



  • zaken1 09/25/12 11:16 am PST

    The most common cause of this type of problem is that a diode shorts out inside the alternator. Many mechanics do not understand how to check alternators for shorted diodes; which is why they say it is good when it really isn't.

    When an alternator diode shorts; the alternator drains power out of the battery when the engine is shut off. You can stop this from happening by charging the battery, driving the vehicle normally; and then disconnecting one of the battery cables whenever you shut the motor off. Reconnect the cable just before the next time you use the vehicle.

    Of course; that does not fix the problem; it just prevents it from interfering with your use of the vehicle. The way to fix the problem is to replace the alternator with a remanufactured unit. If you buy one from a typical discount parts store; there is a large risk of getting a defective or improperly repaired unit. There is a huge scam about rebuilt electrical parts going on out there; which is a shameful commentary on business ethics in our country. As a result; the only consistently safe place to buy a remanufactured alternator is NAPA Auto Parts stores.

    Here's a relatively easy way to proive whether the alternator is bad: At a time when the battery is well charged; disconnect the battery ground cable. Wait 5 minutes, and then briefly touch the ground cable clamp to the negative battery post; from which it had been disconnectedt. Watch for a spark when the cable clamp touches the battery post.

    If there is a spark when the battery ground cable clamp touches the negative battery post; do not connect that cable. Instead, disconnect the heavy power cable from the alternator, and wrap the metal terminal on the end of that cable with a thick rag or cover it with electrical tape, so it cannot touch any matal objects. Then touch the battery ground cable clamp to the negative battery post, and watch for a spark. If there is no spark this time; the alternator is defective, and should be replaced.

    If there is still a spark; there is a short somewhere else in the vehicle's electrical system. An auto electrical specialist would be the kind of person to fix this.


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