Edmunds Answers



  • zaken1 03/02/10 3:36 pm PST

    All too often; when people test an alternator, they neglect to test for shorted diodes. If an alternator diode has shorted out; it will drain the battery while the car sits unused. But an alternator with this kind of damage will often still pass a voltage output test. If you have an electrical tester with an ammeter in it; one test you can make (after the battery has been fully recharged with a battery charger; not with a brief jump from another vehicle) is to disconnect the battery ground cable, and connect the ammeter between the battery negative post and the cable clamp (with the negative meter lead going to the battery post). With no electrical items turned on in the car, and no lights on; there should be less than 0.350 amps reading on the meter. If the meter reads any higher than that, there is an excessive drain from something. If the meter reading is too high; then disconnect the power cable to the alternator, and recheck the meter reading. That will tell you whether the alternator is the source of the drain. If the meter shows there is excessive drain, and it is not coming from the alternator; try removing each fuse in the fuse blocks, one at a time; and see whether the drain drops below 350ma when one fuse is removed. It removing a fuse makes the drain go down to normal; there is a shorted or defective component in that particular circuit.

    If you don't have an ammeter, you can test the alternator by disconnecting the power cable to the alternator; letting the car sit for a week after that, and then see whether the battery has power. Sometimes, a new battery will be defective; so if it goes flat during storage when the alternator is disconnected, I would have the battery tested or replaced by the store where you bought it. 

  • zaken1 03/02/10 4:28 pm PST

    I just realized that I misstated the acceptable current drain level. It should be 35ma (.035 amp); not 350ma. My age is apparently showing.

  • saturn_sc2 03/02/10 10:10 pm PST

    it could be three things 1 your battery 2 your altenator 3 your starter. to find out if its your altenator take the cables off your battery if your car dies thats it if not check you starter they hook it up free at auto zone

  • zaken1 03/03/10 2:49 am PST

    The person who recommended testing for a bad alternator by disconnecting a battery cable while the engine is running apparently has not tried this themselves many times (or else never understood why his alternators were failing so often). This is something that alternator manufacturers warn against doing; because it is practically a guaranteed way to blow alternator diodes. On some brands of vehicles; disconnecting a battery cable for even one second while the engine is running will immediately blow a diode.

    You see, the battery must always be connected when the engine is running; because the alternator needs the constant load of the battery to absorb voltage pulses. No other electrical item on the car will stabilize the alternator output in this way. When a battery cable is disconnected, the alternator voltage will suddenly increase past safe levels; because the voltage regulator is made to not respond instantly to changes, in order to avoid making the lights flicker or create unstable pulses in the electrical system. The alternator output is normally kept at about 14 volts; but it can instantly jump to 70 volts or more when a battery cable is disconnected. And that is more voltage than alternator diodes can handle; so they pop when that happens. So please don't try that.

  • kiatech1 03/03/10 5:56 am PST

    You said it,its allso more than the headlights,stereo or any outher accessory thats on at the time can handle.


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