Do I disconnect the negative or positive charge on the battery to the car to check alternator?

I think my alternator is going out. I have been checking the internet, but I get both answers. Do I disconnect the negative or positive charge on my battery to check if my alternator is going out?? I know that I need to disconnect one of those then start the car, if it doesn't start it's the alternator. Right?

13 Answers

  • Joe
    Lv 5
    9 years ago
    Favorite Answer

    That is something we did before cars became so computerized. Now - DON'T DO IT.

    Some people tell you that you can check your alternator by disconnecting it from the battery to see if the alternator can produce enough current to keep the engine running. Disconnecting the battery will subject the voltage regulator (and computer and audio equipment...) to significant voltage spikes which may cause an otherwise good alternator to fail. Even if there were no damaging spikes, this test would not indicate whether or not the alternator was good because the engine will easily run with a weak or failing alternator.

    Simple Test:

    If you want to see if your alternator is producing current, turn on your headlights when you're parked and the engine idling with the headlights shining on a wall (at night). Notice how bright they are. Then turn the engine off. The lights should get dimmer when you turn the engine off. If the lights get brighter when you kill the engine, the alternator was not charging sufficiently. When doing this test, the lights should be the only load (turn the stereo, a/c and other accessories off). With a heavy load, an otherwise good alternator may not be able to produce sufficient amounts of current at idle.

    Source(s): Worked on cars for over 55 years.
  • paul h
    Lv 7
    9 years ago

    Don't do it.

    Disconnecting the positive battery cable to check the alternator output used to be an old school unreliable method to test the alternator...some people suggested holding a screwdriver near the rear alt bearing to see if it was magnetized while the engine was running which indicated it was working but power output could still be low... on modern cars, disconecting the battery cable can cause power surges/spikes and fry sensitive electronics like the computer That can get expensive..And disconnecting the positive battery cable doesn't work while still need battery power to start the car....the alternator simply provides power to run the car's electrical systems once the engine starts and recharge the battery after starting so it's fully charged for the next starting cycle. A vehicle can run off the battery power alone for a short time but with a bad alternator, it will drain down the battery and cause the car to run rough and die.

    It's best to have the alternator output tested as well as the battery... load tested...many parts stores offer to do this for free.

  • Anonymous
    5 years ago

    Don't know what year but you may want to check the connections on the cables to make sure both ends are tight and also check the tensioner on the serpentine as if it is weak it will loosen the belt and cause the alternator to slow down. You might have the alternator checked at a second source, I really place my money on the regulator. Once again most regulators are in the alternator but if you have an older vehicle it may be outside the alternaotr and this would then be the problem

  • 9 years ago

    It does not matter which one you pull BUT First start the car and with the car running you disconnect the positive (usually) but any interruption to the battery will do If the car stalls when you do that it is the alternator

    Source(s): Mechanic
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  • boater
    Lv 4
    9 years ago

    Disconnecting like that is hard on the alternator, if you must it should be the negative, that wire goes on last and off first. The positive one can short out on you if the ground is still hooked up. Go to a shop to have it checked properly and safely.

  • keyt
    Lv 4
    4 years ago

    How To Check Your Alternator

  • 9 years ago

    wrong ,it wont start if the cables are disconnected you can go to any major autoparts store and they will test it for free or get a dc voltage meter place the red lead on the red battery post ,touch the black to the negative battery post should read between 13-15 volts while the car is running

  • 9 years ago

    the old school way is to unhook the positive and if car stalls after awhile its the alternator. Go to a chain auto parts store they offer a free check.

    Source(s): mechanic for 20+ yrs
  • 9 years ago

    Im not sure but I know the people at the auto supply store can check your alternator for you, you might want to call them or go down there to see.

  • 6 years ago

    you would do the positive as some vehicles have extra ground cables hooked up and still keep running if negative is pulled as its grounded else where........ just be careful with the positive cable to not touch anywhere metalic and after make sure car is right off before putting back on

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