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alternator going bad?

Discussion in 'General Chevy & GM Tech Questions' started by loic, Aug 20, 2009.

  1. loic

    loic Rockstar

    i just bought a 98 tahoe, the battery indicator on the dash shows 13 v while driving, if i turn the a/c on, it goes down to 11v. is my alternator going bad? if so how easy/difficult it is to change it? and of corse how much $ i'm looking at ?
  2. vncj96

    vncj96 Epic Member 5+ Years 1000 Posts

    its very easy to do it yourself, but before you do, start the truck, then disconnect the battery and if it dies then you know its the alternator if it keeps running its your battery or the connection to the battery somewhere
  3. loic

    loic Rockstar

    ok, I'll try that today, thanks !
  4. 2COR517

    2COR517 Epic Member 5+ Years 1000 Posts

    That old test is generally considered ineffective these days. You need the battery there for a solid baseline voltage. You can also damage your alternator that way.

    Your symptoms actually sound normal Does the voltage go up as soon as you rev the engine a bit?

    You can also go to an autozone or whatever and they will do a free on vehicle load test of the alternator.

    It is also a good idea to make sure your major electrical connections are nice and clean and solid.

    Battery, starter, alternator, grounds......
  5. Jimmiee

    Jimmiee Epic Member 5+ Years 1000 Posts

    X2 on a bad test method. Not only can you damage the alternator you can damage solid state modules and the PCM.

    The first thing to check is the battery cables. Very common for corroded cables to cause charging problems. Once the cables are clean then you can test the alternator buy turning on all accessories that have an off on switch including the A/C and rear defog. Then watch the voltage with a volt meter hooked to the B+ terminal on the alternator. At 2000 RPM you should see at least 14.5 volts. If you still see 11 or 12 volts you can bet the alternator is bad.
  6. loic

    loic Rockstar

    I went to a checker, the guy check the alternator but at Idlle without any accessories on and read at the battery 13.4 V
  7. vncj96

    vncj96 Epic Member 5+ Years 1000 Posts

    I have done it on mine numerous times with no problems:neutral:
  8. Jimmiee

    Jimmiee Epic Member 5+ Years 1000 Posts

    Go buy a lotto ticket, NOW! :)
  9. 2COR517

    2COR517 Epic Member 5+ Years 1000 Posts

    Well, it's definitely charging, at least some. Static charge on a lead acid battery is 12.6 volts, after you drain the surface charge. You can buy a digital multimeter pretty cheap. It's a very handy tool to have. With the truck off, check your battery voltage. Put your headlights on for a minute or so, then shut them off. Take another reading. It will probably be a bit lower, but should be just about 12.6 volts. Now start the truck, and take another reading. Have someone rev the motor to 2 grand or so and take a reading. Then turn on lights/heater/etc and take your measurements at idle and revved up. A good alternator should be putting out better than 14 volts. This may vary as you change motor speeds, and turn accessories on and off.
  10. Did you ever figure out

    Loic did you ever find out what was going on?

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