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When testing an alternator on a car, it’s important to first check to see if a battery has enough voltage. Use a test light on a car’s alternator with help from an auto mechanic in this free video on car repair and diagnostics. Expert: Tom Brintzenhofe Bio: Tom Brintzenhofe has been a mechanic for the past 17 years. Filmmaker: Reel Media LLC

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19 Responses to “Car Repair & Diagnostics : How to Test a Car’s Alternator”

  1. Zorritototote2000 says:

    this guy is drunk or what

  2. jerseyRJ says:

    so do u touch negative battery terminal and alternator terminal for the reading?

  3. crackboompow says:

    He kind of “shotguns” the alternator. he didnt check for amp output, good effort though, I hope all the fuses were checked before alt. replacement……

  4. gamzmasta says:

    theyres a easier way, its my way!, lol.
    on a serious note, just disconnect the battery terminals while the car is running, if it turns off the alternator maybe dead, if it still runs your ok, check fuses and wires before buying a new 1, final test when you have taken it out the car, conect it direct to a battery and it will spin like a fan if it is working be carefull when trying this, e.g make sure you use jump leads or something, and that you wont injure ur self

  5. bluegap81 says:

    GOT NAP?

  6. woodterm says:

    gamzmasta, you almost got me with this one… lol

  7. roarpony says:

    If I don’t see a patch that says ASE on his shirt that means it’s BS. lol I wouldn’t recommend performing this procedure if you’re not an expert. He did the volts but I’m wondering what “is here”? An alt… has volts and amps. Don’t play with amps if you don’t know any about current. Gamzmasta I wouldn’t recommend your procedure either. Never, never disconnect a battery while it’s running.

  8. hkssupra86 says:

    i’m automotive wireman..i special list repair truck ,forklift,car,excavators..eletrical system….
    u work like shit~~~

  9. thenightwolf80 says:

    This guy speaks the truth whether you believe it or not. When connecting your DVOM from the battery to the Alt. it doesn’t matter which one is connected where color test lead wise so long its from the batter Neg. terminal to the Alt. Hot lead ( not the connector ). You should get 13.5-14.8V (DCV) anything under or over that the Alt. is defective.

  10. albrainya says:


  11. TBWdracula2003 says:

    do you think any mechanic has an ASE patch on EVERY SINGLE PIECE OF CLOTHING they wear? no. your retarded

  12. roarpony says:

    The battery acts as a buffer and stabilizes any voltage spikes that may cause damage to the vehicles electronics so disconnecting the batt is not a good choice. Do a voltage output and curren output test, a voltage drop test, a full field test if the output test fails. Make sure the batt is fully charged.

  13. roarpony says:

    A retard with an ASE badge on my shirt then. lol I’m not trying to be rude but if you want others to do the right procedures I would recomend you to know the facts. Back in the days you could do that I admit but now it’s all about electrical parts. The deadline for an ASE test is on Sept 30th. I have to redue mine on EP.

  14. angelvoicedee says:

    There has been no mention of the exciter feed from the dash light i.e. a blown bulb/ interrupted power supply . Or voltage drop test between the neg battery terminal and the alternator chassis.
    Some diesel engines use one of the alternator terminals for the engine rev counter .
    This can be a factor to be aware of .

  15. youdragon says:

    i have a stock gmc truck think my alt can handle 4 100watt off road lights?

  16. zillsbury says:

    thanks govna!

  17. STFUNOWlol says:

    If you’re talking about 400 watts total, and assuming you have a standard 12 V battery, you’d be talking about a 33 amp draw on your battery. Considering that the cranking amps of a battery are in the hundreds, and the operating amp of a battery are much lower, I would imagine that it would be fine.

  18. stankwho says:

    the last part..were did you plug the multimeter after you did the light test on the alternator?

  19. youdragon says:

    if i put a 150 amp in place of my 105 amp alt will it kill my battery by over chargeing??? and if so hwo can i fix that? im putting about 520watts of off road lights onto my truck and need a alt that can handle it but those light wont always be on and i dont whant to damage my battery, any tips?

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