Ghetto Testing for a Bad Alternator

[Dirtymech]

I heard from somebody that a way to test to see if you're alternator is bad is to start the engine and disconnect the negative battery cable.

Any truth to this? Sounds kinda crazy to me.

[curtis73]

The old way to test it was to remove the postive cable, but it wouldn't really test anything and it would almost certainly fry the diodes in the voltage regulator. The vehicle's ignition system only requires about 15 amps, so the only thing that would tell you is that the alternator is capable of producing about 15% of its original output.

The most ghetto way I can think of is to test the battery voltage with the car running and the car off. Often times a bad alternator will only put out about 11v, so the battery will read higher with the car off. It should be 12-12.7v off and 13.5-14.4v while running. If the alternator is bad sometimes it tests 11-12.5 running. Not always conclusive, but usually linked

[Dirtymech]

Thank you curtis. That's all I needed to know =]

[jveik]

lol run the battery dead by leaving the lights on and then jump start the car and see if it will keep running off the alternator with the headlights on and all the power accessories and stuff and then see if the battery is charged at all after like 30 minutes of driving, if the car wont start on its own by then, the alternator is probably bad lol...