[ZiGeNX77]

Alright so I know what an alternator is, and what it does. I know that one way to test an alternator is to start the car and then disconnect the negative terminal of the battery. This test will help determine if the alternator is still working to charge the battery, or if the car is just relying on the battery by sucking power from it. My question is, is this a good way to test it or is this potentially damaging to your car? I read that it may damage your electrical components such as your ECU. Is this true?

Oh and don't say take it to a shop. That is a given. I want to know how to diagnose the problem on the spot for future reference. My mom's car had a problem, but I got a new battery and it fixed it, but it got me thinking.

[jphase]

well one way i know is to take the alternator and hooking it up to the battery.. but taking one out in some cars are PITA.. if u think u can do this, i can provide a step-by-step..

[wangta]

i awlays thought you were suppose to disconnect the positive terminal, but i dunno never tried it.

[taoh]

It's not a good idea to disconnect the battery terminal because the battery is supposed to act like a filter and prevent electrical spikes from hitting your electronics.

The better way to test an alternator is to use a voltmeter or ammeter (ammeter should support at least 100 amps). Depending on specifications by your manufacturer, the alternator is supposed to generate a given amount of amperage at a certain RPM.

With a voltmeter test, you can check to see if the alternator is supplying anywhere between 12~15V at a given RPM, usually any measurements outside of that range is suspect for a bad alternator.

You'll have to check a service manual or some kind of reference to see what the testing conditions are for your alternator.

If you have access to an alternator testing machine, that's your best bet. Kragen will usually test alternators for free.

[some-azn-guy]

all i did was check the battery voltage(using a dmm or a voltmeter) with the car running.

with headlights and all accessories on, the battery should show a voltage of no less than 12.5v at the terminals. preferably, the reading should show a value above 13.0v but nothing over 15.0v

anything lower than 12.5v means that your alternator is either:
1) going out on you
2) not strong enough to begin with

anything over 15.0v is overcharging and this condition should not be taken lightly, as it will likely cause potential problems with electronic components and might even cause the battery to blow up

i'm not an automotive tech so whether or not you believe in my methods is up to you.