BLKXJ33 said:
Easy and fairly accurate way is to take it to your local parts store (advance, or autozone) They will do it for free, just make sure you grab someone who knows what they are doing. If you want to do it yourself grab a volt meter or watch your voltage gauge if the vehicle is equipped. Watch the voltage as you crank the motor... It should move from about 11.5-12.5 volts (engine off) up to ~13-14.5 volts after the engine is cranked... Anywhere out of that range and you should probably be concerned. I would put a load on while your testing to see how much draw from the electrical system... example, turn your headlights, wipers, and radio on and up to see if the voltage drops significantly while your motor is cranked. Make sure to check if your belt is tight enough/no slipping and/or your electrical connections are secure as well... that can cause a misdiagnoses of a bad alternator.
Easy? Yes. Accurate? Hardly. I used to work in a parts house, and I've had to service a couple of those benches. They test voltage output, but not current. Oops...
Disconnect a wire? Sure - if you want to buy control electronics afterwards. While that would work on vehicles with points and condensers, I sold a bunch of ignition modules to people who decided to try that (and blew the early ICMs,) and it's only gotten worse.
You want to test your alternator yourself? Here's what you can do for a "scratch test" that won't cause you any trouble...
1) Grab your DMM and a notebook and crayon. You don't have a DMM? Get one - you should have one anyhow, and you can get a decent one for about fifty bucks at Sears and better hardware stores.
2) With the engine OFF, check voltage at your battery. Write it down. It should be 12.0VDC or higher (12.6VDC is nominal, I've seen anywhere form 12.2-13.4VDC from a good battery with a full charge.)
3) Start the engine. If you want to check your battery and starter, have someone else crank the engine while you check voltage with the ignition coil disconnected - it should be at least 9.6VDC. Reconnect the coil and start the engine.
4) Check voltage at the battery. It should be significantly higher than when the engine was OFF - around 13.5-14.5VDC. If it's over 15.0VDC, you're overcharging - and you'll blow out your battery in short order. If it's "Engine OFF" voltage or lower, you're not charging. If that's the case, check the voltage
at the output post on the back of the alternator - since you may have a wiring fault instead (there is a fuse or a fusible link between the alternator and the rest of the vehicle. That may have blown on you, for instance - and the alternator is trying to do its job without being connected to anything.)
If voltage at the battery is not good but the voltage at the alternator is; shut down, disconnect the output lead at the alternator, and check resistance between that ring lug and the battery + terminal - there should be "none" (there are conductor losses - but it should be less than 5 ohms total. If it's more, check connections for corrosion. If it shows open, find and repair the fault - it's probably the fuse.)
If voltage is not present at the alternator output, then plan on replacing your alternator.
If voltage at the battery and at the alternator output post are both good, start turning on electrical accessories while monitoring voltage at the battery. It should dip slightly as you turn things on (say, a few hundredths of a volt for each accessory,) but it should remain above the engine OFF value you recorded at first. You are now checking the current output of your alternator - which those parts house benches can't do. The smaller shops that just do starter/alternator rewinds usually can (mine does,) but the larger chains don't bother - and I shan't go into why. Turn on your radio, lights, fans - whatever you've got that's electrical. Voltage should remain a few points above the engine OFF value, even with everything turned full on.
5) Shut down. Check your battery voltage with the engine OFF immediately, and again about fifteen minutes later. It should be comparable to what you wrote down in (1), above. You're doing the second check to make sure your battery is taking the charge fully, and not getting a "surface charge" - which is useless.
Of course, this all begs the question - why for do you suspect your alternator? What trouble are you having?