Bad alternator?

[igidy]

I have recently performed some tests on my alternator (which I suspect to be faulty). I took it in to a couple of local auto parts stores and it tested good at both places (showed N/A for diode check). I decided to check the current output from the amp and here is what I found. At idle, with all electronics off, the current was approximately 12 amps. At 2000-2500 rpm the current was approximately 12 - 12.5. Still at 2000-2500 rpm, the lights were turned on and the amps jumped to around 40 amps. The blower was then turned on and the amps jumped to around 50 amps. I had the revs increased and the current maxed out at around 58 amps. Here's where it gets really funky.... I switched my ammeter over to a/c and sure enough i measured 3 amps (A/C!!!!) at 2000-2500 rpm. I know these things should NOT be putting out a/c.... how could it test good with a funked up prob like this. This alternator is most definitely bad isn't it?

 

[beba]

try disconnecting the battery when the car is running, but parked please, if she keeps going your alternator is fine, if she stalls out get a new one
and you can just disconnect the red cable

 

[Justin2129]

i have an alternator if you need one

 

[pcb]

^^ Two thumbs up for the disconnecting the battery technique. It's fast and easy and can test for a bad alternator that some bench tests can't detect.

 

[P-Funk!]

Doesn't the battery provide primary power to the ECU and the field charge for the alt? Meaning this test would not work - but fail 'every' alternator?

Has someone tried this yet? I would not -the power spike to the ECU scares me...

 

[pcb]

Yea,... I did it to prove to my friend that the car would die if the battery was disconnected. I was wrong and my car kept running (2002 P5).
I didn't notice any ill effects from disconnecting the neg. battery cable. I reconnected the battery cable with the engine still running as well. The car will die if the alternator is bad.
I guess it's kinda an old-school backwoods technique but it's super fast and easy to do compared especially to removing the alternator for a bench test.