General Alternator Question

[Clatch]

I have a question concerning alternators. I recently posted a thread, “Rainy Electrical Problems” in which I had some electrical problems going on. A fellow member tipped me off in the post that it might be the alternator. So I took the car up to my local mechanic; he diagnosed it and said it was a bad alternator. However while trying to do my own diagnoses; I drove the car around my neighborhood, from exit to exit on a highway twice, and up to AutoZone and back. The car had to be jumpstarted from AutoZone as that was the only time I turned the car off and it would not start again. The last time I had a bad alternator, the second I took the jumper cables off the battery, the car died; or when I fully charged the battery the car wouldn’t make it out of the neighborhood before it died. So how can I have a bad alternator when I was driving around town?

***Note the car would not start after being shutoff***

[terrormaschine]

have you checked the battery? maybe its not keeping the charge.

[benemorius]

A good battery should be able to run the engine for some time - easily long enough to get you around town a bit, but by that time it may not have enough charge to crank the engine back up. Unless you have reason not to, you should trust your mechanic. It's stupidly easy to diagnose a bad alternator/battery when you know what your'e doing. It doesn't sound like you have a multimeter, but that would be the best way to diagnose it. Get just barely enough of a jump to start the engine, then drive around town to see if it dies on you - up to an hour. If it dies, it's probably the alternator. If it doesn't die but then won't start back up after you kill it, then it's probably the battery. That's oversimplified, but it's something you can DIY if you want to help confirm your mechanic's diagnosis.

[Q45tech]

Alternators are 3 phase units. They have three parallel windings combined together.............each winding provide 1/3 of the current.........common to have a blown diode in a winding giving you RIPPLE and reduced capacity.

[Clatch]

Thanks guys for the in-depth input, I went ahead and had the alternator put in. After I had it done I asked the mechanic the same question I just asked you and he gave me the same answer benemorius gave me; the battery just simply had enough power to run me around town and that they usually last about an hour or so. Thanks agian for the in-depth info!

[DaphneAL240vert]

How would one go about checking an alternator with a multimeter? The alternator just went out(I think) on my '94 240sx convertible, but I want to make absolutely sure that's what is wrong with it before I go buying parts and spending time on it. As it is I'm going to have to negotiate it away from my mechanic on Monday. Also is there anything i need to know that isn't readily obvious about changing it out (like bleeding the air on the return line for coolant when you work on the radiator), like maybe I have to be standing on my head while i do it, or amputate my smallest toe and donate it to charity to make it work... Thank you guys for your help...

Frustrated 240 owner in Daphne,Will