How to check an alternator??
 

Does anyone have a good easy way to check an alternator? The kid at AZ (and those guys are looking younger and younger every time I go in there. maybe I'm just getting older) told me alternator was bad after I already dropped $150 on a new battery. (the battery was 10 years old, so I don't feel too bad about putting in a new one - I think I got my money's worth out of that battery) But for some reason, I just don't trust guys. Maybe I just need to take the alternator to another store or my small shop mechanic to have it tested before I sink another $200 in a new alternator.

Alternators fail a couple differs ways. One failure is caused by high miles the bearings and brushes wear out and it makes noise or quits charging. More often lately I have the rectifier fail. The alternator still charges but it will not make 14.2 volts, instead it makes about 13.5 and makes some AC voltage. Sometimes the light will be on at idle and go out when you rev it up. When the rectifier fails it can discharge the battery overnight. The digital charging system testers qualify the alternator, but don't necessarily tell the salesman what is wrong. They aren't going to sell you the repair parts just a replacement unit. An alternator shop will fix what's wrong but with labor at $100.00 hour it is cheaper to put on a Chinese reman.

Hope this helps

Yes that does help, thanks. According to the tester that the kid at AZ was using, the diodes checked good but the output was too low. I can see how the diodes being bad would drop the voltage and be reason for a bad alternator. But how can the alternator put out less than what the load is demanding, i.e. what failure mode in the alternator causes it to not be capable of meeting the full load demand? I'm assuming that these alternators are multiple pole machines (although I do not know that for sure) and if so, if you lose one of the poles, then the output capacity is reduced. That's about the only way I can see that the alternator is not capable of producing its rated output.

Update - went out to a different AZ and asked them to check the alternator - checked fine. Been driving the dang thing all day and no problems. I'm thinking the kid was just trying to hack me for a battery and a generator. That's why I don't trust those places.

I'm glad to hear that things seem to have worked out. This mornings question had me thinking I needed more info from you.

A second opinion is always a good idea. For the most part, any of the 3 major parts stores here offer free testing on the car. The Zone, Advance and O'Reily all do it here, free of charge. And in all the years I've been farting around with cars, I rarely have had to replace an entire alternator. I'd say 95% of the time a new set of brushes was all they needed and 5% of the time a bearing needed replacing. You used to be able to purchase these parts separately at a GOOD parts store, like NAPA. It's been 10 years since I needed a set of brushes, so don't know if they still sell them nowadays or not. And I agree 100% about the "help" at most parts stores nowadays. You don't get much for minimum wage and these kids don't know squat about cars.

I was always told an easy alternator check was to run your car at idle with the headlights on. Push in the accelerator and if your lights dim, odds are you have a weak alternator.

I turn on everything possible meaning rear defrost air conditioning headlights and radio then check the voltage which the alternator can hold rev it a little see if it rises. It should have a minimum of 12.5 and 13.5 is more like it and not rise when you rev it up otherwise the rectifier diodes are going bad and it should be replaced. A battery should not go below 10 volts when cranking to start or it's going to fail soon