testing alternator capacitor
 

My alternator is not charging the battery.
I pulled it and O'Reilly said it had a bad capacitor.
Can the capacitor be tested through the three prong plug or do you need
to tear the alternator apart to get to it?

Also, what is the black part next to the three prong plug?
It is showing an open circuit between the ground and the end of
the wire. The insulation broke off when I removed it to test.

Voltage regulator is good.
I had this alternator rebuilt three years ago locally.

Jeff

The capacitor is nothing but a noise supressor. The alternator can live happily without it.

That would be the capacitor.

Is the capacitor the same as the diode? or is the diode internal?

A capacitor is to a diode what a muffler is to a fuel injector. One is essential to output; the other just keeps the noise down.

From what I've been reading the capacitor in an alternator
has three diodes and a ground. Ohm resistance should only flow in only one direction
across the diodes. The capacitor turns AC current to DC current.
Please correct me if I'm wrong.

Are you saying that an alternator doesn't need a capacitor?
Also, that doesn't answer my question about whether or not I
need to take the alternator apart to check the diodes in the capacitor.

Thanks,
Jeff

You are wrong.
Diodes convert ac to dc. Capacitors just smooth out the spikes.
There are no diodes in the capacitor.
Who ever is giving you the story about the capacitor is pulling your leg.
Best you go see a good auto elec or read a book on basic electronics.

The word you are looking for is Rectifier.
http://en.wikipedia.org/wiki/Rectifier

I don't know if a bad capaciter could cause a charging issue (like a short circuit) but it could be unplugged to see what happens.
I don't recall my Alternator having a Capacitor.

Easiest way to check out the diodes is with the to use a multi tester. Use the continuity setting, and with the alternator disconnected, touch one lead to the alternator connection that the battery wire is usually connected to, and with the other lead, touch the alternator case. If you have continuity, one or more of your diodes is failing.

I can see where the OP is getting confused. The higher quality rectifier circuits do, indeed, make use of capacitors. The capacitor serves to make half and full wave rectifers generate a more constant DC output voltage. Otherwise you'd have positive (or negative) going half sinusoidal waves only. This isn't very efficient and could possibly wreck havoc with sensitive electronics among other things. Think of it like a "miss" in our engines, buy only invisible.

The alternator could very well function a long time with a bad capacitor (assuming it has one). But it wasn't designed that way.

Pimpernell, Thank you for answering my question.
I am checking just continuity and not ohms resistance, correct?
In the photo above, I get a continuity reading between the prong closest
to the D+ and the alternator body. The other two prongs do not show continuity.
Bad diode in the rectifier? The black capacitor shows neither continuity or ohms resistance.

911- thanks for the clarification regarding the rectifier. The fellow at O'Reilly
told me I had a bad capacitor, not a bad rectifier. The alternator is going to the shop next week.

Grumpy Ol' Layback40--Hey, I'm just trying to learn a little about checking these alternators from
you nice folks without having to read an entire book on basic electronics.

Jeff

[QUOTE=Rooster300SD;2957056]Pimpernell, Thank you for answering my question.
I am checking just continuity and not ohms resistance, correct?"

Yes, just continuity between the battery connection of the alternator and the alternator case. This does not guarantee that all your diodes are fully functional, but will show if one has shorted out, and allowing current to drain off via the alternator case to the engine block.

In my case, one or more of the diodes went bad, and when the engine was off, in a matter of 5-10 hours, the battery would be almost dead. I disconnected the battery, charged it, and it had no problem holding the charge. Disconnected all the wiring to the alternator, and did the above test, and found that the alternator was draining off current. New alternator installed, and everything has been fine. Good luck!

FYI
 

Automotive repair training book on line, you can download it.
Construction Mechanic Basic Volume 02 - Construction methods and practices


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