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Alternator testing? off the car

Is there a relatively simple method to test an alternator while itís off the car?

Iím finishing up a reseal and a degunking project, and while Iíve got the fan off the car, thought it might be best to test it. (It looks overly difficult to replace once itís assembled and in the car).

For some of my (ordinary) cars, I would typically take the alternator down to my local AutoZone store. Theyíd look it up in a book, pull out a small wiring harness from a drawer, and strap it into a machine and give me a thumbs up or down. Talked to the guy at the store today and he says his machine canít handle it. He suggested another store might have a different machine that could test it.

Any suggestions?
Old 06-29-2011, 01:09 PM
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Not really a good way to test it other than on an alternator test bench. Your guy probably didn't have the right pulley, the wiring connections are a no-brainer.

Testing these things requires a big electric motor capable of spinning the alternator while under load- probably about 3hp at a couple thousand RPM. You need a monster clamp to hold the alternator down so when you energize the field it doesn't come loose and fly across the room. You also need a big load that simulates the load the alternator will experience during operation. Think of a heating element from an electric stove and you get the idea.

Anyway, the best way to test it if you cant find a Bosch specialist is to put it in the car and measure the voltage at the battery. It is probably fine, and if you find any problems an R&R of the alternator is about an hour job (if you follow directions and do things like disconnect the battery before disconnecting B+, etc.)

I do not recommend doing anything like this: http://www.youtube.com/watch?v=RQxkAEirtV0
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Last edited by 304065; 06-29-2011 at 03:01 PM..
Old 06-29-2011, 02:59 PM
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1. Put the alternator in a vice.
2. Hold the red wire in one hand.
3. Use your drill in the other hand to hold your drill against the alternator shaft and spin it up.
4. Make sure you touch the alternator casing.

Not sure if that will work but try it and let us know if the alternator is working ;-)
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Old 06-30-2011, 05:05 AM
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Quote:
Originally Posted by JohnJL View Post
1. Put the alternator in a vice.
2. Hold the red wire in one hand.
3. Use your drill in the other hand to hold your drill against the alternator shaft and spin it up.
4. Make sure you touch the alternator casing.

Not sure if that will work but try it and let us know if the alternator is working ;-)
The alternator rotor likely does not have enough residual magnetism to "self-excite." So you would have to put voltage on the field to get power out of it.
The casing won't shock you, it's the ground. You would have to put a load on B+ to get real current.

If you did get the full amperage across your heart it would probably kill you, which is why I don't recommend testing these things on a piece of wood like the gentleman in the video.
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Ex-'88 Carrera (Sold 3/29/02)
Ex-'91 Carrera 2 Cabriolet (Sold 8/20/04)
Old 06-30-2011, 05:21 AM
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Of course my comments are in jest...please dont do this or any variant of it....
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Old 06-30-2011, 05:43 AM
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Thanks guys. Just didn't want to miss an opportunity to bench check it before reinstalling if possible. Doesn't sound like it is. I guess it's going back on the engine and it'll be evident once I get it all back into the car and running.




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