Farm Equipment - Quick 12 volt generator test

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Quick 12 volt generator test

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Category: Allis Chalmers
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Topic: Quick 12 volt generator test

Posted By: ChuckLuedtkeSEWI
Subject: Quick 12 volt generator test
Date Posted: 17 Mar 2010 at 8:03pm

Refresh my memory, what is the quick way of testing a generator, to see if it works. I know you ground one terminal and read the other and it should be double the voltage. I just can't remember which one to ground and which one to test and I want to make sure I do it right and don't screw things up doing it backwards.

Replies:

Posted By: bigfish_Oh
Date Posted: 17 Mar 2010 at 8:31pm

Can you do it like a 9 volt battery?...... put your tongue on it?

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1941 WC sat for 29 years,started & dynoed 27 h.p.
1957 WD45 Grandpa bought new,factory p.s.,added wfe
1951 WD, factory p.s.
1960 D14 HnMk IV BkHoe 4 sale
2014 HD Tri Glide
2009 GMC CC SLT Dually

Posted By: jjwo
Date Posted: 17 Mar 2010 at 8:33pm

Hook a battery to it and it should run like an electric motor if it is good. I'd have to go look at one to remember how for sure but I don't think you can hurt them.

Posted By: jjwo
Date Posted: 17 Mar 2010 at 8:35pm

PS, that's a generator - not alternator.

Posted By: Butch(OH)
Date Posted: 17 Mar 2010 at 8:47pm

oops, see below

Posted By: Butch(OH)
Date Posted: 17 Mar 2010 at 8:49pm

If it will motor it will charge. You motor it with a set of jumper cables + to the A term and - to the field terminal (this is for negative ground, if positive ground reverse or youll have the thing polorized backwards.)Needs to be on the bench or belt completely removed as they dont produce enough torque to overcome much resistance to rotation.

Posted By: D-17_Dave
Date Posted: 17 Mar 2010 at 8:56pm

With the engine running at or above half throttle, ground the field terminal and this will induce FULL charge rate. Look at the Amp meter for positive charge rate or put a volt meter on the battery and see if the voltage rises. If not then the gen. is bad. If it does charge then you have faulty wiring, a bad regulator, or a bad cutout. Depending on how the engine is wired.

Posted By: Brian Jasper co. Ia
Date Posted: 18 Mar 2010 at 7:56am

Make sure the wires between the regulator and gen are ok. Grounding the "F" terminal will make it charge it's maximum with the engine running. The other test is take the belt off and with your jumper cables connect the neg terminal (pos ground reverse polarity for neg ground) to the "ARM" or "GEN" terminals and it should spin like a motor. While spinning, ground the "F" terminal and the gen should slow down but keep on spinning. If it does all this, your gen is good and you either have a wiring or regulator problem.

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"Any man who thinks he can be happy and prosperous by letting the government take care of him better take a closer look at the American Indian." Henry Ford

Posted By: Brian G. NY
Date Posted: 18 Mar 2010 at 8:56am

I know this is oranges and you're talking apples, but you jogged my ol' memory.

Years ago, a guy at a local junk yard told me how he tested alternators on running cars to determine whether to junk them or put them in stock.

With the engine running, hold a large screwdriver or other steel object near the center of the back of the alternator.

If the screwdriver is pulled by magnetism, it is working.

Seems too simple.

Posted By: Dave(inMA)
Date Posted: 18 Mar 2010 at 5:02pm

Tested a gennie today (buddy's green machine) by grounding the field terminal and measuring voltage between A terminal and ground. Result was zero volts. Pulled the gennie apart and found that a wire had come unsoldered from on of the brush assemblies. I think when he fixes that, the gennie will work a bit better!

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WC, CA, D14, WD45

Posted By: Gerald J.
Date Posted: 18 Mar 2010 at 5:53pm

That screwdriver test works on an alternator because the rotating field coil magnetizes the shaft when it has field current. And it has field current when the regulator is working right. That test would also pass an alternator with a shorted regulator, but the lamps on the vehicle would be blowing right and left and the battery might be spewing fumes and steam.

Gerald J.

Posted By: ChuckLuedtkeSEWI
Date Posted: 18 Mar 2010 at 7:40pm

thanks for the help. I tested it tonite with the tractor running and grounded the F terminal and test the A terminal without the wires on and with a multimeter. Got nothing. I guess she will have to get fixed, or maybe it's time for a alternator as most of the wiring on this thing is toast, and the cover for the voltage regulator is missing so I'm sure that's junk also.