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12 volt generator farmall MD

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Rodney Axford

11-18-2007 11:09:58




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Does anyone know what has happened to a 12 volt generator to cause it to start charging 18 volts? I noticed my ammeter wasn't showing any charge so checked gen with an ammeter, 8 amps which is right for that generator. Checked with a volt meter, 18.7 volts. that'l ruin a good battery mi pronto. Shut engine down and checked battery, already reading 14.4 volts. Checked my volt meter on my pickup, 14.3 volts so my meter is ok. Thanks in advance for any info. Rodney

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Janicholson

11-18-2007 13:18:32




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 Re: 12 volt generator farmall MD in reply to Rodney Axford, 11-18-2007 11:09:58  
I'm going to take a different tactic. If your meter is reading 14+ volts on a vehicle with the engine off, it may not be calibrated good enough to use. Thus your 18 volts might be much less in reality. Use a different meter to second opinion yours. A fully charged battery won't read over 13 volts when sitting not being charged. You may have a problem, but check the meter first. JimN



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John T

11-18-2007 16:28:13




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 Re: 12 volt generator farmall MD in reply to Janicholson, 11-18-2007 13:18:32  
Darn good point Jim (his meters accuracy), a full charged battery is around 12.6 volts (2.1 per cell) and once a charger or genny is removed I dont see it as sitting there at 14 volts. That plus his ammeter not showing charge sounds a lil suspicious.

John T



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John T

11-18-2007 11:42:12




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 Re: 12 volt generator farmall MD in reply to Rodney Axford, 11-18-2007 11:09:58  
Rodney, Sounds more like a Voltage Regulator then a generator problem to me. If all esle is correct n working if you dead ground the gennys FLD post that forces her into high max charge so perhaps the wire from the gennys FLD post to the FLD terminal on the VR is shorted to ground,,,,, ,,,,or the VR is bad like an internal short or the field current control relay is stuck permanent in high charge n not chattering n controlling. Maybe it has a contact thats stuck/burned/welded shut???

See what happens if you remove the wire on the gennys FLD post?? That should make her charge very weak if any but if she still high charges that way, Id suspect the end of the field winding internally near the Field post is shorted to case.

If its NOT any VR or wiring problem like any of the above and is indeed the genny itself THEN I SUSPECT THE FLD POST STUD/NUT WAS OVERTIGHTENED AND BROKE THE INSULATOR WHEREBY THE FLD IS ALWAYS DEAD GROUNDED N SHES ALWAYS MAX CHARGING

BETTER CHECK THAT OUT !!!!! !!!!! !

Let us know what you find

PS Im assuming youre measuring battery voltage cuz the open unloaded genny output dont really tell you all that much, its what it does when coupled up to a batery that matters. Im a lil confused about you say the ammeter isnt showing charge yet the voltage (at battery???) is so high cuz the ammeter must not be workign or isnt wired right. If she really max charging the ammeetr ought to be indicating current flow into the battery, so insure its wired right n working and where youre taking those high readings cuz if its different then my assumptions above WE GOTTA RECONSIDER MY DIAGNOSIS

John T

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Bob

11-18-2007 11:25:35




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 Re: 12 volt generator farmall MD in reply to Rodney Axford, 11-18-2007 11:09:58  
Your voltage regulator has stuck contacts, is otherwise defective or not grounded to a good chassis ground.

Also, the "FIELD" wiring between the regulator and generator coulld be shorted.



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