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I have a 1988 520HC, about 2 years a new 20HP Onan engine was installed, on the voltage gauge it stays right on the bottom left of the green at 12 volts, it wont charge any higher, have checked with other meters, when engine not running it checks at about 12.6 volts, when I start the engine it drops to about 12.4 volts. Wondering if anyone had any ideas, could it be the rectifier or the stator, I have a wiring diagram, I thought when engine running the voltage should actually rise to around 15 to 16 volts, anyone got any ideas on things I can check to see if it is charging properly, havent had it long, it must be charging a little because the new battery is still starting it.
 

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Vet-Wrencher
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If the meter is pushing positive AT ALL, your charging. If your certain it's not charging i would get a multimeter and probe the battery to see if there is a voltage jump between it running and shut off. If no change i would suspect eaither loose connections which is possible or ultimatly the rectifyer/regulator which REGULATES your charge. See if you can get a jump in voltage by wiggling the fuses or wires and watch the amp meter. If it jumps, there's your problem.
 

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Discussion Starter #4
Found the service manual on the tractor but not on the Onan engine, in the tractor manual it tells about the charging system but no details on how or where to check tnx
 

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Battery voltage while running should be between 13.5-13.9 depending on battery state of charge. If all connections are clean and tight then check the stator output. The two outside leads at the regulator are the AC leads from the stator.

With engine off disconnect them and connect your DVM set to Ohms to each lead, you should read continuity. If open the stator is bad. Then check between each stator lead and ground. Reading should be infinity or zilch. If good reconnect DVM leads to stator leads and change DVM to AC volts and start up the engine.

At idle you should have about 25-30 volts AC. Full throttle reading should be about 50 Volts AC. If the stator checks out then the regulator is bad.
 

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Sorry to jump in here but I'm having the opposite issue - voltmeter periodically pegs all the way right on my 520, then will settle back to about 14 volts while running, then peg right, then back, etc. Tried jiggling the wires while running to no effect so I don't think it's a loose connection. I can also faintly here a change in pitch to a higher frequency from some electrical component (regulator??) when it pegs right. I would like to test the regulator but I have no idea where to find it on my P220 engine. Also, what's the procedure for checking the regulator output? Thanks for any help.
 

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I have a 1988 520HC, about 2 years a new 20HP Onan engine was installed, on the voltage gauge it stays right on the bottom left of the green at 12 volts, it wont charge any higher, have checked with other meters, when engine not running it checks at about 12.6 volts, when I start the engine it drops to about 12.4 volts. Wondering if anyone had any ideas, could it be the rectifier or the stator, I have a wiring diagram, I thought when engine running the voltage should actually rise to around 15 to 16 volts, anyone got any ideas on things I can check to see if it is charging properly, havent had it long, it must be charging a little because the new battery is still starting it.
Mine did the same thing. Found one of the "pins" in the engine to harness "Molex" connector was funky and had heated. Found it when I wiggled the harness and the charged jumped up. Pulled apart and cleaned and applied dielectric grease and it's been good since.
 
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