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Old 08-17-2022, 07:01 PM   #1
Mischief Managed's Avatar
City: Hampton Beach, NH
Vessel Name: Mischief Managed II
Vessel Model: 1992 Tollycraft 44 CPMY
Join Date: Oct 2018
Posts: 523
1999 Onan 6.5MDKUB makes no power/low power


I'm working on a genset issue for a friend and thought I'd see if anyone can share some insight.

Facts and thoughts:
  1. Engine starts/runs/stops perfectly and genset has < 400 hours
  2. Genset is wired for 240V two phase 60Hz
  3. Genset suddenly stopped making AC
  4. Main breaker is fine
  5. Voltage regulator passes bench test (diode check per service manual)
  6. Voltage regulator will produce up to 76 volts at the field terminals when "tricked" into operation. By "tricked" I mean: I disconnected the field wires from the voltage regulator and connected a volt meter in their place and started the genset. I then applied 13.4 volts to the field coils with a power probe. This made the generator produce 2.6VAC on both legs of the 240VAC output and it also made the voltage regulator ramp up to 76 volts DC to the field. To me, this seems like normal voltage regulator operation.
  7. If I "flash" the either of the stator coils while the genset is running, it will immediately go from 0VAC to 30VAC on each leg and the engine will bog down severely and stay bogged down until the field breaker trips a few seconds later.
  8. I can reset the field breaker and repeat step 7.
  9. If I reset the field breaker and start the genset up, it will run at normal speed and not produce any AC power.
  10. Resistance from field coil wire to field coil wire at voltage regulator end is 33 ohms. Book says there should be 20 to 24 ohms between the slip rings, so I think the added 9 to 13 ohms is due to the brushes and is probably OK. Getting to the slip rings looks like a real hassle... The fact that the field breaker trips in step 7 leads me to conclude that the brushes are probably fine.
  11. My gut says it's got to be a short to ground in the stator and I don't have the proper insulation test tool (megger) to check that. Any other ideas?
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Old 08-17-2022, 09:51 PM   #2
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TowLou's Avatar
City: North NJ
Vessel Name: Bassey
Vessel Model: 17' Bass
Join Date: Aug 2016
Posts: 362
Something is funny in the alternator head. Putting 13v to the field should give you more of a output. Be mindful of the polarity you put the field as well. You can reverse the field.
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