The majority of modern RV generators utilize slip rings and carbon/graphite brushes to transmit magnetizing power to the generator rotor coils. This current/voltage signal comes from the control circuit during starting and the voltage regulator during operation.
A characteristic of this setup is that, under light or intermittent loading conditions, the brush material is deposited on the slip rings and moisture changes it chemically, forming an insulating layer on the two slip rings. This layer requires the regulator to work harder and use more power to “punch” through this high resistance layer to magnetize the rotor and maintain the 120 volt generator output.
The layer can build up to such a level that the field flash circuit can’t get through it and it may overload the voltage regulator to the point of burnout of its’ power circuit. The brush material can also cause the brushes to stick in their holder and stop electrical flow to the rotor.
SO – what can we do to fix this condition?
1. Run the generator at least 50% of full load – 80% is better. Run it at least 4 or more hours at this load level.
2. Periodically take resistance readings and clean the brushes and slip rings if necessary.
Tools needed are a multi meter, Service Manual, ¼” drive socket set with 5/16, 3/8, and 7/16 sockets as a minimum. Socket or Torx screwdrivers, electrical tape.
Ring cleaning material such as: a. Commutator stones, coarse and fine and a short piece of rubber heater hose to hold stones. b. small piece of wood and a “Scotch Brite” pad.
The following procedure is for the ONAN BGE, NHE, BGM and NHM series RV generator sets but with slight modification, can be used on other units.
Uncover the brush rigging.
1. Remove the air filter cover and filter.
2. Remove metal or plastic cover over brushes at upper rear of generator case behind air filter.
3. Mark the top wire on the brush block (holder) with a magic marker and mark the top of the brush block the same way.
4. Remove both wires and tape separately to insulate them from each other and the metal parts of the genset.
5. Measure the resistance from one brush terminal to the other with the multi meter and compare the reading with the “rotor ohms” spec in the service manual.
6. Example: On ONAN spec ‘D’ and later, the reading should be between 20 and 26.5 ohms. Any less and you have a rotor problem $$$$. Any higher and you need to clean the slip rings and brushes. Read the service manual to be sure.
7. Remove the brush block with the brushes in it. Lay aside for now.
8. Look at the rings on the rotor. They should be a light chocolate brown in the center and a copper/brass color on each edge. If a dark brown/black streaked mottled color, clean as follows.
9a. Wrap a piece of “Scotch Brite” around a small piece of wood (popsicle stick, part of a paint stirrer, etc.) and secure it with a rubber band.
9b. Press a commutator stone into a piece of heater hose.
10. Read step 4 again. The two wires will have 12 volts on them during the cleaning step.
11. Press and hold the start switch, the engine should run. If you let go of the switch the engine will stop.
12. With the engine running, push the “Scotch Brite” or stone against the slip rings until they are a bright shiny copper/brass color. If using stones, finish with the ‘Fine’ stone.
13. Stop the engine and use the “Scotch Brite” or stone to ‘rough up’ the bottom surface of each brush. Don’t change the curved shape, just rough up the shiny surface a little.
14. Check the brushes for freedom of movement in the block and compare brush length to the manual specification.
15. Reassemble the brush block in the generator – Magic Marker mark to the top. The block will break if installed upside down.
16. Recheck ohms with the multi meter and record the reading for use next year.
17. Reconnect brush wiring, install cover and air filter assembly.
18. Bow three times in the direction of Fridley, MN (ONAN Hq) and hit the start button. It should keep running. If not, you probably need a regulator.
If you have a Kohler genset, the procedures are the same except the brushes and slip rings are accessible at the rear bearing of the generator under a sheet metal cover.
On Generac side by side units, the brushes and slip rings are on top of the belt driven generator under a plastic cover.
Dropping the screws that hold the brush block into the generator is a big NO NO, and will cause you to learn new words and/or force a trip to the RV generator repair place — BE CAREFUL.