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.
Generator Jim
Jim Latour
myke
my onan5500 gold turns over but dies when i let off the swich i pulled out the brushes thay look good hou do i test them do i ohm brush to brush??? ive checked everything else !!! the roter ??how do i check it???? please help im dieing here
free reverse phone lookup free full results
Fckin’ awesome things here. I’m really glad to see your post.
Thanks a whole lot and i am looking forward to contact
you. Will you please drop me a email
Rotary joint
I am trying to get the off of my steering wheel to install on an e36 M3 wheel. The on the M3 wheel came right off after removing the three small nuts. After removing the three nuts on the M5 wheel, the is loose, but something is preventing it from coming off completely.
Bill
If you are ever in Virginia Beach and need a good GenSet repair shop, call George at Generator Technologies @ 757-361-0202.
I had been having issues with my Onan 5.5 for over a month, when we pulled into Virginia Beach I tried the Onan dealer but they could not get me in for at least a week. I called George and they worked me in the next day and pulled the GenSet out of the coach and found the root cause of my ongoing issues. The GenSet was repaired and we were on our way by 6pm. By the way, George was willing to work late to fix our gen and help us get on the road again. Thanks again to Generator Technologies.
FX Williams
Jim: My 2005 Generac gas 6.5K genset is driving me crazy. It works fine at 120 VAC for up to 1/3 rated load. Above 1/3 rated load, the out voltage drops then usually surges back up then drops then back ect. at 1/2 rated load, the out voltage drops way down to the 4o’s VAC and my system drops the load. I have cleaned the brushes and rings and have replaced the AVR = no change. My brush holder terminal to terminal resistance stopped is only about 5 ohms. when running, the resistance jumps all around from open (usually open) to 20 ohms. Question 1) is my only 5 ohms stopped rotor resistance my problem (field winding problem)? Question 2) is the bouncing around, usually open, running rotor resistance my problem?Thanks a lot. FX Williams
john gushulak
Jim I have a old kolar generator . model no k662ep serial no 33111b gen no 5rm21 4531a serial no 270589. where could i find a owners manual or a repair manual and how old my generator is . runs like a cloke. thanks john.
kookimebux
Hello. And Bye. 🙂
John
Hi Jim,
Thanks for the great genset cleaning instructions. I will be buying a new or 1 year old diesel pusher in ’09 and am wondering if I should be looking for a particular model of genset. You list several models of gensets in your blog, are some models better than others? If so, I sure would like to know the strong and weak points of each.
Cheers,
John
Warren
My Marquis 5.5 BGM is putting out 1.2 volts at the circuit breaker on both the 20and 30 amp circuits. I can’t get any voltage at the brushes. Am cleaning brushes and rings this am but expect regulator is problem. Any suggestions?
Jim Latour
There could be a couple of things wrong with your unit. The cleaning procedures in the blog were for only the models listed and you didn’t say what model of 5 kw you had.
With that in mind, the problem on a BGE, BGM, NHE, NHM could be either a need for slip ring cleaning or a new regulator or both. If the 57 volts only shows up during starting, it could be a bad regulator and the voltage is coming from the field flash circuit. I would try the cleaning first as it doesn’t cost anything except time and go from there.
Generator Jim
PS: Sorry if I haven’t been blogging for awhile. We’re just finishing up round two of bladder cancer and I’m finally starting to feel better. Round one was in 1997.
pete
Jim thanks !! I have a Onan 5 that is putting out only 57 volts, i am going to
try the cleaning, I hired a tech who checked it out for 85.OO and the informed me he doesn’t work on gen sets. Any other info you can give would be helpfull.
John
Jim,
Thanks for the technical tips. It’ll come in very handy for me as I do all my own servicing. Retired from the trucking industry, will be getting a new diesel pusher next year and want to do all the preventive maintenance myself. I’ve found out over the years that if you want it done right, there is no substitute for doing it yourself provided you have the knowledge and manual.