Onan 5500 Producing 2.0 volts. Pics inside

Voltage regulator, field current, open winding etc.

Suggest a gen service tech next.

The slip rings look good. A consistent chocolate brown all the way around is what you want but I'm with Jerry on this one. The control board, when its working properly, will vary the field voltage to regulate the output voltage to maintain a constant 120VAC with a changing demand, at least I think it's field voltage. Been awhile since I went thru motor/generator training.

Your best course of action at this point is to remove the generator from the coach and take if to a small engine/generator repair shop rather than a RV service center. Have someone who does nothing but generators take a look. Pulling the generator yourself isn't difficult although it probably weights close to 200 pounds. Save a few hours of labor time the shop will charge you.

Thanks to both of you for the replies!

Im fairly competent when it comes to electrical and electronics, been doing it most of my life. the generator engine itself is working fine as it runs quick and quiet, so this surely a matter of diagnosing elecitrical components gone awry.

My next plan is to test the dc field voltage while the unit is running to see if its the regulator. If I am correct in my readings it should show at least 12v but more likely quite a bit more as im not producing power. If I dont see a few hundred volts being thrown at the brushes then it should be the regulator, yes?

Im new to the whole onan generator thing, but it seems somewhat straightforward as far as the generator portion itself goes. Hoping that anyone here with some in depth knowledge can guide me if im making a mistake!!

Reset the output breakers on your generator. The 2 VAC that you are reading does not mean that there is an actual contact.
Usually the generator circuit breakers are right at the output on the generator itself.

rgatijnet1 wrote:
Reset the output breakers on your generator. The 2 VAC that you are reading does not mean that there is an actual contact.
Usually the generator circuit breakers are right at the output on the generator itself.

Thanks for the advice! I wish it were that simple. Resetting the breakers was the first thing I tried. The voltage I measured was on the hot wire going to the breaker from the generator itself, and again at the transfer switch. the genset is definitely running 2.0vac.

Here is a little more info that you can use to trouble shoot your gen set. This was provided to me by Generator Jim when my Onan failed.

1. Pull the air filter and remove the brush cover from the generator housing behind the filter.
2. Unplug the two wires that feed the brushes and tape them to prevent shorting them out. Make sure the brushes are free in their holders and the slip rings are fairly clean.
3. Obtain two jumper wires. Connect as follows:
3a. Connect the bottom brush to the frame of the generator set.
3b. Connect to the top brush and leave it hanging free - but safe.
4. Open the two circuit breakers on the output of the unit.
5. Connect a multimeter on the buss bars as you did before.
6. Start the unit, observe the multimeter, and connect the one loose brush wire to the battery positive cable on the generator set starter solenoid.
7. This will put 12 volts DC on the stator. It should generate about 65 volts no load at the generator output.
8. If no output voltage, bad rotor and/or stator.
9. If voltage, you have a bad voltage regulator or problems in the wiring to the regulator.
10. Go buy a service manual.

Richard

Little tip, if you find the regulator to be bad, company named Flight Systems offers a regulator that is rebuildable, but first, you could send the old one to them to check it out. They offer a bunch of stuff for generators & electronics.
Neil

As above you actually "full field" the rotor by applying the 12 volts through it. 65 volts would be correct for one phase of the two phase unit.
The regulators job is to control/vary this rotor voltage to keep the output constant.

You could try taking the coach to a Cummins/Onan dealer. They have several in California (Fresno, Bakersfield, Ventura, San Leandro, etc).

I tested the voltage at the brushes... from what I understand one brush is supposed to be a +12v (or more depending on needed field strength) and the ohter is supposed to be grounded?


for some reason they were both showing +5vdc.

Also, when testing the resistance between the rings and the ground, I am supposed to find one mega ohm? I found no resistance whatsoever, and when tested for conductivity, there was none. its like there was no pathway whatsoever to ground from the rings.