Onan Marquis Gold 5500 Generator Just Quit

Genny had been running for about 10 minutes when it died. It will run on restarts until the start switch is released.

Checked oil level - OK

Error code blinks: 27

There is a 20 and 30 amp breaker on the genny. Neither were tripped.

However, if I turn off the 30 amp breaker it will start and run. It dies if I flip it on. I turned off the main breaker at the AC panel with no change.

Any ideas?

Tom

Just guessing here-bad cable from genny to panel or ATS, or bad ATS?

What is ATS?

Note that the breaker doesn?t ?trip? as if overloaded.

Error Code 27 says the control board is not sensing voltage from the generator.  That along with low oil pressure are two conditions that will make the engine shut down after you release the start switch.

Sounds like dirty slip rings or worn brushes that are blocking the excitation current from reaching the armature.  Or a bad control board that likewise can't generate the required field current.

Note that dirty slip rings can increase the strain on the control board, leading to it's failure.

RV generators differ from the "usual" textbook model in that the field coils receiving the excitation current are on the armature and the output is taken from the stationary coils surrounding it.

With the 30 amp breaker off there's no load on the generator so the limited field current can let the voltage build normally, but the generator can't get enough excitation to maintain that voltage when a load is applied.

Flight Systems makes replacement regulator and control boards for Onan products, both the RV and commercial lines, and they have long offered free troubleshooting guides on their website.

They now have a good YouTube video on their website showing how to check the generator's internal condition using a multimeter.

If all that fails, you can also get free tech support by appointment.

Thanks Lou,

Brushes make sense and I need to figure out how to access them. The genny was under a fairly heavy load this morning when it died.

I?m not sure what increased load would be on genny when I close the 30 amp breaker. The A/C panel had all breakers off so I don?t know what would be powered.

Armed with my meter and service manual I?ll head out soon to see what I can discover. Had to take a break to deal with a credit card fraud attempt. You know, when it rains...

Tom

The load could be anything, including the load of the transfer switch coil itself if there's a bad restriction.  But dying while under load suggests a regulator failure, perhaps due to trying to force high current through a bad field connection.

Check out the videos on the Flight Systems website, they're simple but really informative.

They don?t make it easy to get to stuff. The book calls for 29.x ohms on the stator brushes. I get30.4 which I deem ok. Pulled the brush away and they look good and within specs for length. The slip rings look good too. I would liked to clean them but all I had was a pencil eraser. It didn?t do much. Pretty sure all?s well there.

Will check out the videos next.

Heli_av8tor said:

They don?t make it easy to get to stuff. The book calls for 29.x ohms on the stator brushes. I get30.4 which I deem ok. Pulled the brush away and they look good and within specs for length. The slip rings look good too. I would liked to clean them but all I had was a pencil eraser. It didn?t do much. Pretty sure all?s well there.

Will check out the videos next.

Click to expand...

Tom, have you checked the very simple thing first - are you below 1/4 tank on fuel?  It shuts down at that point but I don't know if it will even start so just asking the question.
Bill

Thanks for the thought. I?m aware of that and over 3/4ths tank.

Looking more and more like the combo control / regulator board for (gulp) $329 from Flight Systems for aftermarket part.

Heli_av8tor said:

Thanks for the thought. I?m aware of that and over 3/4ths tank.

Looking more and more like the combo control / regulator board for (gulp) $329 from Flight Systems for aftermarket part.

Click to expand...

Gulp is right but that would not even buy a single battery for my portable oxygen concentrator - $399.  But you and I have the same generator so now you have me sweating that out too..................lol

Per my PDF copy of the manual
PMA SENSE LOST FAULT?CODE NO. 27
(Controller unable to sense PMA frequency)

Not voltage but frequency... Voltage is a different code.
possible causes: Bad control board. Temps below -40 (or some other arbutary number darn close to -40 and since it ran 10 mniutes I doubt that)  bad brushes/slip rings. bad generator (other issue) loose wire.

Of all those my money is on the control board.

Thanks John for looking up the error code. For some reason my manual shows it differently. (See Picture)

I too am betting on the control board but keep thinking of things to check before dropping the cash.

I'm puzzled by the genny running if I don't turn on the 30A breaker. The sense line connects to the genny side of the breaker. Turning the breaker on doesn't trip it or give any indication of a short. Shore power works ok. I'm going to load thee genny with the MH wiring disconnected to see if it still quits.

Heli_av8tor said:

They don?t make it easy to get to stuff. The book calls for 29.x ohms on the stator brushes. I get30.4 which I deem ok. Pulled the brush away and they look good and within specs for length. The slip rings look good too. I would liked to clean them but all I had was a pencil eraser. It didn?t do much. Pretty sure all?s well there.

Will check out the videos next.

Click to expand...

After checking the field winding and slip ring resistance at the Molex plug, the video recommends cranking the generator and verifying the resistance stays constant as the armature rotates.

One failure mode is a "rolling short", where varnish has come off of the armature winding and gravity's pull only makes it short out at a specific point in the rotation. This will be shown by the resistance dipping to a very low value as the armature rotates.

The resistance jumping high points to a bad spot on the slip rings or brushes.

Be it voltage or frequency (By the way My Generator is the same model).

The suspect list is the same.

Problem found! 

Thanks everyone for your input.

I did the crank test and found the field resistance steady at 29.3 ohms.

It kept bothering me that with the 30A breaker off the genny would run, but instantly die when turning the breaker on.

I disconnected the wire to the motorhome from the breaker and alligator-clipped a 100 W trouble light to the terminal. When I started the genny and turned on the breaker it kept running and illuminated the lamp. Hmmmm.

The AC cable from the genny goes to a four-square junction box and then to the transfer switch. The transfer switch lives in a 5" cube attached to the back of the AC/DC load center which lives under the bathroom lavatory. It was a struggle to get it out. I had to unhook numerous wires.

Once access was gained the problem was obvious. The hot wire from the genny went to a connection block. The connection must not have been tight and overheated, melting the connecton block. This allowed the wire to touch the grounded mounting bracket.

It will take some time to put it all together but the fix will be free.  I can't help but wonder how many parts and hours a dealer would have thrown at it before finding the problem.

Tom

Great news Tom.  I am sure you are relieved to see it was a loose connection that caused the problem.  Now I won't be sweating quite so much.

Bill

Glad to see you figured it out.  Persistence pays!

Glad you found the problem Tom.  If I have an issue, I'll look you up. 

Anytime Gary.

Finally got all the wiring repaired and tucked away about an hour ago. While the main panel was out I couldn?t run the generator that HueyPilotVN loaned me. Batteries had dropped to 47%. Hate that they went that low, but...

Hope my genny doesn?t keep you up. I?ll kill it at ten. I?m sure it?s not the sweet music to others that it is to me right now 

Heli_av8tor said:

Anytime Gary.

Finally got all the wiring repaired and tucked away about an hour ago. While the main panel was out I couldn?t run the generator that HueyPilotVN loaned me. Batteries had dropped to 47%. Hate that they went that low, but...

Hope my genny doesn?t keep you up. I?ll kill it at ten. I?m sure it?s not the sweet music to others that it is to me right now 

Click to expand...

Tom, a bit off topic but could you give me you thoughts on the CRV toad.  I know the past couple of years they have not made one that can be towed 4 down but the car has always interested me for a toad.  Our little Sonic is very nice and easy to tow but it the word 'little' describes it. The CRV looks to be quite a bit better.

Bill