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Old 10-03-2018, 09:01 PM   #1
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Onan generator fuel pump voltage source

Does anyone have a wiring schematic or service manual for the 12.5 KW Onan generator (model HDKCB) in my '13 Anthem? I am trying to find out what the voltage source is for the generator fuel pump. I would expect the 12 volts to come from a relay in the gen set but I am told the electronic control board inside the generator supplies the voltage directly from the board.


My generator is stalling randomly due to fuel starvation. The pump randomly looses voltage and quits pumping. It is not the pump or filter. Both have been replaced, checked and rechecked. If I jump the pump directly to 12 volts the gen will run forever. A test light on the open pump feed wire will glow very dimly and at times be normally bright randomly indicating a change in the voltage feeding the pump from the source.


The official fix from the local Cummins-Onan service is a new control board for $1000. I'm considering using the pump voltage source to trip a relay and hot wire the pump directly from battery voltage thru the relay. When the source voltage drops it is still sufficient to hold a relay so the pump would continue to run yet still be controlled by the gen set.



I am just curious as to the real source of the power for the pump. It is almost impossible to trace inside the generator as Onan uses all black wire for everything. No colored wires at all so you have to physically trace the black pump feed wire and not all the harness is accessible unless you take the gen completely out and undress it. PITA but effective at keeping the DIY mechanics from meddling I guess. There are relays inside the gen so I am curious as to why the control board would feed the pump directly or if that is really the situation.


Any input appreciated.
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Old 10-04-2018, 05:42 AM   #2
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Cool

Well, I can tell you, that main harness does in fact feed voltage to the fuel pump. I had the same problem earlier this year.

While at a rally, I had Cummings check it out and they found a loose pin in the main plug, that feed power to the fuel pump. The tech tightened that pin and I have not had any more problems with my generator shutting down.

If you remove the coolant tank, the main harness is right behind it. If you jiggle the harness with one hand and prime you generator with the other, you can determine if the harness is your issue, since you can hear the pump working or not. Easy fix, if it is....
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Old 12-16-2018, 12:17 PM   #3
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Onan Generator Problems

This summer I took the long way (2 new fuel pumps, re-wired, etc.) stuck in Mesquite Nevada triple digits......to finally discover that Cummins had a service bulletin put out on the plastic plug holding (10 pin I believe) wires going to the generator. It was getting corroded or dirty and lowering the voltage (mine was 10.7 volts) and causing the generator to fail. Much time and effort spent, time lost etc. ….. 1 new plastic plug, 20-30 minutes labor and all fixed. Hope this isn't too late to help.


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Old 12-16-2018, 06:04 PM   #4
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See attached wiring diagram. J2-24 at the controller to ground energizes the fuel pump circuit, it also powers up the battery charging alternator. Has the BC alt. been checked not to be drawing too much current and causing the voltage to decay over time? I think that it is worth a try to isolate that circuit to confirm. Otherwise the board appears to be the issue.
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Old 12-16-2018, 08:37 PM   #5
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bad control board in onan generator

Thanks for the info Jose. The problem did end up being the control board. It ended up costing $1k to get the coach out of hock at the local Cummins Bridgeway. I took apart all the connectors looking for problems and loose pins but could not find anything. The control board voltage to the fuel pump ended up decaying further over time and loosing power more frequently.


I had diagnosed and was convinced it was the board but ended up taking the bus into the CB shop to have them replace the board to get their guarantee on the repair. If I just bought the board and it was not the problem I'd be stuck with it and there was no warranty on the board if I replaced it so I had Cummins Bridgeway do it. Gen has been good so far.


In a bit of irony the dash mounted hour meter failed when the new board was installed! Just coincidence I'm sure.....
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Old 12-17-2018, 08:33 PM   #6
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I had the exact same problem on my generator. Found the problem to be the same as Dr Q. I found that by moving the harness slightly where it plugs into the board solved the voltage problem. Pins were clean, just a poor connector. I removed the rubber gasket for a better connection, and sealed it up on the outside. Two yrs and no problems.
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Old 12-18-2018, 09:42 PM   #7
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Al, what you experienced is an all-too-common problem with Onan 10K & 12.5K diesel generators.

I had the exact same problem with the exact same symptoms with 10K generator in our 2010 Tiffin Allegro Bus. Once I determined the issue was being caused by the control board supplying low voltage to the fuel pump, I of course replaced the control board. But I also replaced the fuel pump, just in case it had been compromised by the low voltage.

While I was replacing the fuel pump, I wired in an extra hot and extra ground wire, cutting them long enough to make it easy to do two things:

1) Check the voltage going to the fuel pump by simply removing the same access panel that is removed to change the oil filter.

2) Hot wire the fuel pump in the event of an untimely failure of the new control board.

Cummins-Onan has to know this particular failure is occurring on an all-too-frequent basis. We should hope they're doing something about it, but I wouldn't bet a whole lot of money that they are.
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Old 12-18-2018, 09:46 PM   #8
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Quote:
Originally Posted by Jose67 View Post
See attached wiring diagram. J2-24 at the controller to ground energizes the fuel pump circuit, it also powers up the battery charging alternator. Has the BC alt. been checked not to be drawing too much current and causing the voltage to decay over time? I think that it is worth a try to isolate that circuit to confirm. Otherwise the board appears to be the issue.
The battery charging alternator is optional equipment on these generators and I'm not aware of very many (if any) motorhome manufacturers who install Onan generators with an alternator.

That said, I haven't checked to see if the 12.5K generator in our 2010 Cornerstone has an alternator. I doubt it does, but if anyone knows for sure I'd appreciate the info.
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Old 12-19-2018, 04:45 AM   #9
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Is there another plug the needs to be sealed in addition to the main generator plug?
The main plug is terrible as t is not protected at all and in a bad location to get any kind of road spray.
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