Onan APU Overheating

JustUsTrucking · Aug 30, 2010

Hey All: Have an 07 freightliner with a Onan CMQD 5500. Worked fine for 3 years, all of a sudden it overheat's. Take the cover off, it work's just fine. Thinking there must be a mechanical reason it worked just fine for 3 years with the cover on, but all of a sudden will run for 5 minutes then shut down, if the cover is on. Besides finding a new cover that allow's better ventilation, I'd rather find the mechanical problem that's causing it to happen when it has not happened before. The pic show's the cover on the APU. Thank's for your help.

ATeam · Aug 30, 2010

You are half way there with your diagnosis. If the unit worked well for three years with the cover on and now does not, you can eliminate the cover as the cause and rule out the need to modify the cover. The cover did not change. Something else did.

Coolant is the first thing that comes to mind. Has it been changed in three years? What is the coolant level? The unit has an air filter on the underside. Has it been changed at regular intervals? Same question for the oil and oil filter.

If any of these have been neglected it could increase the operating temperature to the point where the cover that used to be able to vent the unit is no longer able to do so.

Start by checking the easy stuff. Move on from there if you must.

davekc · Aug 30, 2010

Could be several things assuming that the unit has the appropriate amount of anti-freeze.
Check to ensure the fan is coming on to ventilate it and the radiator is clear of all debris. If not sure, take some oven cleaner to the radiator and spray it off with a garden house. Nothing with too high of pressure.
Next step is to check the heat sensor and connections. Might need to be replaced. If you have a temp gun (to ensure no overheating) you can jump the terminals to see if it is the switch. Lastly would be a stuck thermostat. Thermostats usually last longer than three years so it isn't the first place I would check. Kinda the same thing with the radiator cap. But it is worth a quick look for seal and to see that coolant is moving so you know the water pump is working.

JustUsTrucking · Aug 30, 2010

Thank's Dave....will do.

JustUsTrucking · Aug 30, 2010

The unit is "serviced" every 300 to 400 miles, which I assume includes new filters. Brush block assembly has been replaced twice in it's life, slip ring's cleaned a few months ago.

Doggie Daddy · Aug 30, 2010

You have the exact same unit that we have,and it it the same age as ours. We are also experiencing the same problems as you are and after many trips to the shop (Cummins-Bridgeway in Cleveland,Ohio) have not found out what is causing it.

So far we have replaced the radiator cap,the thermostat,and the water pump 5 times. Cummins-Bridgeway is my home shop for repairs and so far they have not been able to cure this problem,or to tell why it has had to have so many water pumps put on.

I have called technical support in Saint Paul,Minn to ask them about the water pump problem,and was told that in all likelyhood, the water pumps may not have been bad. There is a weep hole on the body of the water pump, and after a new pump is put on it seem to function correctly,that is it only leaks a very small amount of coolant from the hole.

The longer the pump is on the more the amount of coolant coming from the weep hole increases. The Onan tech told me that the problem could be with the radiator cap,the radiator hoses,the thermostat,debris in the fins of the radiator and that is probably what is causing pressure to build up in the system and causing the increase of coolant coming from the weep hole.

The only thing that we have not done so far is to replace the radiator hoses. To me the overheating problem and the water pump problem are related. When new,this unit could and did run non stop in the desert heat of Tuscon and we were quite comfortable in the truck.

There has got to be a sensor on the unit that is going bad and is shutting it down for a overheat condition,even when that condition does not exist.

If the outside temp is in the 90's or above the unit will shut down in just a couple of hours. After the sun goes down and the temps drop a bit it will usually run all night until about noon the following day.

We use this unit a lot,and after 3years and 1 month of ownership we now have 8781 hours on it. If I can find the answer to the problem,I will pass it on,please let us know if you find out anything.

greg334 · Aug 30, 2010

If you followed Dave's suggestions, and it still overheats, replace the radiator, reverse flush the engine and put a new thermostat in while the system is open.

I went through this 6 weeks ago, the tech and repair manager at one Cummings dealer said 'it wouldn't be the radiator' and they changed out everything except the radiator. When it got back into my hands, I pulled the unit off the truck, pulled the thing apart and just replace the radiator. There was so much crude in the radiator and it was getting soft for some reason. It wasn't so much being dirty and plugged on the outside but the inside seem to have an issue with some build up.

Just a note, the water pump is a solid piece, the worse that happens is the thing seizes or leaks, as long as there is a belt that is moving it, it will move coolant.

davekc · Aug 30, 2010

I would still check the sensors. Don't know this particular unit, but there could be two. One for the coolant temp, and one for the fan. Screw the coolant one out and clean it, or replace it.
I would also monitor at what temp the fan is coming on.
After three years, it would not be uncommon to have the coolant system sensor crudded up.
If your careful, you can jump the wires and see if it still runs past the cutoff point. If it does, then you know it is the sensor.
5 waterpumps seems high. It certainly sounds like it is something else.

ATeam · Aug 30, 2010

There are thousands of Onan Quiet Diesel generators in use on RVs. Do a Google search on "Onan Quiet Diesel Overheating" and other such phrases to find dozens of posts by RV people who have similar issues. It might help you think of something you have missed so far.