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Old 01-09-2019, 07:17 AM   #1
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Onan 12.5 Overheating Issue

Generator would run for a while and then shut itself down from overheating. Probable causes are low coolant, broken belt on water pump, bad thermostat or faulty coolant temp sensor.

Had plenty of coolant so that was not issue. Removed then top cover of generator so you could access the engine and found loose belt on water pump. To tighten this, had to remove the rear radiator hose and the inlet air pipe. Once these are removed you can get access to bolts for belt adjustment and tighten. Tight fit but accessible with long extension for socket. While in there I changed the thermostat and coolant temp sensor which were less than $100 for both and put back together and no more cooling issues. Engine now runs at 175 degrees. I did drain the coolant and replace with new to finish the job.

If your belt is broken you will have to do the above and remove the back cover, cooling fan and spacer off the water pump for access to the belt. Not a fun job but not that hard. Just takes time.

Hope this may help someone out there who may be contemplating this work.
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Old 01-09-2019, 11:53 AM   #2
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I shredded the belt on mine at a dog show a few years ago, parked in a field. Cummins didn't carry the belt because they had a mandatory retrofit to replace one of the belts with a mechanical drive - it would cost over $800. As it was Friday afternoon and I needed a generator for the weekend it wasn't an option. They provided the pages from the shop manual and the belt sizes so I could source them from an industrial parts supplier.

Changed it out in a field, took about 4 hours once I had the parts. Last year I had my coach into Cummins because the bearings in the Stator had failed and they had to pull the whole unit. I asked about installing the direct drive (the part was not expensive and the genny had to come out so why not?).

The tech told me it was a good time since the stator was already off and all they had to do was dismount the engine to move it forward to get at the belts. I was surprised and he said this was standard to change the belts on one of these (I have a 10K with a Kubota 3 cylinder diesel). I told him I'd done it in a field with the generator in place and he was impressed. It took them less time to remove the unit and dismount the engine and stator that to fix it in place.

Clearly not a repair friendly design.
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Old 01-09-2019, 03:47 PM   #3
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Laughed out loud on that one, yes, clearly not repair friendly design! Been there done that, multiple times
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Clearly not a repair friendly design.
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Old 01-09-2019, 05:49 PM   #4
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Thanks for posting.
I would be concerned about why the belt was loose?
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Old 01-09-2019, 06:37 PM   #5
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Thanks for posting.
I would be concerned about why the belt was loose?


I had the belt tear itís self up earlier this year and suspect the person that did the work failed to properly tighten the bolts on the alternator.

If you want something done right, do it yourself.

Lesson learned.
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Old 01-10-2019, 05:34 AM   #6
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Yea really. My exposure came from the dealership where I purchased my first Monaco with the 12.5 Onan. They installed a new belt using a tractor "green" belt which lasted about 8 hours use time and broke. They claimed the green tractor belts are tougher than the Onan normal Gates style belt. I call BS on that one!
Changed my second belt as PM 1500 hours or so later.

Changed the nose bearing in the generator end cap 2 times in 3500 hours although the second time the squealing I thought was the nose bearing was actually just the darn belt needing tightened

I had the old Onan with the skinny belt.
New Onan has the wider belt but I have to tighten it about every 200 hours or it will squeal. I bet if I left it squeal a while it would shred!
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Originally Posted by vito.a View Post
Thanks for posting.
I would be concerned about why the belt was loose?
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