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Old 05-08-2009, 03:46 PM   #1
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Generator problem after service

I had the 1000 hr service done on my Onan 7500 QD generator about 3 weeks ago along with motorhome chassis service – oil & filter, fuel filter, air filter, & coolant change. Yesterday was the first time we had tried to run the generator since. It was about time to exercise it so I figured we could do so while driving. It started normally, and I noticed it was not providing ac power to the coach. I went out and banged on the transfer switch as I have had them hang up before, but that did no good. I opened the front hatch and found the breaker on the generator was in the OFF position. I turned it on, went inside and confirmed we were getting power, and then the genny died. I went out and restarted it, and it ran about 50 seconds and died. The light was flashing what I interpreted as a “1 code” …which means “high temp.” I didn’t think to press the stop button to get the secondary code as I didn’t have the manual handy to decipher it anyway. I couldn’t see how we could have high temp condition after running a couple of minutes. So I started it again, and again it ran just 50 seconds and died. When it shut down I heard lots of gurgling sounds …”aha” says I …”air in the coolant?” I didn’t have time to deal with it so we made our short day’s drive with windows and vents open.

Today I confirmed again that both the oil and coolant recovery tank levels were normal. I checked the new air filter for proper installation & air flow. Then I removed the plate that hides the coolant pressure cap, removed the cap, and could barely see some coolant way down in the tube. I started the generator and it ran for several minutes, then the coolant tube did a lot of spitting and I could no longer see any coolant. I went and got some green coolant and filled the tube to the top, replaced the cap, and then ran the generator under load for about 1/2 hr. It appears the cause of the shut-down and high temp error code was just an air lock in the cooling system. I called the guys who did the service. The tech said they measure how much they take out against how much they put in, and they normally run the generator for quite awhile to ensure all the air is out, but did say it was possible they didn't get all the air out. Based on what I described, he agreed with me that an air lock was likely the problem. I added right at 1 qt. of coolant.

April wasn't a good month for us with service experiences -- two major service expenses at two different service locations and I've had to do follow-up fixes with both!
Paul (KE5LXU) ...was fulltimin', now parttimin'
2022 Coachmen Leprechaun 319MB
towing 2017 Jeep Grand Cherokee Limited
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Old 05-08-2009, 04:41 PM   #2
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I just replaced the coolant in my 7.5 Onan.
You need to add the new coolant slowly, a little at a time, to allow air to burp/escape out of the vent hose, that connects just below the opening of the fill hose. I'm sure a lot of service people rush the filling procedure, which causes air to get trapped.
Tom, Pat and Buster (the Boston terrier)

2004 DSDP 3807, 370HP Spartan
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Old 05-09-2009, 06:28 AM   #3
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Onan also advises running the engine for a few minutes with the radiator cap off after refilling (see coolant change procedure in owner's manual). This lets air bubbles out. I actually had to do that a couple times after I changed the coolant in mine.
Gary Brinck
Former owner of 2004 American Tradition and several other RVs
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Old 05-12-2009, 01:13 PM   #4
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From your description, I don't believe for one min that your service folks ran the genny for a 'long' time. JMHO

Callsign: KD8W
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Old 05-14-2009, 05:41 AM   #5
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look under the coach at the radaitor to make sure there isn't stuff blocking the air flow this happened to mine it and it would run for a short time just like yours. good luck
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Old 05-14-2009, 06:47 AM   #6
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Just another testimony as to why, for those of us that are able, we do our own preventative maintenance.

I have run across this type of problem several times in my lifetime, and usually it has been with routine service like oil changes, lubes, etc.

AF Chap-- You absolutely did the right thing by a close follow up and fixin' the problem yourself-- Atta Boy
Max H,
2002 Newmar Mountain Aire, 37', 3778, W-22, 8.1 Vortac, Ultra Power upgrade, CAI (cold air intake), Taylor wires, colder plugs, Koni shocks.
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Old 05-29-2009, 07:57 AM   #7
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It sounds like the cooling system service was not performed properly. The small black cover by the circuit breaker must be removed to access the radiator cap. Remove the radiator cap and fill the radiator with a 50/50 mix of antifreeze. Reinstall the cap and refill the reservoir, This should correct your problem.
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