Onan 7500QD Gremlins

Mine did a very similar thing and it was a loose fan belt. One other common cause is a faulty temp sensor. It's also possible that the radiator which is at the bottom of the genset has dirt in the fins.

Thanks Dick,

At 18 years of age and around 3000 hours of service nothing would surprise me.

Hope my issue is a simple as yours was!

FormerBoater wrote:
Thanks Dick,

At 18 years of age and around 3000 hours of service nothing would surprise me.

Hope my issue is a simple as yours was!

With that many hours doing a complete service including coolant flush and new thermostat might be worth considering....thermostat is pricey but worth considering. FYI, the radiator is plastic and over tightening the drain plug can ruin it...as my service center found out.

Actually,
The radiator is NOT at the bottom of the unit. It sits at a 45 degree angle, about mid-height of the unit. The air, for cooling, is brought in through the bottom, through the fan, around the stator, routed through the radiator, and finally cools the exhaust/muffler and, then finally exits the bottom of the unit, towards the rear.
There are two distinct sides of that QD 7500. There is a major bulkhead partition that separates the two. In this manor, the air for cooling has a very specific route, because it cools more than just the radiator.

Now, as for your code 33. Yes, it can be a bad temp sensor and, is has been for many on here and other RV forums. It's not always the remedy for a code 33 but, it does cure a lot of them. You see, the temp sensor is the main controller for both cold and warm operations of that little 14HP Kubota engine.

And that sensor, resides in a lousy spot. There are two halves of the thermostat housing. Onan calls that part something else but, I forgot what. I call it the thermostat housing. And that housing is at the rear (actually front) of the gen, as you face it from the front of the coach. The sensor resides in the lower half of the two parts. And, it looks like, and is about the same size as a lawn mower spark plug. It has a hex head for the wrench to be used on. And small, threaded stud that the temp wire attaches to.

Then, the business end, is a round, solid piece, about 1/2" in diameter and, about 1" long or so. The housing where it resides is cylinder, about 5/8" or so, in diameter. So, there's very little clearance around that sensor and, its housing.

The problem is, the coolant circulates all through that little engine just fine. And it passes through the thermostat just fine. But, where that little cylinder and sensor is, there is very little circulation of coolant and, even when the coolant looks crystal clear or green (anti-freeze), there are great possibilities that there will be corrosion/build up in the cavity surrounding that sensor due to the low flow of coolant.

When that happens, there becomes a solid substance that blocks the coolant and, sensor becomes over heated. Guess what happens next? Yep you guessed it, you get CODE 33. And, when that sensor gets extra hot, it tells the little ECM for that little Kubota: "Shut down, we're over heating". When in fact, the gen is running at a perfectly fine operating temp.

What's the answer? Well, to remove and replace that sensor. How hard is it to do that? Well, it depends on if your generator is on a slide out tray or not. If it's on a slide out tray, you've got half the battle won. If not, you're in for some serious work.

Once the outside paneling is off, then comes the part of the sensor removal. On some, like mine, it fought me all the way, and it got real ugly between me and it. There were some choice words that fortunately, no kids were around to hear.

But, on some, that sensor spins right out. If you're lucky, yours will be that way. Hope this little explanation helps some.
Scott

Scott,

Thank you for the detailed explanation.

My Onan is on a slide fortunately.

I hope that my sensor will spin!!!

FormerBoater wrote:
Scott,

Thank you for the detailed explanation.

My Onan is on a slide fortunately.

I hope that my sensor will spin!!!

You're most certainly welcome Sir. Here's a few pictures of what mine (generator, sensor etc.)when it was apart for the service/repairs. Don't be too alarmed at what you see. It looks like a land mine went off in the interior. I did the sensor, belt, head gasket and thermostat. Oh, and oil and antifreeze too.

And, I had to replace the rotor during that time. Later, I replaced the stator. All in a days work.
Scott

If you decide to get inside the generator to R/R the temp sensor, just heat it a little bit with a propane torch, then spray it with some good penetrating spray (I used Deep Creep) and it will come out easily.

Also, at 3,000 hours, if you already have it apart, I would replace the belt, coolant hoses, the T stat, adjust the valves, and get the radiator cleaned and checked.

smlranger wrote:
If you decide to get inside the generator to R/R the temp sensor, just heat it a little bit with a propane torch, then spray it with some good penetrating spray (I used Deep Creep) and it will come out easily.

Also, at 3,000 hours, if you already have it apart, I would replace the belt, coolant hoses, the T stat, adjust the valves, and get the radiator cleaned and checked.

Hey SML,
Ha! yeah sure that little sensor will "come out easily". As you can see in the pics, mine was fighting me 'till the last thread. I used everything in the book to get it out. And, as you can see, it broke off in the efforts. And yes, that was with heat and penetrating oil, (several brands)too. The straw that broke the camels back was, a little trick I thought up but, was skeptical.

I purchased a can of "Freeze" from a place called "Marshalls" on Miramar road in San Diego. It's like a smaller sized Home Depot, lowes, ACE Hardware, and Tractor Supply, all rolled up into one store but, only on a smaller platform. They had this stuff called Freeze.

I bought some, buzzed home and, heated the area around the stub that was left from me using Gorilla efforts to get that sensor out and, when it was thoroughly heated, I sprayed this "Freeze" right on the left-over stubble. I then grabbed it with my smaller pair of Vice grips and, out it came. Whew!!!!!! That was one tough little bugger.

But, you can see why it was so tough. It was completely corroded all the way around those threads which, locked that sensor into position and, was not going to let it go.

I failed to mention earlier that, in the onset of my gorilla efforts, I put so much torque onto that little sensor that, I cracked the lower T-stat housing and, didn't know it. So, I got it out, cleaned up the threads with a tap, installed the new sensor, replaced each and every part of that generator with the exception of the outer panels and, started filling it with antifreeze.

DRIP, DRIP, DRIP, DRIP, DRIP....... What the he..... is this? I checked and re-checked all my brand new clamps, hose ends and everything else that had anything to do with the cooling system and, NOTHING. But the drips, actually a small, steady stream of Antifreeze was dripping out the bottom of the unit.

So, in comes my mirror on a stick. Whaaaaaaaaaaaaaaat is this? Low and behold, it's a CRACK in the lower T-stat housing that's impossible to see, without the use of a mirror.

So, on the phone to Cummins/Onan and, ordered up some parts, including a head gasket 'cause the only way to replace that little, lower T-stat was to remove the head!!!!!!!!!! Oh well, all in a days work.
Scott

P.S. Below is a pic of that lower T-stat housing. You can see the crack in the lower section of it.