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Old 10-12-2016, 06:30 PM   #1
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Onan 7.5 Quiet Diesel Generator Problem

Always starts and runs good for however long you'd like, but don't shut it down and expect to restart it again, while it still hot or even still warm, as it will shut itself down after 5 minutes and send a code 24 (Controller did not sense temperature change during first 5 minutes of operation). Corrective Action: See and authorized Onan dealer.
Had a Cat dealer order me a sensor that is supposed to be the most likely culprit, so now the question is, has anyone personally done this and where might this part be located? I've looked a every drawing I can find and can't find any reference to this particular sensor.
The genset doesn't slide out the front like some and really hope I won't have to drop it for access. The top cover has limited clearance and looks doubtful for being able to remove such. Let alone work on any parts contained within. Then again, towards the front might be possible if one can remove the cover.
Anyway, any insight to the problem will be appreciated and am putting off taking it to Cummins, as a last resort.
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Old 10-12-2016, 06:58 PM   #2
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Originally Posted by tropical36 View Post
Always starts and runs good for however long you'd like, but don't shut it down and expect to restart it again, while it still hot or even still warm, as it will shut itself down after 5 minutes and send a code 24 (Controller did not sense temperature change during first 5 minutes of operation). Corrective Action: See and authorized Onan dealer.
Had a Cat dealer order me a sensor that is supposed to be the most likely culprit, so now the question is, has anyone personally done this and where might this part be located? I've looked a every drawing I can find and can't find any reference to this particular sensor.
The genset doesn't slide out the front like some and really hope I won't have to drop it for access. The top cover has limited clearance and looks doubtful for being able to remove such. Let alone work on any parts contained within. Then again, towards the front might be possible if one can remove the cover.
Anyway, any insight to the problem will be appreciated and am putting off taking it to Cummins, as a last resort.
Well Sir,
The "Sensor" that most on here have had to deal with, including yours truly, is the temperature sensor, located in the lower thermostat housing. It's at the front of that little Kubota diesel, on top and to the left, as you look at the front of the coach. It has caused a myriad of problems for many of us. Depending on year/version, it usually throws out a code 33 which is, "Overheat" condition. And, in many times, it's (the generator) is not over heating. The cooling system is simply not circulating coolant around that sensor due to the fact that the tip of it, resides in a bore, about an 1/8" larger than the sensor itself.

What happens in many cases is, due to the lack of circulation and turbulance of that coolant in that confined area, some corrosion will build up around that sensor to the point that, it completely fills the bore and makes contact between the sensor and the housing. The sensor now cannot receive and send accurate information that little diesels ECM.

So, you get false readings, codes and whatnot. And, as a result of some of those false codes, the engines ECM is instructed to shut down.

That little sensor is about the size and shape of a small, lawn mower spark plug. Here's the BAD part. On some of those little diesels, that corrosion can get bad enough to the point that, it litterally locks that sensor in the aluminum housing. When that happens, it's a like a battle between Patton and Rommel. But, on some, it zips right out, just like a spark plug should. If you're lucky, it comes out the latter way.

Now, this sensor is the usual problem but, it's not guaranteed that, that's YOUR problem. The problem with yours, and many other diesel coaches that share that 7.5QD Onan is, quite a few of them are NOT on slide out trays. So, the effort that is needed to get that sensor out, is compounded by the the fact that the gen, in many cases if not all, needs to be dropped, in order to perform the surgery needed.

I could and have, posted pictures of the battle I had with that little sensor. I won! IT was defeated but, it was a fierce battle. So, keep us informed on what you find out.
Scott
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Old 10-12-2016, 09:38 PM   #3
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I did mine and it's not really easy acccess even with a slide out.. yes if yours doesn't slide it will have to be dropped. Pull the thermostat housing and replace the temp sender. The bulb of the sensor will break off in the housing and will have to driven or drilled out.
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Old 10-13-2016, 08:33 AM   #4
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Also had the temp sensor problem and shutdown. My hose/thermostat housing was corroded which required replaced housing and temp sensor. No need to try to removed old corroded or broken sensor just buy a new housing not that expensive and corrosion will start again as soon as replacing just sensor. I found one housing bolt would nor clear front cover of engine. Choices would be remove head, or disassembly front case, or cut bolt to remove. I choose to cut bolt and install a shorter bolt when reassembling. I was able to cut bolt using a hand held hacksaw blade.

I think that improper maintenance (not changing antifreeze) can contribute to this common failure.

Fireup post is an excellent explanation and written way better than I could do it. thanks Fireup
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Old 10-13-2016, 09:22 AM   #5
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Well Sir,
The "Sensor" that most on here have had to deal with, including yours truly, is the temperature sensor, located in the lower thermostat housing. It's at the front of that little Kubota diesel, on top and to the left, as you look at the front of the coach. It has caused a myriad of problems for many of us. Depending on year/version, it usually throws out a code 33 which is, "Overheat" condition. And, in many times, it's (the generator) is not over heating. The cooling system is simply not circulating coolant around that sensor due to the fact that the tip of it, resides in a bore, about an 1/8" larger than the sensor itself.

What happens in many cases is, due to the lack of circulation and turbulence of that coolant in that confined area, some corrosion will build up around that sensor to the point that, it completely fills the bore and makes contact between the sensor and the housing. The sensor now cannot receive and send accurate information that little diesels ECM.

So, you get false readings, codes and whatnot. And, as a result of some of those false codes, the engines ECM is instructed to shut down.

That little sensor is about the size and shape of a small, lawn mower spark plug. Here's the BAD part. On some of those little diesels, that corrosion can get bad enough to the point that, it litterally locks that sensor in the aluminum housing. When that happens, it's a like a battle between Patton and Rommel. But, on some, it zips right out, just like a spark plug should. If you're lucky, it comes out the latter way.

Now, this sensor is the usual problem but, it's not guaranteed that, that's YOUR problem. The problem with yours, and many other diesel coaches that share that 7.5QD Onan is, quite a few of them are NOT on slide out trays. So, the effort that is needed to get that sensor out, is compounded by the the fact that the gen, in many cases if not all, needs to be dropped, in order to perform the surgery needed.

I could and have, posted pictures of the battle I had with that little sensor. I won! IT was defeated but, it was a fierce battle. So, keep us informed on what you find out.
Scott
Actually code 24 (Controller did not sense temperature change during first 5 minutes of operation). Corrective Action: See and authorized Onan dealer.
Does sound like you have the answer I'm looking for, including it's location. Now since I'm smaller than average, I might just give it a go, if all I have to do is remove the front cover. There is some space between it and the front clip and just maybe between working from the top and from down under, I can get it off.
As you know the main body of the genset is green and the front of the unit is black, so if I understand you correctly, then I must remove this black cover for access.
Now did you say that you had pictures of your ordeal and if so, I would like seeing them, as well.
I do have the sensor and it is as you say, a spark plug look alike of sorts.
Once I get ready and in to it, I may want to send you a PM if that's OK. Right now, I have more pressing problems with it, so will probably get those out of the way first. Meanwhile, I'm learning all I can about everything and somewhat overwhelmed by the newness of this big diesel and all I've had to do so far. Not like the familiarity of my old gasser, either and did get to know it pretty well, with doing most everything myself over a 9yr period.
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Old 10-13-2016, 10:15 AM   #6
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Tropical36,
First off, gaining access to that sensor, while the gen is in place, is going to be a bear. Knowing what I know and, what needs to be done, especially if that sensor decides to go to war with you, in all reality, you're waaaaaaay better off dropping that gen out the bottom and getting out to where you can work on it.

There are three sheet metal sides/panels that must come off and, getting those off while the genset is in there, good luck. In reality, it's really not all that hard to drop that gen. You need to raise the coach to a point that you'd clear that gen as it rolls out on a floor jack or, whatever you'll be using.

If that sensor fights you the way mine did, you'll need to do a lot of work, as one other poster states. IF, that sensor is locked into its threaded bore, it can be one of the toughest extractions you'll contend with. It was with me for sure. And, as another poster stated, the thermostat housing is a two section deal. The sensor is in the lower half. And, thanks to some seriously intellegent engineering on Kubotas part, that lower T-stat housing has a bolt that cannot be removed without either removing the cylinder head or, removing the front cover of the engine.

I chose the cylinder head removal. It was the least technical and actually easiest of the two remedies. It was a no big deal for me. I had such a difficult time in trying to get it out that, I put so much torque on it that I actually cracked the lower thermostat housing and didn't know it. Long story short, I finally got the pieces of it out, cleaned up all the threads, inserted the new sensor, buttoned everything up (which was more than 3/4 of the generator and, started filling it with anti-freeze.

Drip, drip, drip, drip......... What the h.....????? I finally found where the stream of a/f was coming from. It was the lower t-stat housing that was cracked. CRAP!!!!!!! So, guess what, it all came back apart and, that's when I had to pull the head, just to get that other bolt out.

But, this is a horror story that, while it can happen, you might get lucky and yours will simply unscrew without issue. So, while it will ad some time to the project, you'd be way better off yanking that gen out to work on it, than trying to snake your way around up in that cavity working, almost blindly. I know I wouldn't do it that way. Your choice.

I'll post some pics later today. Going to breakfast with the wife.
Scott
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Old 10-13-2016, 01:30 PM   #7
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Hey guys, thanks for these posts. Guess what I'll be doing this weekend!
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Old 10-13-2016, 05:54 PM   #8
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Tropical36,
First off, gaining access to that sensor, while the gen is in place, is going to be a bear. Knowing what I know and, what needs to be done, especially if that sensor decides to go to war with you, in all reality, you're waaaaaaay better off dropping that gen out the bottom and getting out to where you can work on it.

There are three sheet metal sides/panels that must come off and, getting those off while the genset is in there, good luck. In reality, it's really not all that hard to drop that gen. You need to raise the coach to a point that you'd clear that gen as it rolls out on a floor jack or, whatever you'll be using.

If that sensor fights you the way mine did, you'll need to do a lot of work, as one other poster states. IF, that sensor is locked into its threaded bore, it can be one of the toughest extractions you'll contend with. It was with me for sure. And, as another poster stated, the thermostat housing is a two section deal. The sensor is in the lower half. And, thanks to some seriously intellegent engineering on Kubotas part, that lower T-stat housing has a bolt that cannot be removed without either removing the cylinder head or, removing the front cover of the engine.

I chose the cylinder head removal. It was the least technical and actually easiest of the two remedies. It was a no big deal for me. I had such a difficult time in trying to get it out that, I put so much torque on it that I actually cracked the lower thermostat housing and didn't know it. Long story short, I finally got the pieces of it out, cleaned up all the threads, inserted the new sensor, buttoned everything up (which was more than 3/4 of the generator and, started filling it with anti-freeze.

Drip, drip, drip, drip......... What the h.....????? I finally found where the stream of a/f was coming from. It was the lower t-stat housing that was cracked. CRAP!!!!!!! So, guess what, it all came back apart and, that's when I had to pull the head, just to get that other bolt out.

But, this is a horror story that, while it can happen, you might get lucky and yours will simply unscrew without issue. So, while it will ad some time to the project, you'd be way better off yanking that gen out to work on it, than trying to snake your way around up in that cavity working, almost blindly. I know I wouldn't do it that way. Your choice.

I'll post some pics later today. Going to breakfast with the wife.
Scott
You're most likely right and seldom do things go your way, even when you do have all the room you need. Having said that, I may make a vague attempt for removing the front black cover, if possible for a looksee and can still drop the unit if I have to and as I'll most likely need to do. However if the above should go well, I have a good 12" along the whole front of the unit in which to work and which is accessible from the top and the bottom, as well.
By the way, I'm looking forward to the pictures you've promised.
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Old 10-13-2016, 06:09 PM   #9
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I have a 12.5 Onan and had the same failure. The good news was the Onan is on a slide and the sensor wasn't seized. The bad news was I got a look at the belts and they were shot. Another "no fun at all" job.
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Old 10-13-2016, 06:43 PM   #10
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Scott, did you mean you pulled the valve cover instead of the head? I'm to the point of getting a wrench on the sensor. I'm going to try from the front.
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Old 10-13-2016, 11:30 PM   #11
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Scott, did you mean you pulled the valve cover instead of the head? I'm to the point of getting a wrench on the sensor. I'm going to try from the front.
racerdave,
Nope, not the valve cover, I do mean the HEAD. As was briefly explained in another post here, Kubota, in its infinite wisdom, designed that part of the cooling system in that, that lower thermostat housing is held on by three bolts. Two of the bolts are easy to get out but, the third one cannot come completely out due to the fact that it's too long and, it hits a part (I forgot what it hits, it's been a long time since I did this operation).

So, in order to get that last bolt out, you do one of three things:
1. Manage to cut that bolt off and possibly use a shorter bolt when re-assembling those components
2. Remove the entire front of that little engine which, also involves removing part of the fuel injection and timing rack.
3. Remove the cylinder head which, involves just disconnecting the three fuel injector lines from each cylinder.

#3 was the easiest way out for me and, the least time consuming. In reality, from the very start to the time I actually had the head off and was laying it on the work bench, was right at 45 minutes. Pretty simple.

Once the head was off, I could simply remove the third bolt and lower t-stat housing. I ordered one up and, a head gasket and, a sensor and that was it. It went back together, ultra easy. I found the specs for the valves and adjusted them accordingly.

Put it all back together and, it's been running just fine now for about 3-4 years now.
Scott

Below is a series of pics, showing various stages of what that 7.5QD Onan looks like stripped down, at least most of the way. If any of you have any questions, surely ask.
Scott

Pic #1 shows the gen tore down to the point where I had the best access to that almost destroyed sensor.


Pic #2 shows a closeup of how it sits in its mounted position, in that lower t-stat housing. You can see I've tried various ways of purswading that little guy to come out, he resisted all the way!!!


Pic #3 shows what was left of that sensor after the war was over and I won. I also shows what a new one looks like for comparison.


Even tore down to this point, still did not afford me good access with the correct tools, to get the remaining part of that sensor out. I had to go further with the tear down, so I could use specific vise grips etc. on what was left of it. And the "Bulk-head" divider kept me from using those tools. So, it came out too.



Pic #4 shows the condition of the bore, where the business end of that sensor resides, inside that lower t-stat housing. That is the primary reason it would not come out easily. It was captured by that corrosion.


This last pic shows what it looked like with a new sensor installed, in the "cracked" lower t-stat housing. The crack, is on the back side that I cannot see while working on it. That's why when I put it all back together, I had a massive anti-freeze leak because, I never saw that crack.
Scott

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Old 10-13-2016, 11:54 PM   #12
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racerdave,
Pic #4 shows the condition of the bore, where the business end of that sensor resides, inside that lower t-stat housing. That is the primary reason it would not come out easily. It was captured by that corrosion.
just curious, any ideas on why so much corrosion? how many hrs on gen? was annual coolant swap or was service per onan spec? Just wondering how to avoid, or take preventive action to reduce the likely hood.

BTW, great job for sharing!!!
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Old 10-14-2016, 07:49 AM   #13
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Holy cow! Hopefully mine will just come out. Getting parts today after work. And if things go south I'll be getting more on Monday!

Thanks for this info
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Old 10-14-2016, 12:38 PM   #14
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just curious, any ideas on why so much corrosion? how many hrs on gen? was annual coolant swap or was service per onan spec? Just wondering how to avoid, or take preventive action to reduce the likely hood.

BTW, great job for sharing!!!
Kerryvan,
Well, my thoughts on the reasoning for such a large amount of corrosion in that particular area are mainly the fact that, even when that bore is completely clean, as in new, the probe of the sensor is very, very close to that bore size. And, the fact that the water pump, while in the vicinity of that sensor, simply does not have enough turbulance to "wash out" that area between the probe and the bore.

I think it, the anti-freeze, for the most part, simply scurries on by that opening and, that means there's less chance for a complete anti-freeze change in that bore area. When that happens, you get stagnation. And stagnation can provide the "breeding ground" for corrosion. The anti-freeze was always in prime condition.

I've not done the actual flush and cleaning that maybe prescribed by Onan on a timely basis due primarily to the fact that it was always up, looked and tested (per the little hydrometer thing) to be in great shape. So, I never messed with changing it. And, as for the hours, If I recall at that time, it was right at 205.
Scott

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Holy cow! Hopefully mine will just come out. Getting parts today after work. And if things go south I'll be getting more on Monday!

Thanks for this info
racerdave,
Yes, I do hope yours comes out without a hitch. I really don't have a real poll on just how many come out hard like mine and, come out easy or, in between so, I can't really tell you that you SHOULD not have any problems. I wish I could. I think it's a matter of either luck and or, maybe even if a person's kept up on the maintenance. Keep us informed of how your job goes.
Scott
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