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Have a 2010 John Deere x748U. Over the past month it has begun to just shut down after about 1 hour of full throttle mowing with my 62 inch deck. Last summer did have some problems with it overheating under these conditions, but it just shuts off now with the temp gage at about the normal setting. Changed oil and oil filter immediately when this began to happen. No improvement. My dealer says it may be a defective electronic dash. Any ideas anyone?

You don't give us much info to work with, so I'm going to ask for more info.
1. What are the model and serial numbers of the unit?
2. What is the brand and the model and serial numbers of the engine?
3. How does it react just before shutting down?
4. What have you tried to resolve the problem?
5. What has your dealer done to resolve the problem?

From what you posted, the first thing I would do is fire your dealer. He does not sound to competent, but maybe he has done other things that you don't tell us.

Unit is : John Deere x748 Ultimate Model No. *M0x748A051079*. Engine is: Yanmar Diesel Model No. 3TNV76-XJLTE, S/N *CH3011D28495*. There is absolutely no hint of the impending shutdown other than the fact that the temp gage will have risen to just slightly above the middle of the area between C and H. Upon the shutdown the oil dipstick seems to be hotter than I would like. The oil is hot as well and almost always has tiny air bubbles in it. I told the dealer rep that air bubbles in the oil stream are an absolute no-no in autos, but he said it is 'normal' with mowers as the oil sloshes around quite a bit. The oil being used is John Deere Plus II, 15W-40, the oil filter is the John Deere specified filter. The mower's engine, radiator, oil cooler, deck, etc. are blown clean with an air blower per the JD Manual periodically during every mowing.

Racer's Edge said:

Have a 2010 John Deere x748U. Over the past month it has begun to just shut down after about 1 hour of full throttle mowing with my 62 inch deck. Last summer did have some problems with it overheating under these conditions, but it just shuts off now with the temp gage at about the normal setting. Changed oil and oil filter immediately when this began to happen. No improvement. My dealer says it may be a defective electronic dash. Any ideas anyone?

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:welcome: to LMF!

My knowledge of diesels is limited, but I do know that air bubbles in the oil is not good. Air does not lubricate. Also, air in the oil is an indication that the oil is getting very hot. I would be a cooling system check. I don't know if your engine has a temperature cutoff switch, But if it does that could be your problem. I would give the the cooling system a good flushing, particularly the radiator, which may be partially plugged. I would also get the temp gauge tested, as it may not be reading correctly. Again I say that I have limited knowledge of the Yanmar engines. Good luck, I hope some others chime in as I need to learn more.

Racer's Edge said:

Have a 2010 John Deere x748U. Over the past month it has begun to just shut down after about 1 hour of full throttle mowing with my 62 inch deck. Last summer did have some problems with it overheating under these conditions, but it just shuts off now with the temp gage at about the normal setting. Changed oil and oil filter immediately when this began to happen. No improvement. My dealer says it may be a defective electronic dash. Any ideas anyone?

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Does the temp gage always read normal or does it start out low then rise to normal?

I agree that it most likely is the temp shut off switch.

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Thanks Guys, for the comments and suggestions. To answer a few of the questions-- The Oil Temp Gage starts at the C or Cold setting and rises to about halfway now. The tractor has a coolant overflow container with a mark for the 'max' fill when cold. Last summer it was overheating when stressed by mowing tall grass for an hour and/or longer periods--the temp gage would go into the red area and it did shut down on me about twice before i learned to watch that gage, and when it approached red--shut it down and let the engine cool down. Thru trial and error, I discovered that the mark is below what creates a balance in the system. So I fill it slightly higher now.
The change in the overflow container level seemed to stop the radiator overheating. Also the fins on the radiator are clean externally, but I have sprayed cleaning water thru both the radiator and the oil cooler. Have blown out the outer air cleaner til I can see daylight thru it, lubed all three spindles, etc. The dealer rep says the shutdown is accomplished by a signal to a pump to stop fuel flow! What triggers that signal is the $100 question.

I think you answered your own question. What triggers this signal? Overheating the engine would seem to be the answer. Before you do damage to the engine which will cost big bucks I would do two things. Replace the temp gauge and flush the cooling system with an appropriate cleaner. Your engine is telling you that it's too hot and shuts down, the gauge is not reading properly.

Thanks for the comments and suggestions. Did take my JD748 tractor to the dealer. It has approximately 250 hours on it and the warranty runs to 750 hours. The dealer rep said the tractor did the same thing with them after about 45 minutes of heavy mowing. They did changeout the temp gage by replacing the electronic dash. The new gage did not indicate overheating but the engine stopped as usual in another mowing test that duplicated my conditions. Did suggest they consider a coolant system flush. They said they will consider this, but at present they are awaiting further suggestions from the John Deere Technical department.

Wish I had a wiring diagram of this unit. Would like to see if the temp sensor is wired to shut down the engine. Does anyone out there know where we can get one?

Racer's Edge said:

Thanks for the comments and suggestions. Did take my JD748 tractor to the dealer. It has approximately 250 hours on it and the warranty runs to 750 hours. The dealer rep said the tractor did the same thing with them after about 45 minutes of heavy mowing. They did changeout the temp gage by replacing the electronic dash. The new gage did not indicate overheating but the engine stopped as usual in another mowing test that duplicated my conditions. Did suggest they consider a coolant system flush. They said they will consider this, but at present they are awaiting further suggestions from the John Deere Technical department.

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Diesels must have adequate cooling or you run the risk of warping the heads due to that extreme heat from the injectors. Very important that they do not run LOW on water or oil due to their cooling effects. On an average diesel the combustion temperatures can reach temperatures hot enough to melt aluminum. I've actually seen the tops of pistons on big rig Cummins engines have a spiderweb pattern, or ripple pattern due to the injectors firing too hot and puddling the aluminum. I've even seen holes the size of quarters or bigger burned through the tops of pistons in them as well. But all diesels run hot in the heads and that's why it is critical to have adequate cooling into that head and also cooled adequately in the radiator. Another thought ...Is the oil level normal when you checked it and saw the bubbles and the oil seemed super hot? I'm wondering if somehow the oil level sensor might be messing up due to the heat and thin oil and causing it to shut the engine down? I haven't seen the diagram and not sure if that Yanmar has an oil shutdown as well. Wouldn't hurt to check. Hope this helps.

I also believe the problem is related to the engine running hot. After the engine has done one of its shutdowns, a certain symbol is displayed on the electronic dash. My JD Operator's Manual says that the presence of that symbol indicates among other things--low oil pressure. A savy parts person at another dealer told me the Yanmar oil pressure sensor drops ( signals low pressure) at 3-6 psi. But according to JD Technical, neither low oil pressure nor high engine temprature will directly trigger a shutdown. Theoretically, it is supposed to just light up that symbol mentioned above.

Spoke to the dealer rep yesterday. He said they have determined that somehow the system is getting a signal to shutdown the fuel flow. He said they were being visited that day by the John Deere District Service Rep. Upon examining the facts and the tractor, the factory rep suggested getting the fuel pump and some other parts of that system checked out by a speciality shop. Perhaps they will find evidence of overheated fuel injectors as mentioned in a previous post!

Welcome to LawnMowerForum!

I moved your thread to the Small Engine & Mower Repair forum. :smile:

Got my mower back from the dealer and have mowed about 5-1/2 hours with no periodic shutdown problems. The John Deere District Service rep had visited my dealer and somehow that resulted in a test of supplying the mower with diesel fuel from a separate tank. They tried it and the mower cut grass for 3 hours without stopping. They took out the built in tank and found that the in tank pickup filter was slowly clogging up. I didn't even know it had an in-tank fuel filter! The complete stoppage was taking about 45 minutes to one hour and then came the shutdown. The Deere in-tank assembly has a mounting plate, a pickup line with screen filter and an excess fuel return line from the fuel injection. According to the JD dealer, the entire unit must be replaced to make the repair.

They cleaned the tank, replaced that pickup assembly and put it all back together. They say the unit then mowed for over 3 hours with no shutdowns. BTW, I buy my off road diesel at a local Cooperative which sells so much fuel that they supply several local gas stations as well as farmers, contractors, and other off road users. I am currently using 2, 5, and 6 gallon plastic containers so the fuel is almost always fresh. Have now found that my 'cans' appear to have gotten dirty over time and need to be replaced and/or thoroughly cleaned. This experience shows me that clean fuel is a must for diesels. But even with that, maintenance of the supply containers is required.