John Deere F935 (1997) diesel lawn mower not staying running

bzim

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  • / John Deere F935 (1997) diesel lawn mower not staying running
I'm starting to run out of things to test. I bought this at an auction because it ran, but assumed I'd need to put work into it. After getting it home I found that it would run for 10-20 minutes and then shut down. Took it to the local dealer expecting them to find and fix the problem. Long story short, they sent it back to me knowing that it wasn't working and charged me $900 anyway. I think they assumed that I didn't want to spend the money on a new fuel solenoid and TDM, which they determined were the problem. In the mean time, I have learned a lot about diesel engines and lawn mowers in general to try to repair this myself. I've even had a diesel mechanic come look at it for me. Still can't figure out what the issue could be. Here's what we've ruled out:

-TDM (replaced)
-Fuel solenoid (replaced)
-Fuel pump (tested)
-Alternator/Battery (both tested)
-Water pump (tested)

When I turn the key, the solenoid does not click, however, if I manually push it in, it will stay in until I touch the pedal to go forward. Then the solenoid closes and the machine shuts down. What is strange is that I can manually keep the solenoid open with a bungee cord and the machine will run for 10 minutes, but then loses power for 10-20 seconds before it then shuts off. After that, I need to wait 20 minutes before I can start it again. For what it's worth, the electrical system has been jerry rigged in all kinds of ways by the previous owner. I think that has something to do with it, but I can't figure out why it runs for a while and then shuts down even with the solenoid wide open.

Other than what I've learned over the past couple months, I have no mechanical acumen, so if anything above doesn't make sense or if I'm missing key information, please let me know. But if anybody has any other ideas for things that I can try, I would be all ears. Thanks in advance!
 

sgkent

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  • / John Deere F935 (1997) diesel lawn mower not staying running
Since you say the wiring has been tampered with, I would start by getting the proper manual and wiring diagrams from JD, and then tracing out each wire to see if it is in the correct place. If the wire diagram is electronic then have a couple large copies printed you can work from. If it is in the manual in paper, get it scanned and have some large copies made. Then check off each wire on the copy as you verify it. Once that is done if the behavior is the same, just remember that a diesel engine needs fuel and air to run. The compression and heat should keep it running as most diesels run wide open all the time and vary the fuel injected to vary power. A pressure gauge should tell you if the needed fuel pressure is there, and I would guess the injectors are electronic so a noid light should tell you if the injectors are getting a signal. If this is some mechanical injector monstrosity that runs with cams and gears, then all bets are off. It could be as simple as a floating piece of rag in the fuel tank, or a bad fuel pump - meaning it works fine until it heats up.
 

slomo

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  • / John Deere F935 (1997) diesel lawn mower not staying running
Water cooling doing it's job? Any dead cylinders? Fuel tank been dumped, blown out, reverse blown out and super cleaned? Diesel filters good?

All the new parts "tested"? Ran an IR temp run over the block and hoses looking for hot spots? Thermostat sticking? Thermostat verified opening at proper OEM temp? Leaking fuel injectors? Poor spray pattern? Time to replace injectors? Something killing fuel delivery?
 
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bertsmobile1

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  • / John Deere F935 (1997) diesel lawn mower not staying running
Since you say the wiring has been tampered with, I would start by getting the proper manual and wiring diagrams from JD, and then tracing out each wire to see if it is in the correct place. If the wire diagram is electronic then have a couple large copies printed you can work from. If it is in the manual in paper, get it scanned and have some large copies made. Then check off each wire on the copy as you verify it. Once that is done if the behavior is the same, just remember that a diesel engine needs fuel and air to run. The compression and heat should keep it running as most diesels run wide open all the time and vary the fuel injected to vary power. A pressure gauge should tell you if the needed fuel pressure is there, and I would guess the injectors are electronic so a noid light should tell you if the injectors are getting a signal. If this is some mechanical injector monstrosity that runs with cams and gears, then all bets are off. It could be as simple as a floating piece of rag in the fuel tank, or a bad fuel pump - meaning it works fine until it heats up.
Because of the power in numbers, what he said.
Buy the JD technical manual for your mower
It will have full details of how to test the mower then how to fix it including where all of the parts are hidden
Usually the JD tech manual has a full engine manual for the engine fitted that is better than the OEM engine makers manual
Electronic fuel injection engines can be shut down by the lift pump shutting off, the injectors shutting off , the pressure pump shutting off ,the fuel line solenoid shutting off .
A little tool we use on the farm gear is a LED instrument light with a long pigtail and some aligator clips on the end
We hook this up to various components and see when it turns off or lights up as the case may be
An electric pump might work fine till it gets hot then fail so it has to be tested both hot & cold .
The fuel tank vent could be blocked so it is a simple vacuum lock
And as already mentioned, floating crud in the tank blocking off the outlet.

But buy the file ( the digital down loads are a lot cheaper than the hard print ) , pour your self a good strong coffee then sit down and read it cover to cover
When finished you will know more than most service techs.
JD usually include a "theory of operation" at the beginning of each section so you can understand what is happening and why which is a big plus when you ned to work out what is not happening.
 

bzim

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  • / John Deere F935 (1997) diesel lawn mower not staying running
Thank you for the ideas. Good stuff! I had thought about getting the technical manual to look over wiring schematics, but couldn't understand how that could be connected to the engine getting enough fuel. Some of these ideas highlight a possible connection though, so I will get to work testing. The debris in the fuel tank idea leads me to another idea...this model comes with dual tanks with a switch. One of them has been empty since the day I got it and I've only used the one since it already had the fuel in it. Maybe I should try the other tank and see if that changes anything?

Also, I found a fuse that was blown, so I replaced it hoping that might lead to something, but in the subsequent test drive, the mower deck wasn't cutting, which I quickly realized was due to the belt coming off (which it has never done before). I took the mower deck covers off and reset the belt in place and started the mower again. As soon as I engaged the PTO, the belt came off again and got chewed up pretty bad. After further inspection, it looks like one of the belt pulleys is bent. Based on the angle that it is bent, I don't think it happened when the belt came off, I think it had been that way. I didn't do anything that could have caused it to bend, so I'm wondering if it has been like that since I got it and just never noticed. Could it be that the added torque on the PTO caused by the bent pulley was causing the power failure back at the engine? Or is this me just grasping at straws?

You guys are awesome. Thanks again for your help!
 
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