John Deere 790 stuck in the middle of the field, dead, won't stay running.

[LouNY]

Or just remove the fuel shutoff and see if she will start,
just be aware it will not shutoff if it starts.
So low idle high gear stall it.

 

[Richard]

I'm not familiar with your tractor. I had some similar issues with my JCB (except I couldn't take it to higher RPM's sitting still for a "long" time)

Upshot for me, I have a water separator bowl under/near the fuel filter. Turned out for me, the separator was full of water so I only had a sliver of fuel residing on top of it. As long as my demands were pretty small, it would trickle through fine. Once I created any kind of real demand (like drive across the field) it would die and 15/20 minutes later, it would start again and idle perfectly fine....but once the demand was created, it would die again.

I was a bit embarrassed when I discovered it was choc full of water. I don't check that too often (like never it would seem!)

 

[ranger danger]

The problem is actually two fold.

#1. just to get it started is a hit or miss. Right now it won't even start regardless of anything I do... then randomly it'll just start like nothing was ever wrong. So that's part one.

#2. Once I do get it started it runs great. Until I try to engage anything. Any gear or the PTO. Anything that would normally transfer power to something. The hydraulics work fine. (bucket).

#1 i can't help with but, #2 sounds like the safety switch under the seat. I'd try hotwiring that switch.

 

[Chewwy]

JD 790 wiring diagram

does anyone have access to a wiring diagram for a 2002 JD 790 tractor? closest i can come to one is buying a $180 shop manual. thank you

www.tractorbynet.com

 

[LouNY]

Post #6 of the thread linked above is the wiring diagram;
https://www.tractorbynet.com/forums/attachments/790_electrical-jpg.457134/
this may get you to it.
Interesting to me is that this uses a time delay unit to control the fuel solenoid.
Makes me wonder if it is using power to shut off the engine by activating the fuel solenoid.
It seems that your tractor has several safety switches,
A neutral start relay, but you are cranking so that seems to be working, controlled by the time delay unit.
Then a seat switch, a mid pto switch, a rear pto switch and a neutral switch which tie in with the delay unit.

 

[CobyRupert]

Do you know how safety shutdown normally worked? Did they work?
Interlocks may include some combination of seat switch, neutral switch, parking brake and pto switch, etc... Usually an empty seat switch shuts tractor down, but an empty seat switch may be canceled out by a parking brake (set) switch, or only work in combination with another switch.
I can’t think of a scenario where if a switch other than the seat switch failed that it would shut down the tractor.
Seems like all roads lead back to the seat switch.
…now if the brake switch , PTO switch, neutral switch (or whatever) also fails (or sticks closed) intermittently, tractor may not shutdown for the broken seat switch because it erroneously thinks the parking brake switch is still set, or the pto is off, etc..(a case where two wrongs make a right).
Then when the other switch intermittently corrects itself, the broken seat switch shuts tractor down.
…well that’s one possibility. There’s a few more.

 

[LD1]

I agree it sounds like a wiring/safety issue.

5 switches to look at
1. Seat
2. PTO
3. Neutral
4. Clutch
5. brake

And as it has been eluded to....they usually work in conjunction.

You say you bypassed the seat switch....how so? Just a jumper wire? Was it a 2-wire or a 3-wire switch.

Not sure on the deere, but on alot of compacts the seat switch is actually a 3 position switch. It senses operator presence, seat presence, and seat flipped up. The reason is for running something off the PTO that requires no operator....like a PTO generator, or a PTO log splitter. The MFG's realize it would be a PITA to have to have someone in the seat. SO you CAN actually have the PTO on and NOT be present on the seat of the tractor, but you must flip the seat forward immediatly upon getting off of it. Thus a 3-position switch....and 3 wires. So a simple jumper wire in there may be causing the hit or miss issues.

The clutch switch is usually only a starting thing....butt in seat, in neutral, and clutch pressed = start
The neutral switch is both a starting thing and off the seat thing. In conjunction with brake...and in neutral...with PTO off you should be able to get off the seat. So the seat switch and clutch switch are ignored if in neutral, brake on, PTO off
The brake switch is only looked at when getting off and tractor running. But may also need set for start
The PTO switch has to be off to start and you have to be on the seat. But doesnt really interact with the neutral, clutch, or brake switch


SO in one way or another all 5 switches interact with one another and either disable the start solenoid/relay, or cut the fuel off to kill the tractor (fuel solenoid).

Id start checking continuity of all 5 switches, both engaged and disengaged and see if that reveals anything

 

[Matt621]

Okay I am 99% sure I'm battling two issues.

Tried to start it today. Started from square one. Everything in neutral, PTO disengaged, hi/low in neutral. Would not start. Rowed thru the gears put it back on neutral, rechecked PTO, rechecked hi/low. Even cycled the throttle a few times. Turn the key and it started right up. Okay, so I let it idle for a few minutes... put it in gear and it did not die. Drove it around, worked fine. Okay so I'm going try and finish what I was doing yesterday. I worked about 40 minutes. All was fine. But I did not want to turn it off, so I jumped off it and to move some stuff out of the way. (I had bypassed the seat sensor). It was idling for about 10 minutes. Got back on it, and it drove fine, till I tried to pick up a bucket of stuff. Just low on power. Oh, "I'm still in high." So I shifted it to low and tried again.. then it died. But not like before. Before was instant. This time the idle slowly went down... and then tried to keep going and then died.

I tried to restart and it tried, but then could not keep going.

So, what this means is:

Idling for several minutes seems to have something to do it. It did the same thing yesterday when this happened.

And there is an issue with some switch because when it won't start it really won't. It's like a kill switch.

If I can get over that (just by luck it seems) it'll run fine till I let it idling for sometime. After that, it'll die like it's running out of air or fuel. I did check air filter. Dusty but by no means clogged. I'll replace it anyway. So there is likely a fuel filter someplace.

So now the question is: are their manuals for these things like Chilton and Edmunds for cars? I really need a shop manual to know where to look for filters, fuses, switches, wiring diagrams, etc.

Thanks

 

[Matt621]

Regarding the seat sensor. It's a 4 wire, all molded together. I took it apart. It has 3 positions:

1. no one on the seat, nothing is engaged. Open circuit.
2. someone on the seat. The two outside wires are shorted/connnected, closed circuit on the two outside wires.
3. Pull on the switch and the two inside wires are shorted/connected, closed circuit on the two inside wires.

#3. for for PTO operations.
#2. is for normal tractor operations.
#1 is for no one is on the tractor.

So I just twisted the 2 outside wires together and capped the two for the PTO since I'm not using it at this time.

 

[Matt621]

Thanks for the wiring diagram!!! Really helpful. First thing I did was go to the seat switch and what do I see? It goes to something called a "time delay control module." Great. What the heck is that? It goes to both the seat switch and the fuel relay. Anytime I read "timing" and "delay" and "control" all in the same sentence, I cringe.