John Deere 790 PTO engaged kills engine

tractorroadie · Nov 5, 2016

Hello,

Long time lurker, tractor geek and shade tree wrench turner. I've recently acquired a 790 and 990 from an estate. Going through the basics and the 790 starts and runs like a top. Upon engaging PTO, engine immediately dies. Has to be electronic. I've ordered the manuals but won't be home for a week to get into it. I'm guessing it's a safety lockout not engaging but wanted to see if anyone else has had this issue.

990 hasn't run yet. I'm sure you'll be hearing more about that one here somewhere as well. Appreciate all the experience.

Tractorroadie

dieselcrawler · Nov 5, 2016

Seat switch or associated wiring is likely the issue. On my 3005 (new number on a 790) the motor dies if you get off the seat with PTO running... Maybe its as simple as seat switch wiring is dissconnected?

tractorroadie · Nov 5, 2016

From the reading I've done here I was aware of this switch. Seems there is a procedure to run the PTO out of the seat, but even in the seat, I couldn't get it to engage. I'm sure it's electrical because you can use the PTO engage handle like an on/off switch for the motor. Hope to get into the diagram and fire up the fluke. I'll definitely start at the seat

dieselcrawler · Nov 6, 2016

A fellow 790 owner posted a link to a wiring diagram in post #1 of the thread in the link below. It clearly shows wiring to the seat switch, should be of some help in your troubleshooting process.

http://www.tractorbynet.com/forums/...2-help-interpreting-wiring-diagram-using.html

Post back with any further questions and updates... Let us know how it turns out!

RoyJackson · Nov 6, 2016

tractorroadie said:
Hello,

Long time lurker, tractor geek and shade tree wrench turner. I've recently acquired a 790 and 990 from an estate. Going through the basics and the 790 starts and runs like a top. Upon engaging PTO, engine immediately dies. Has to be electronic. I've ordered the manuals but won't be home for a week to get into it. I'm guessing it's a safety lockout not engaging but wanted to see if anyone else has had this issue.

990 hasn't run yet. I'm sure you'll be hearing more about that one here somewhere as well. Appreciate all the experience.

Tractorroadie

There were three versions of the 790, so make sure you use the right schematic for your tractor. The serial number, IIRC, is on a plate just above the rear PTO shield and the schematics go by the serial number range.
There are two safety switches to consider. One is the operator presence (dead man) switch under the seat. Just flip the seat up and it's right there.
There's also a PTO safety switch, IIRC. On my 790, it was located on the left side of the transaxle housing.
I can't recall if there was a separate neutral safety switch, or if that was part of the PTO switch.

RobS · Nov 6, 2016

On my 790 there is an override for the PTO seat switch. You flip the seat up and flip over a bent metal rod sort of thing under the seat. It's for running something like a PTO generator. I've never used it so I'm not completely sure how it works...

tractorroadie · Nov 6, 2016

Appreciate the help. I'll look at the schematic. As far as the serial number, upon checking tractor data my serial number is higher than the last option listed. So I'm assuming it was one of the last ones off the line. Bummer it's the lower HP machine, but for the price I paid, it doesn't matter. Here is the serial number if the more informed among us have a comment as to exactly what year machine this is.

dieselcrawler · Nov 6, 2016

RobS said:
On my 790 there is an override for the PTO seat switch. You flip the seat up and flip over a bent metal rod sort of thing under the seat. It's for running something like a PTO generator. I've never used it so I'm not completely sure how it works...

I knew there was an over-ride of some type, but never needed it, never looked. This has prompted me to check mine when I go down to the barn.

RoyJackson · Nov 6, 2016

dieselcrawler said:
I knew there was an over-ride of some type, but never needed it, never looked. This has prompted me to check mine when I go down to the barn.

IIRC, the override switch was used for off-seat PTO operation. I bought a wood chipper when I had the 790 and off seat use was 2 or 3 times a year...pretty handy. The 790 has been gone for 6 or 7 years...still have the wood chipper though.

RoyJackson · Nov 6, 2016

tractorroadie said:
Appreciate the help. I'll look at the schematic. As far as the serial number, upon checking tractor data my serial number is higher than the last option listed. So I'm assuming it was one of the last ones off the line. Bummer it's the lower HP machine, but for the price I paid, it doesn't matter. Here is the serial number if the more informed among us have a comment as to exactly what year machine this is. View attachment 487008 View attachment 487009

TractorData.com John Deere 79 tractor information

dieselcrawler · Nov 6, 2016

RobS said:
On my 790 there is an override for the PTO seat switch. You flip the seat up and flip over a bent metal rod sort of thing under the seat. It's for running something like a PTO generator. I've never used it so I'm not completely sure how it works...

I knew there was an over-ride of some type, but never needed it, never looked. This has prompted me to check mine when I go down to the barn.

dirt ditch · Nov 6, 2016

Looking at tractordata.com your 790 is probably one of the first John Deere 790's sold as the serial number is LV0790G29XXXX. I'd bet their information is not complete. tractorbluebook.com has 790's listed back to 1999 but no serial number information. Your 790 also has the exhaust pipe coming out of the side panel like the original 790 with the 30 HP engine. For reference, I bought my 790 new December 2007. Serial number LV0790S19XXXX.

2000: 390001
2001: 470001
2002: 570001
2003: 670001
2004: 790001
2005: 890001
2006: 900001
2007: 190001

tractordata.com - John Deere 790
tractorbluebook.com - John Deere 790 4WD

RoyJackson · Nov 6, 2016

dirt ditch said:
Your 790 also has the exhaust pipe coming out of the side panel like the original 790 with the 30 HP engine.

Mine was a 2004 model with the 2nd engine (of the three used in the 790)...27 Engine and 24 PTO HP. It also had the vertical exhaust.
I thought all the 790's had the vertical exhaust...not sure about the 3005 machines though.

tractorroadie · Nov 6, 2016

Hey dirt ditch. I was wondering that myself, but couldn't find any info on 2wd with numbers that low. I didn't think these machines were that new but was assuming the counter tripped over to 2xxxxx after the listed starting number for '07. Make sense they are older but now I've ordered the incorrect manuals. That stinks. Well, see what shows up and how that matches to the physical when I get home in a week.

dirt ditch · Nov 6, 2016

RoyJackson said:
Mine was a 2004 model with the 2nd engine (of the three used in the 790)...27 Engine and 24 PTO HP. It also had the vertical exhaust.
I thought all the 790's had the vertical exhaust...not sure about the 3005 machines though.

Thanks for correcting me on the exhaust. I know the later 790's exhaust came out under the left side blowing forward of the tractor.

dirt ditch · Nov 6, 2016

tractorroadie said:
Hey dirt ditch. I was wondering that myself, but couldn't find any info on 2wd with numbers that low. I didn't think these machines were that new but was assuming the counter tripped over to 2xxxxx after the listed starting number for '07. Make sense they are older but now I've ordered the incorrect manuals. That stinks. Well, see what shows up and how that matches to the physical when I get home in a week.

Well, it is possible that I am wrong on this. I would think a JD dealer would be able to look up the model year to be sure.