4020 Powershift pressure test results are in. Diagnosis needed.

[Scott 730]

I acquired a 71 Diesel PS a few years back in non running condition. Supposedly had sat for maybe 8+ years. Got it running after a cleaning and re seal of the injector pump. Changed hyd filters and pulled sump screen which was nearly pristine. Oil and filters have been completely changed twice since then. From day 1 there would be random times that reverse would not engage. Usually first thing in the morning, then once it decided to engage it would be good the rest of the day. Some days you could start it in the morning and reverse works right away. Well now I think it has gone into permanent limp mode. Rigged up a pressure gauge and took some system pressure and B3 pressure checks. System pressure is right around 160# in all forward gears, forward neutral, and neutral. In reverse neutral and all reverse gears, system pressure drops to about 125#. Testing B3 yields only 20# in neutral reverse and all reverse gears. Wondering what the Deere Doctors here have to say. The prior random engagement seems rather puzzling. Thank you.

 

[JIM MN]

There is a sticking shift valve.. Transmission shift valve housing AR2335 has a shift valve AR52251 valve sticking when cold but frees up when warmed up.. May need to take valve out and clean up...

 

[620 John]

I'd pull the transmission control valve assembly off and use a rubber tip blow gun and blow air into the B3 oil port. If you can hear air leaking into the main case then you know you've got an internal problem with B3 which could be piston seals or rubber sealing rings underneath the brake pack.

The pic I have here is out of a 4640 tech manual. However, if I did my homework correctly Parts shows the same valve used on almost all the 8 speed power shift transmissions so this should apply to your 4020. I didn't look in the 20 series tech manuals but I do remember trying to find this in a 30 series book but it wasn't there so I had to refer to the 40 series book.

Last but not least IF you need to tear it apart, I HIGHLY recommend testing the transmission lube pressure while shifting through all the gears, turning PTO on and off, and turning diff lock on and off. Lube pressure will tell you a lot more about what's going on inside the transmission than system pressure. Even though you know you've got an issue with B3 you STILL want to check all the other elements before tearing anything apart. There may be more areas with excessive leakage then you already know. I know guys who have had to re-split tractors because they didn't thoroughly diagnose problems before tearing it apart the first time. On top of that you might find questionable parts when you get inside that may or may not cause excessive leakage. If you perform the element leak tests properly it will tell you if you need to be concerned about a particular component.

 

[Scott 730]

I HIGHLY recommend testing the transmission lube pressure while shifting through all the gears, turning PTO on and off, and turning diff lock on and off. Lube pressure will tell you a lot more about what's going on inside the transmission than system pressure.

Can you clarify lube pressure vs system pressure?? 2 terms, same thing?? I have the TM and it shows the test ports on the shift valve with one labelled system pressure, which is where my 160# reading comes from. My 71 has the later shift valve with 2 accumulators BTW.

Does B3 require a single or double split?

 

[Scott 730]

There is a sticking shift valve.. Transmission shift valve housing AR2335 has a shift valve AR52251 valve sticking when cold but frees up when warmed up.. May need to take valve out and clean up...

This latest episode of not working happened after the tractor had been used for several hours of stop and start use. Then on 1 start up reverse acted up. So not really temperature dependent. Tractor was at operating temp when reverse quit.

 

[620 John]

Can you clarify lube pressure vs system pressure?? 2 terms, same thing?? I have the TM and it shows the test ports on the shift valve with one labelled system pressure, which is where my 160# reading comes from. My 71 has the later shift valve with 2 accumulators BTW.

Does B3 require a single or double split?

Lube pressure is basically what's "left over" from system pressure. System pressure is what operates the clutch packs while lube pressure is there to lubricate bearings, gears, ect. System pressure is what you've been testing which is supposed to be in the 140-160 PSI area. Lube pressure is considerably lower than that. The light on the dash monitors the lube pressure and according to the specs is supposed to light up if the lube pressure gets down to around 13-18 PSI. Ideally testing lube pressure to check for a leak is the better way to go. The reason for this is because it's possibly to have a small leak and the system pressure can stay relatively the same or not change at all. Example: Let's say the system pressure is 160 PSI and the lube pressure is 30 PSI while no internal leakage is present. Then the tractor is shifted into a gear with a clutch pack that has a slight leak. In this case if the leak is small enough, the system pressure could remain at 160 PSI and the lube pressure drops to 10 PSI. The reason this happens is because lube pressure is "left over pressure" from system pressure. So when there's a small leak, the system circuit actually robs the "left over" to maintain the 160 PSI. Consequently the lube circuit suffers the loss in pressure first. This scenario doesn't happen very often but when it does it comes back to bite because the problem went undetected by testing system pressure.

In your case you've got a bad enough leak in the B3 circuit to where it shows up while observing system pressure. But that doesn't mean ALL the other circuits are still good with minimal leakage. It appears the 4020 TM is pretty vague with testing this system. They seem to rely on testing system pressure to find internal leaks. My guess is since this was Deere's first powershift they didn't have good test methods developed at the time. When they went to the 30 and 40 series tractors they give clutch element leak tests based on lube pressure. I can't find any place it says where the lube pressure can be checked. In the 30 and 40 series tractors the lube pressure test port is located in the upper RH front corner area of the clutch housing. I'm looking at parts advisor and I see a plug located in a similar location on the 4020 (#24 in the pic). Maybe a guy can test lube pressure here??? Can't hurt to put the gauge in there and try. It's the same style plug where you tested B3 and system pressure. Maybe someone will chime in here with more experience on this.

BTW I went through a 4640 about a year ago to fix B3 and that required a double split. Don't know if it's the same for 4020. Had about $7500 in parts on that one because every disc and plate was cooked and had to go the salvage yard for a B2 housing because the discs wore grooves into it.

 

[Scott 730]

620 John, Yes I have a plug near the brake pedals and "lube" is cast into the trans housing next to it. Thank you for the clear explanation. It helps alot.