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John Deere Combine

mjohnson2981 on Thu June 18, 2009 9:33 PM User is offline

Year: 198X?
Make: John Deere
Model: 6620
Engine Size: 466
Refrigerant Type: R-134a (was 12)
Ambient Temp: 84
Pressure Low: 23
Pressure High: 213
Country of Origin: United States

Hello all!

I am working on a John Deere 6620 Combine. I'll explain the system.

Compressor: Denso 6E171
Metering: TXV (OEM was externally equalized, Local A/C shop said the hose was an oil bypass and wasn't needed. Currently Internally Equalized)
Condensor: Tube & Fin - Cooling fan is solid (not clutched)
Evaporator: Tube & Fin
Hoses: Condensor & Compressor about 3 feet of hoses. Evaporator has probably 12-15 feet of line.

The system was totally removed last year. Condensor & Evaporator pressure tested. All other componets (hoses included) were new.

The system would be cool but not cold last year. After thinking about it all winter, I decided to open the TXV an extra 1/2 turn because it is no longer equalizing from the evap pressure.

I have charged the system with 3# - 2oz of R134a.

At this charge:
Ambient: about 84 - High Humidity
Vent Discharge: 53 deg
HS (at compressor): 215 PSI
LS (at compressor): 23 PSI
LS Temp (at compressor): 62 degree
HS Temp (exiting condensor): 123 degree
I figure the superheat is about 36 degrees and subcooling is about 13

I know I've screwed witht he TXV, but I'm thinking undercharge EXCEPT for the readings at 2# 8 oz. look improved over the current charge.

At 2# - 8oz.
Ambient: about 84 - High Humidity
Vent Discharge: 47 deg
HS (at compressor): 206 PSI
LS (at compressor): 22 PSI
LS Temp (at compressor): 57 degree
HS Temp (exiting condensor): 121 degree
I figure the superheat is about 32 degrees and subcooling is about 12

I have several questions:

1 - Was my thinking about adjusting the TXV wrong?
2 - Is the system undercharged?
3 - Does anyone know if anyone has an Agriculteral list of charge weights?



Chick on Thu June 18, 2009 9:44 PM User is offlineView users profile

The best thing to do with that condenser is of course go back to R12, but that being said, you are close to fully charged, and can go as high as 2.5 times ambient on the high side with no problem. You might want to add an electric pusher fan on the condenser, that will bring the high side down and help in condensing the refrigerant going thru the tube and fin condenser, a parallel flow condenser will also help. I'd try adding a few more ounces of R134a, but don't expect a refrigerator... working in the fields I'm sure is hot, removing the moisture from the air is just as important..
It should be noted I never worked on one of those and don't expect to in the city, but I'm sure more help will be along.. Hope this helped somewhat..

Email: Chick


Freedoms just another word for nothing left to lose

extramile on Fri June 19, 2009 10:15 AM User is offline

If you go to and on the left click on ag forums then click on the machinery link you can post your problem and likely someone will answer. I had a 7720, same series just a little bigger then the 6620 and you could hang meat in those cabs, system worked really well. Had it so long ago I can't remember much about the system.

mk378 on Fri June 19, 2009 10:37 AM User is offline

Put on a new TXV, externally equalized, as close to stock as you can find. This will raise low side pressure (reduce superheat) because it will properly compensate for pressure drop in the evaporator. Especially the tube and fin evaporators with multiple flow paths and a manifold at the inlet typically have a restricting orifice in the manifold, and an externally equalized valve is essential.

Do as much as you can for condenser cooling. Besides the condenser, make sure the radiator, oil cooler, etc. are not clogged up with dust and chaff. Frequent cleaning is necessary on off-road equipment.

If you keep the TF condenser, don't be afraid to charge it. Operating pressure will be higher on a converted system.

mjohnson2981 on Fri June 19, 2009 2:14 PM User is offline


I have a TXV from JD. The reason the AC shop went with the internal equalized TXV is because the aluminum threads on the evaporator came off with the steel nut from the TXV.

I have found the piece through JD, just have to aluminum weld it on.

Does anyone know if I could find this part anywhere else? It's the o-ring type equalizer line.


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