Kubota Tractor A/C Help

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Kubota Tractor A/C Help

Postby jeffmc134 » Sun May 10, 2009 1:58 pm

2004 M4900 Kubota Tractor
NipponDenso Compressor SCS06

Ambient Temp 80F
Hi Side 225 psi
Lo Side 40 psi (factory spec is listed as 21-28)

Compressor does not cycle. It has been working when i first start the tractor then it will cycle out and not come back in. However yesterday during my test it never kicked out.

I have checked the magnetic clutch by jumping it out and it works everytime. Engages with 12V and Disengages without 12V. Appears to be fine.

Any Ideas?
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Re: Kubota Tractor A/C Help

Postby Nacho » Sun May 10, 2009 8:04 pm

Check the A/C cutoff device (either a thermostat or a low or high pressure switch). Before that, spray water in the condenser to bring pressures down. This should also improve cooling markedly. 225 for just 80°F, at least in a car, is a high start. This means that the pressure could easily go up is there is a condenser airflow problem.
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Re: Kubota Tractor A/C Help

Postby Texan4Life » Mon May 11, 2009 12:05 am

I live on a farm and been around farm equipment my whole life.

First clean all the coolers - radiator, condenser, and oil coolers. I see plugged coolers time and time again especially with smaller equipment (ppl who aren't used to tractors) They think they can run them like cars.

Never been around that model kubota. Hopefully (large american tractors do) they have put the oil cooler(s) and condenser on a hinge so that it will swing out to aide in cleaning. I highly recommend using compressed air. It gets it much cleaner with less mess that a garden hose in my opinion. You will be amazed at the dust that comes out. Take your time. I have spent a good 30 min to an hour cleaning rads before, you just keep finding dust! I recommend a air gun like this one (about 2ft long): http://www.harborfreight.com/cpi/ctaf/d ... mber=35226 cheap and really makes cleaning a breeze!!

Also see what the pressures and vent temps are at different rpms. say idle, about 1600, and full throttle. It can make a huge difference. Every tractor I have driven with AC preforms the best at full throttle.

Now if your pressures are looking ok (which they don't seem to, especially the low.) and the vent temp isn't that good. I would suspect a dirty evaporator. They suck in the same air as the coolers. Except they are sealed up and incoming air is filtered. So check the cab filter(s) and clean those also. in a worst case scenario you would have the clean the evap. itself (way easier than cars at least!)

Why is the AC being worked on? This can help up diagnose your problems.
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Re: Kubota Tractor A/C Help

Postby jeffmc134 » Mon May 11, 2009 1:51 am

thanks for all the help. i am going to try a new pressure switch tomorrow. the tractor does not have a low and high pressure switch. looking at the wiring diagram the only 2 things that affect clutch voltage to the compressor are the pressure switch and thermostat (thermoswitch). it has a dual pressure switch installed in the inlet line(liquid line) between the receiver and expansion valve.

i did clean the condensor with compressed air. it doesnt swing out but it has a removable screen in front. cleaned the screen and condensor fins. the evaporator is in the top of the cab and i am going to double check the filter there also.


Below is some info from the workshop manual.
Factory Spec:
ac: on
temp : max cool
blower: hi
ambient: 86F
engine: 1500 rpm
Lo Pressure: 21-28 psi
Hi Pressure: 185-242
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Re: Kubota Tractor A/C Help

Postby jeffmc134 » Mon May 11, 2009 2:02 am

Why is the AC being worked on? This can help up diagnose your problems.


The A/C is not working all the time just intermittently. Normally works when the tractor is first started then stops functioning. Ever since i have had the tractor (bought new in 2004) the clutch cycles on and off alot. when the clutch stops cycling the a/c stops working. i have had the air gap adjusted before. i checked the clutch by jumping it straight to the battery and it worked everytime. engage with 12V and disengage without 12V. i think i have a different problem this time.

if the system is working properly it will freeze you out on low at idle speed. if it is not working it is pretty miserable in the cab.
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Re: Kubota Tractor A/C Help

Postby Texan4Life » Wed May 13, 2009 7:18 am

jeffmc134 wrote:
Why is the AC being worked on? This can help up diagnose your problems.


The A/C is not working all the time just intermittently. Normally works when the tractor is first started then stops functioning. Ever since i have had the tractor (bought new in 2004) the clutch cycles on and off alot. when the clutch stops cycling the a/c stops working. i have had the air gap adjusted before. i checked the clutch by jumping it straight to the battery and it worked everytime. engage with 12V and disengage without 12V. i think i have a different problem this time.

if the system is working properly it will freeze you out on low at idle speed. if it is not working it is pretty miserable in the cab.




EDIT: fudge. I typed up a good 3/4 page when I saw your earlier post that I missed that pretty much answered my questions :)

looking at the wiring diagram the only 2 things that affect clutch voltage to the compressor are the pressure switch and thermostat (thermoswitch). it has a dual pressure switch installed in the inlet line(liquid line) between the receiver and expansion valve.


when you jumped the clutch did you have the engine running to watch hi/low pressure and see if the air got cold?

you say the compressor kicks out. Are you sure can you visually see the center spinning and then stop when the clutch disengages? if so I would say your expansion valve is not the problem as the compressor would keep running, but with poor cooling performance. So I would lean toward electrical if that is the case. hmm. if you haven't replace the dual pressure switch yet, I would wait. (I assume it requires evacuating and recharging the system.)

So you have an electrical diagram that includes the pressure switch? If you do you can do further testing to make sure it is the problem first by jumpering (by passing) the switch

what is this thermostat? like the control in the cab? It could be a possibility it is bad, but I wouldn't expect it as you say the compressor works and then cuts out for good. Is this an all mechanical setup or is there an electronic control module (ECM) involved somewhere with the AC?

Just taking you through my thought process. hopefully we can get it fixed shops want $$$. :mrgreen:
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