M125GX air conditioning question

[Fedup]

I'm working on what (from my experience anyway) is a strange air conditioning complaint. The tractor has had this issue for some time, possibly a year or more. The symptom is this. The tractor is used mainly for PTO work pulling a 15 ft batwing. System works fine, blows cold air as expected. For about an hour or so, then the cold air flow drops off and almost stops. It's still cold air, just not much of it. Operator stops the tractor, idles down, turns PTO off. Waits about a minute or so and the airflow comes back up and all is working again. He claims the fan motor sounds like it's working during all this. Back to work and all is well for another hour or so and the cycle repeats.

I don't work on a lot of Kubota tractors, so specific experience is limited. I see in the parts diagrams (no illustrations for any of this) things like "motor, servo thermostat", " motor, servo F/R change", "thermistor." None of these items sound familiar to me in regard to air condition systems I've worked on. Can anyone enlighten me on what those items are, what they do, and could any of them be involved with this situation?

I'm inclined to suspect something fan motor related, but he claims he can hear the fan running during all this. Could there be vent/duct doors in there that can affect where the airflow goes, the controls for which might be suspect? Whatever the problem is, why would stopping, turning the PTO off and idling down have any affect on it?

 

[PoTreeBoy]

I'm not familiar with the control of that system. But if the fan speed remains the same while the air flow decreases, I'd suspect the evaporator coil is icing up. Has the filter been cleaned or replaced? Maybe the coil needs cleaning

 

[Fedup]

Evaporator is clean. I just had it all apart last week, flushed the system, installed new compressor, drier, and expansion valve. From what I can tell the vent ducting is free and clear. It's all under the cab floor, so it's hard to actually see. As for the cab filter, regardless of condition, why would that produce these symptoms after an hour's use? That condition wouldn't change after sitting idle for a minute.

 

[#40Fan]

Sounds like it is icing up to me too. The lack of flow when it happens, confirms it. The faster turning compressor while the engine is at PTO speed will pull the lower side down more than it would at idle. I'd put an ounce or two in it and see what happens. It wouldn't be the first time I've seen refrigerant specs to be slightly off from what the factory says.

 

[Tughill Tom]

Sounds like a Condenser issue, most likely dirty. I'd clean the Radiator, Condenser and Hydro cooler.

 

[jaxs]

I'm working on what (from my experience anyway) is a strange air conditioning complaint. The tractor has had this issue for some time, possibly a year or more. The symptom is this. The tractor is used mainly for PTO work pulling a 15 ft batwing. System works fine, blows cold air as expected. For about an hour or so, then the cold air flow drops off and almost stops. It's still cold air, just not much of it. Operator stops the tractor, idles down, turns PTO off. Waits about a minute or so and the airflow comes back up and all is working again. He claims the fan motor sounds like it's working during all this. Back to work and all is well for another hour or so and the cycle repeats.

I don't work on a lot of Kubota tractors, so specific experience is limited. I see in the parts diagrams (no illustrations for any of this) things like "motor, servo thermostat", " motor, servo F/R change", "thermistor." None of these items sound familiar to me in regard to air condition systems I've worked on. Can anyone enlighten me on what those items are, what they do, and could any of them be involved with this situation?

I'm inclined to suspect something fan motor related, but he claims he can hear the fan running during all this. Could there be vent/duct doors in there that can affect where the airflow goes, the controls for which might be suspect? Whatever the problem is, why would stopping, turning the PTO off and idling down have any affect on it?

"SERVO" is basically something mechanical operated by motor. In the case of ac, damper door to divert air to floor, defroster or dash. THERMISTOR is a temperature sensitive device which changes resistance as temp goes up or down.

I wasn't able to find Model M125GX so no parts list or diagrams to study. The fact airflow volume changes doesn't always mean somethings wrong. The question should be whether cab becomes hot. Modern building hvac systems speed up and slow down in normal operation. It's totally un-necessary for a tractor but if technology is available Kubota will hang it on their machine.

There's simply too many wild cards involved. You need to eliminate the main one by running system through it's paces instead of relying on operator's analysis. I'm with you as far as coil freezing over, it would take much longer than a minute or two for coil to defrost so rule that out. Ditto for dirty condenser and inadequate refrigerant, things don't work that way. I'm not saying pto must be on, the ac has no way of knowing. It's possible operator isn't running enough rpms and as engine temperature rises, ac partly shuts down to save engine damage. If you are able to duplicate it while sitting in your shop at recommended rpm for loaded pto then simple throttling down, my next step would be by-pass fan controls. Tap into wires between controls and fan motor then temporarily install toggle switch. Attach volt meter and leave it throughout tests. Record voltage to two decimals at beginning while system is working normal. When air volume drops, record voltage then flip switch to see if volume returns. If volume drops and voltage is unchanged, the problem isn't in controls. Time for some if and but. If available, use dpst toggle switch, if not, use a pair of spst. Reason being that control can be in either + or - circuit. If volume recovery is as quick as stated, the odds of bad blower motor are slim to non so I'd move on.

Only other thing that comes to mind is a flap deflecting airflow. I ran a Kubota a few years ago that switched air from blowing on operator to blowing near rear of cab. Make sure there's not a secondary vent covered up. If it were my tractor and problem turned out to be wonky deflector, I'd consider disabling before paying $500 for parts.

Interesting problem to say the least, best of luck.

 

[BAP]

Evaporators and Condensers can look clean but can actually be plugged up with crap especially towards the center of them. Use a flashlight on the back side and look from the other side to see if they are actually clean all the way through.