1736 Premium Cab - ac working intermittently
- Thread starter zdman
- Start date
thclimer
Platinum Member
Thanks for letting us know you fixed it.
lpakiz
Gold Member
- Joined
- Jul 25, 2017
- Messages
- 380
- Location
- Greenwood, WI
- Tractor
- MF1754 HST, Cab, 6 foot Front Mount Snowblower
For the benefit of the next person who has similar AC symptoms, check back on my earlier post (7) where I mentioned that the compressor is controlled by several switches "in series." One of those switches is to prevent "over-pressure" which could blow out hoses, condensers or even damage the compressor. (Another switch senses freon pressure "at rest" and won't let the compressor run if the system doesnt have adequate freon, like if it's all leaked out)
What was probably happening here is that when the operator first turned on the AC, the compressor started normally, but since the freon wasn't being cooled down enough, pressure went up and beyond the "over-safe-pressure" limit, so that switch opened. After a time, when the freon pressure eventually fell, that switch closed and the compressor ran again, for a while, until pressure again got too high. The cycle repeats until the cause of the excess pressure (plugged condenser) was corrected.
There is another control switch that senses pressure at the evaporator core in the heater box in the cab. That switch tells the compressor to shut off if the pressure (temperature) is too low (too cold). It may be part of the temperature controls in the cab.
If the low pressure fall too low (too cold) this switch prevents damage and also prevents humidity from freezing up on the evap core into a solid block of ice, resulting in no air flow thru itself.
Also, if air flow thru the evap core is too low because of a failed blower switch or a plugged up cabin air filter, or doors in the duct system not being in the correct position, this switch shuts off the compressor to prevent the evap from icing up. If this switch fails to shut off the compressor, ice can build up on the core. Shutting off the tractor and placing a can under the drain hose will catch this ice as it melts, indicating this condition. That's why adequate air flow out of the cab ducts is important--the air flow must carry the "cold" out of the evaporator core as fast as it's produced, or freeze up results.
Oh, in addition to all these control switches, all the related connections and wires and fuses and every other component in the system must be intact, or no cold air. Hot wiring the compressor temporarily bypasses all these controls to help diagnose the problem. If hot wiring the compressor clutch produces cold air, the mechanics of the system are OK, and the problem is electrical, usually.
And sometimes the system not only interrupts the electrical supply to the clutch, but in some cases, it can also interrupt the ground. If the compressor clutch has 2 terminals, hot wiring could include directly grounding the ground terminal. You must know which is which, or you will blow a fuse.
What was probably happening here is that when the operator first turned on the AC, the compressor started normally, but since the freon wasn't being cooled down enough, pressure went up and beyond the "over-safe-pressure" limit, so that switch opened. After a time, when the freon pressure eventually fell, that switch closed and the compressor ran again, for a while, until pressure again got too high. The cycle repeats until the cause of the excess pressure (plugged condenser) was corrected.
There is another control switch that senses pressure at the evaporator core in the heater box in the cab. That switch tells the compressor to shut off if the pressure (temperature) is too low (too cold). It may be part of the temperature controls in the cab.
If the low pressure fall too low (too cold) this switch prevents damage and also prevents humidity from freezing up on the evap core into a solid block of ice, resulting in no air flow thru itself.
Also, if air flow thru the evap core is too low because of a failed blower switch or a plugged up cabin air filter, or doors in the duct system not being in the correct position, this switch shuts off the compressor to prevent the evap from icing up. If this switch fails to shut off the compressor, ice can build up on the core. Shutting off the tractor and placing a can under the drain hose will catch this ice as it melts, indicating this condition. That's why adequate air flow out of the cab ducts is important--the air flow must carry the "cold" out of the evaporator core as fast as it's produced, or freeze up results.
Oh, in addition to all these control switches, all the related connections and wires and fuses and every other component in the system must be intact, or no cold air. Hot wiring the compressor temporarily bypasses all these controls to help diagnose the problem. If hot wiring the compressor clutch produces cold air, the mechanics of the system are OK, and the problem is electrical, usually.
And sometimes the system not only interrupts the electrical supply to the clutch, but in some cases, it can also interrupt the ground. If the compressor clutch has 2 terminals, hot wiring could include directly grounding the ground terminal. You must know which is which, or you will blow a fuse.
Last edited:
Doug62
Silver Member
Good info to have for cab owners. My AC was acting similarly last year and cleaning the screen also helped it cool better.