More A/C questions

[Fedup]

I spent a few more hours on this M5 111 today. The basics of the system are working now. The charge is right, pressures are good and blowing cold air from the vents.

Problem now is strictly electrical. As in no power getting to the compressor. The control panel in the headliner came out easy enough, but provides little help. The blower switch seems standard, with spade terminals and a harness plug. That you can test if you need to. The heater/ AC controls however, seem to be a sealed package. Looks like the only way into it is to break it.

After studying the parts list a while I came up with a part number for the thermostat. That seems readily available, and surprisingly less than $60 from most sources. It apparently fits many many models. Pictures show it to be pretty simple, and it's described as "preset". If it's preset, and has no capillary tube to sense temperature, then what does the dial in the cab control?

I have no wiring diagrams to work with, so I'm mainly just guessing here as to how the cab controls manage to vary a signal through a preset thermostat to decide when the compressor clutch starts and stops. The thermostat is of course next to the evaporator (gotta love these newer tractor cabs) under the seat. Can't wait to pull that out along with half the floor just to go probing with a test light to find out where power is and where it isn't.

I have to believe most of the newer Kubota cabs use much the same package, so maybe someone out there more familiar with these tractors might care to share some experiences with them. At least the basics of the circuits that control the compressor clutch, and the more common parts you find that need replacement.

 

[Dave_eng]

I think you are making your system out to be far simpler than it really is.

I worry about what you have bypassed to have cold air produced.

There is a frost thermistor to detct frost and amplifier to jack up the thermisor signal, pressure switch to stop operation if no refrigerant or too much, a relay......

I have attached two pages from a M7040 system.

It is rare there is an electrical issue. Rather, the system logic has decided to stop operation and you see this as no power to the compressor.

Invest in a WSM so you dont destroy an expensive part. Many techs are reluctant to work on systems where the owner has already been inside because additional issues may have been created thus masking the original fault.

Dave

 

[whitetiger]

Here is the AC wiring diagram for an M5-111.

 

[Fedup]

Okay, I have a better picture now of what's involved, and something to look for. A lot going on in there, and with all that, you say electrical problems are rare?

Thanks guys.

 

[Fedup]

I'm going to be visiting this tractor again over the coming weeks. Now that the bulk of the heat wave has subsided in this area, and much of what the tractor is used for will slow down soon I'm hoping to revisit the A/C circuits in a bit more detail.

After studying the diagram posted by whitetiger it seems like a good starting point would be the thermostat itself. As I see it, most everything involved with A/C is under the seat at the bottom of the cab. Can anyone confirm that the thermostat and pressure switch are both in that area?

 

[DaveFromMi]

Before replacing the thermostat (temp sensor), check if there is a resistance value that corresponds with the actual temperature.

With tractor shut off, check at temp sensor leads and at other end of sensor wire harness.

If sensor resistance corresponds to an actual temperature that is in the ballpark, the sensor is OK. The wire harness may have an open circuit or loose connection.

 

[Fedup]

I'm not sure I follow you on that. I haven't actually seen this thermostat yet, nor have I found it in any of the parts diagrams, so I don't know yet just what's available for replacement parts yet either. I would have thought the sensor would be part of the unit and sold together as a package. The diagram doesn't show any harness connections involved with it, so I don't know yet what can be disconnected in order to perform any resistance checks.

What I also see in the diagram is that the thermostat works backwards from any I'm aware of. The compressor relay has ignition switch supplied battery voltage to one side of the coil and the the thermostat is involved in supplying the ground for that circuit. Not directly, because last in line is the A/C control panel. Apparently that's it's own little world and has the final say on everything involving the compressor clutch operation. (or non operation) I'm still wondering where in cab is that little gem, and how does one access it?