I'm not an expert but this is what I would do. Check the amount of charge; is there a sight glass? There should be no bubbles after a few minutes of running. If no sight glass, check with guages.
Does the thermostat cycle the compressor on and off?
Is there a air filter that may be clogged?
If all this is correct, then changing or adjusting the thermosat expansion valve like you did should have solved the problem.

 

This truck has done this from day one. Everything has been changed, checked, and reverified, except this switch.

Here is what happens. You start running the system and all is well. Air is cold and everything. After an hour or two the air flow is getting restricted (icing up) and eventually the heat transfer to the evaporator is no longer enough to keep the pressure up and the low pressure cutoff switch cuts off the compressor. Large quantities of water drain out. It gets real warm because the low pressure switch has a wide operation band and the TXV is of the type that does not let any through when the compressor is not running.

Well after half an hour or more the low side will rise enough that the switch will allow it to cut back in and it starts to working for some time before it freezes again. Meanwhile it gets to be like a muggy swamp in the cab, or when it happens in winter, it fogs all the windows so bad it is a hazard to vision.

I have worked around it by replacing the factory low pressure switch with a Ford cycling clutch type from a mid 90s truck but it is not a very good solution because that is for a flooded type evaporator with a suction accumulator and it keeps this evaporator too warm for very good cooling on hotter days.

What I really need to know is how to get at the evaporator temperature switch on this unit. It is not apparent at all. Do I go from the cab or engine side? Does anybody have a part number or wire numbers for it?

And sometimes it does work on its own, but only when it feels like it. I recall seeing some sort of a recall on this issue for a T2000 but I don't know if it applies here.

Thanks
Birken

 

Birken normaly when the AC system is over charged it will freeze up. Like Twister said check with guages to see it your system is at the parameters needed. R-12 should have no bubbles but 134 is different and may have bubbles. If all else fails evacuate the system make sure it holds a vacuum and try recharging sometimes moisture will be in the line and this could also cause it to freeze up. Hope this helps.

 

Any time the A/C system is opened up for repairs, the receiver/dryer MUST be replaced.

 

The low preasure switch is not you problem and never has been it only works on original start. If you have a charge in the system it alows the power to the clutch.
the high preasure switch turns on the engine fan to cool the condenser.
The thermatic switch is in the evaporator core, it shuts the power off to the low preasure switch/clutch.
Be sure the drain tube for the evaporator is clear.
Put the system back as it was and take it to a different dealer, if you do not get any satisfaction there call the Kenworth regional representative.
Do not yell, just explain the situation. It sounds like the thermatic switch is stuck on.

 

The low preasure switch is not you problem and never has been it only works on original start. If you have a charge in the system it alows the power to the clutch.

Yes, the only reason I changed it to the Ford style is so that I could use it to keep a higher pressure inside the evaporator and thereby limit the condensing temperature above the freezing point, since it is easily adjustable. It does not cool nearly as good as it does when it is hooked up properly but at least it does not freeze up (as fast anyway).

the high preasure switch turns on the engine fan to cool the condenser.

Right, it is working fine and head pressures are right where they should be.

The thermatic switch is in the evaporator core, it shuts the power off to the low preasure switch/clutch.
Be sure the drain tube for the evaporator is clear.

You should see the water it pours out when it is pretty well iced up, and you cut the AC power off and let warm air melt it all off.[/quote]

Put the system back as it was and take it to a different dealer, if you do not get any satisfaction there call the Kenworth regional representative.
Do not yell, just explain the situation. It sounds like the thermatic switch is stuck on.

I'd love to but taking anything to the closest dealer is 3/4 of a day for two men. It would probably be a hotel stay for any other dealer. Besides at this point it is so far out of warranty I doubt it would ever be worth it. Nothing would be covered.

The other trouble with this place is, not only will they not fix anything right much of the time, they don't have the time to send over any info on how to fix it yourself either. That is why I am hoping somebody can help me with how to get at the evaporator switch.

Thanks
Birken

 

I worked at Freightliner, but it seems to me if you call KW and ask they should be able to get you a thermatic switch. Freightliner produces a book that has the cross over numbers I have one but only covers up to 99 KW. Maybe you can google alliance and see if it comes up their.
You talk about adjusting the preasure switch to keep the preasure higher to keep the temperature in the evaporator from freezing, the condenser is the core in front of the rad, the hot one.
Many systems you can not run them on MAX, that is only to bring the temperature down, not to be used continually.
The system is engineered to run at certain parameters if you start boosting pressures, you will have failures and warranty will not cover it.
You may want to get KW to get a number for you from the manufacturer that builds their system and you talk to them.
If you are expecting to get the efficiency that the old R12 systems had being able to cool the cab to the 50F forget it, systems are made to cool the cab 10 to 20F, below outside temps a Maximum, that is so the driver does not get pneumonia, and for fuel economy.
Just a thought!

 

I guess what I wrote was confusing.

What I did was to replace the low pressure cutoff switch temporarily with a Ford adjustable one. The evaporator pressure with this system runs about 12 pounds. This will cause it to freeze with the freeze switch not functioning. The original switch like you said is just to prevent the compressor from running when there is no charge in the system. The Ford switch cuts off the compressor at about 35-40 pounds instead. This keeps the surface temperatures above freezing at all times. However it also limits the total system capacity so that when it gets hot out, it does not cool well at all. So that is why I need to fix it right.

Birken

 

Birken I think you are on the right track.
All of the TXV systems I have worked on have some type of evaporator switch or sensor to prevent freeze up.
I have tried finding some info online, but not much comes up for Kenworth A/c systems.

Perhaps if you tried calling Kenworth corporate instead of the local dealer? maybe try getting in touch with the District Service Manage for some better info?

 

IT IS CALLED A THERMATIC SWITCH. it has a probe that is inserted into the evaporator core and is there to stop the core from freezing some are adjustable most are not.
Most are between the power on switch, to the low preasure switch, then to the clutch.

 

Ihave had a couple of similar problems, one with a peterbilt 378 the cycling switch connection would only hold its loa until the clutch coils warmed up and then it would not cycle the compressor. I bent the pins a bit and greased them , worked well after that. t800 only cooled not cold , removed the climate control fuse and it workred great

The evaporator shouldn't freeze if the fan is on the 2 highest settings if the air is too cold the temp setting on the dash should be set lower.this is especially true for variable valve units.

 

If the humidity is high the evaporator can freeze, to a block of ice in a couple of minutes.

 

I learned something yesterday. On the bottom of the freeze switch (under the silver tape) there is a small screw that can be calibrated to different temperatures. Mine was freezing up as well. They used a glass of ice water to adjust the temp to 36 to 42 degrees by putting the probe in the water.