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Block TXV System Strange Issues

Birken Vogt on Sat August 06, 2011 3:35 PM User is offlineView users profile

Year: 2008
Make: Kenworth
Model: T370
Engine Size: 8.3L
Refrigerant Type: R134a
Ambient Temp: 85
Pressure Low: 10-15
Pressure High: 210
Country of Origin: United States

Hopefully the make of truck does not throw anybody off. It is an ac system the same as any car. Using a Sanden U43xx compressor as I recall. It has a block TXV on which are mounted a high and a low pressure cutoff switch wired in series.

The issue is that when running for an extended period the low pressure cutoff will actuate and stop the compressor. This TXV does not seem to let any refrigerant through when the compressor is not running though. It will get up to 50-60 psi (no higher) and the switch will never cut back in. In winter this is dangerous as the heat from the heater evaporates the condensate water and fogs the windows badly. In summer it is merely uncomfortable.

The system works pretty good with the low pressure switch jumpered but of course it eventually ices up completely and then shutting it down temporarily will result in a massive quantity of meltwater being discharged followed by acceptable performance for a time.

I am thinking this system might have a clutch cycling switch for the evap that senses temperature. I have heard of this being done on some cars. The evap looks to be very difficult to access so I haven't tried opening it up yet. The schematic I have is inconclusive as to whether this is the case or not. The controls all lead to a black box with no description of function. The only clue is that with the low pressure switch jumpered, under conditions I know that head pressure is not too high, the clutch sometimes still cycles but only sometimes.

It has been back to the dealer several times for this under warranty. The TXV has been replaced as have the pressure switches with no change in performance that I could tell. Last time they said no problem found and charged a couple hundred bucks, so I gave up. It's an hour and a half drive one-way anyway.

I was able to get some acceptable performance by replacing the low pressure switch with a Ford CCOT switch from a 96 F-250. Even so I had to back off the adjustment screw about half a turn to get it to cut back in due to the TXV not letting anything through unless the compressor is running. I think it still ices in this configuration but not as quickly as before. The adjustment is very critical, too low and it ices, too high and it will never cut back in.

My suspicion is possibly bad system design (this truck was the first year of an AC system change from our other Kenworths of the preceding model T330) or maybe an inoperative evap temperature switch that I need to find. It seems to me that replacing the low pressure switch with a clutch cycling unit with a closer range than the Ford switch so that it doesn't get "stuck" in that TXV closed, switch still open model ought to work by keeping the evap above the freeze point at all times. Or possibly a retrofit evap temp switch.

Any ideas? Summer is on here now.

Thanks
Birken

bohica2xo on Sat August 06, 2011 4:44 PM User is offline

Looks like you have a bad low side switch. If it drops out near freezing (22 psi or so) it should close again before 40 psi. I have seen those switches go bad that way, and develop way too much hysteresis.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

Birken Vogt on Sat August 06, 2011 11:29 PM User is offlineView users profile

KW has replaced the switch at least once and that is the first thing I did too. No change in performance and the Ford switch was also new and even it has almost too much hysteresis in it. That is what makes me think maybe the low pressure switch is just there for dire emergencies and there is some other element tucked inside that normally handles the clutch cycling. That and the fact that sometimes when the low pressure switch is jumpered the clutch still cycles like you'd expect to see.

Know of any switch that I could try that has even lower hysteresis than the Ford one?

Thanks
Birken

bohica2xo on Sun August 07, 2011 1:04 AM User is offline

Use the older adjustable ford switch on the low side. Adjust it down to where the cut-in is below the 45 psi you see on the low side. You should be able to get the adjustable into range, it is my cure for Explorers with a similar problem. As long as t spends some part of the cycle above freezing you should be ok.

Without clear documentation, you may have to open the evaporator housing if you think it has a temp probe. If it does that is not working properly either... Can you see any wiring entering the evaporator case?

The TXV is a "no bleed" type. Perhaps the system was not designed for that, but the vendor started supplying them?

B.



-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

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