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AC high pressure cutoff switch

LincolnF on Mon July 29, 2013 9:37 PM User is offline

Year: 1999
Make: Ford
Model: Escort
Engine Size: 2.0
Refrigerant Type: R134a
Country of Origin: United States

Great forum here.

High pressure line with the high pressure port and AC high pressure cutoff switch. This is the line / tube of output from the condenser and input to the orifice tube. It has a high pressure cutoff switch which screws into the line at a Schrader valve fitting.

Should I be able to remove the high pressure switch without losing refrigerant? I would think I would especially since it was at static pressure for over a day.

Here's my understanding of this Ford (Mazda?) designed high pressure valve operating principles. Please don't laugh too hard at me if I don't understand correctly. My education of such is lacking.

It depresses the Schrader valve in "normal" mode. When the high pressure reaches threshold 1 it signals the PCM to turn on the high speed cooling fan (2 speed fan on these Escorts). When the high pressure reaches an even greater threshold, it signals the PCM to disengage the compressor clutch until the high pressure drops below a certain threshold. I would imagine these thresholds are greater than 300psi.

I assume that when the internal pressure builds sufficiently, the Schrader valve is actually pushed against the high pressure cutoff switch thus activating the function (fan or complete cut-off) at the designed pressure. Does this sound right?

So if I'm removing the high pressure switch, I would think that as it's unscrewed, the Schrader valve would close until the switch is complete unscrewed. At this point the Schrader valve would seal the tube thus preventing leakage.

Not happening with mine. The more I unscrew the more refrigerant, oil, dye comes out. I'll have to keep it connected and take to a refrigerant collection facility before replacing the valve. Or, can I replace the Schrader valve itself?

Is it possible I had the system overcharged even in static state? Overcharged enough to have sufficient pressure to keep the compressor clutch from engaging as soon as I engage AC? I'd see no way I could have >= 300psi in the system at static state.

mk378 on Tue July 30, 2013 9:25 AM User is offline

Why are you messing with the "switch" and assuming it's bad when you don't even know what the pressure is?

If it's a 3-wire "switch" it is actually a sensor that outputs a signal proportional to pressure. The PCM interprets that signal and decides what to do.

Most high side sensors don't have shrader valves. Refrigerant must be removed first before unscrewing.

LincolnF on Tue July 30, 2013 11:11 AM User is offline

So the switch can't go bad? If so, that's great news. I'll screw it back in and let it be. Although does make me wonder why the switches are replaceable if they can't go bad (and makes us engineers envious that someone designed a switch that can't go bad. )

I was "messing" with the switch under the (apparently faulty) assumption that switches can go bad, and hence this particular switch could possibly be bad and stuck in the "excessively high pressure" state even though the last pressure I read on my high pressure gauge was about 275 psi with compressor clutch engaged.

This particular switch has a 4 wire connector and hence a 4 spade switch. According to the Ford factory manual it serves 3 functions:
1. Let PCM know high pressure is fine and sends such a signal to the PCM.
2. Let the PCM know that high pressure is a bit high and send a signal to the PCM for high speed fan.
3. Let the PCM know that high pressure is too high and send that signal (or lack thereof) to the PCM to disengage compressor clutch.

Whether it does this through a continuously proportional signal to the PCM, or through stepped fixed voltage values for the different states, I don't know. I imagine the latter.

Ford also shows what looks to be a Schrader valve in the high pressure tube/line for the high pressure switch. This is of course different from the high pressure port which appears to have a larger Schrader valve installed.

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