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Old 06-12-2013, 11:19 PM   #1
soambah
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How to unplug the AC low pressure sensor?

I’m trying to check if the low pressure AC switch is open. I was able to get to it by removing what I think is the cruise control servo. I can get a hand on the connector but no matter where and how hard I squeeze it just won’t come off. Obviously I am doing something wrong but there is nothing in the TIS manual regarding how to unplug this thing.

Is there some special tool I need? I’m on the verge of cutting the two damned wires, testing the switch and then using a couple of speed nuts to reconnect the wires. I really don’t want to do that so any help would be appreciated.

Please help! The “boss” is constantly complaining about the AC not working and how it makes her sweat in the car.


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Old 06-13-2013, 05:42 AM   #2
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Um.... you need gauges to verify that you have refrigerant. Maybe the universe is trying to tell you something. The low pressure switch will be closed when the engine is off if you have any charge at all. These do not fail often, but they do fail, so if you test the switch for continuity and don't get it- AND you know you have static pressure at around 100 psi depending on the ambient temp. Then it's bad.

AFAIK, you just press the tabs, wiggle and pull. It's probably stuck from not being unplugged in 13 years. You are unplugging the one under the fender- right?

General AC Troubleshooting and Repair.
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Old 06-13-2013, 12:38 PM   #3
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In “Model Specific Discussions > MK1 Focus”, in the thread named “Bypass low pressure switch? “ austinpetemo asked “Where is the low pressure switch for the ac?” In answer, elsolo said “It's low on the firewall, next to the pass side strut tower, behind the PS reservoir.”

Looking in that area I found what I think is the low pressure switch (see photo below). Should I be looking somewhere under the fender?

[IMG][/IMG]

Either way, can you direct me to a photo or engineering drawing of the connector with directions on where to press to unplug the wires so I can determine if the switch is open or closed?

I’m no stranger to automotive AC. I have gauges for both R12 and R134a, a vacuum pump and a “can heater”.

When the engine is cold and not running, the refrigerant pressure will be equal through out most AC systems. Under those conditions I get 50 psi on the easily accessible HI pressure side. I don’t know if 50 psi will close the low pressure switch. I suppose not since the compressor clutch does not engage when the AC is turned on with the engine running. If the switch is open, I want to monitor it while adding refrigerant to the low pressure side with the engine off. Once I see the switch close I can start the engine, get the compressor running and add refrigerant as required. This way the compressor will not be compromised by being forced to run with insufficient refrigerant.

After charging I can look for leaks. However, if the system has not been serviced at all in 13 years then it is possible that it’s current condition is just due to “normal/expected” refrigerant losses.

I realize that there are other things that could cause the compressor clutch to not function but, having found that there is insufficient refrigerant I want to “fix” that first.
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Old 06-13-2013, 08:06 PM   #4
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Does the clutch on the compressor turn on?
Turn the knob to defrost, the fan to #4, and the AC button on

The low pressure switch only needs 23 psi to remain closed
Pins 2 and 4 on the switch should have <2 ohms
Pin 2 on the harness is +10v
pin 4 is the return back to the CPU at pin #41 (grn/yell wire)
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Old 06-14-2013, 05:26 AM   #5
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It might be there on a 2000 model. I thought it was right on top of the accumulator where the port is. That looks like a LP switch (2 wires), but it's hard to tell from the pic.

Yes all AC systems equalize when the compressor is off. With 134, your charged pressure should be about 10 psi higher than the ambient temp in F. So if it's 90F outside, you'll see about 100 psi. If you're only seeing 50 psi either that gauge is bad, or you have nothing but air in there. If you have nothing but air, you need a vacuum pump, and most likely a new accumulator at the least.

This might bode badly for you. The 2000 model requires a much more expensive repair in parts to replace the accumulator. If you check online parts ordering places like RockAuto.com (we can get you a 5% dc) you'll see a note that tells you that models made between 7/99 and 1/01 or something like that require a different kit. The new accumulator comes with a hose. IIRC, the difference is that there are fittings at the new accumulator so you can replace it without replacing the hose going to the compressor. At any rate, parts store prices will shock you, but that might give you a better chance to look at the part itself in case someone has already done this repair- in that case you can use the 02 accumulator which is cheaper.

A new orifice tube is a good idea since the system is down.

Best of luck, but 50 psi static pressure is not a good sign.
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Old 06-17-2013, 01:53 AM   #6
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First I want to thank you all for your excellent suggestions. The A/C now works!

To make a long story short, it didn’t have enough refrigerant. The compressor clutch was not kicking in. Had the equilibrium pressure been near 90 psi I would have gone looking for a fault in the clutch control circuits. However with an equilibrium pressure of only 50 psi it was clear that the system was low on refrigerant. I figured that the low pressure switch might be open. If it was I could monitor it and just add refrigerant until the switch closed. However, I never figured out how to unplug the damned thing!

So I decided on adding refrigerant in 10 psi increments. After each 10 psi increase I would start the engine and see if the clutch would kick in and run the compressor.

The third 10 psi increment got the compressor cycling. Then, I followed the advice to turn on the A/C at it’s coldest setting and highest fan speed and then add refrigerant to the low side until the compressor stops cycling. That did the trick!

I turned the car off and let it cool for several hours. The outside temperature was 78 F The equilibrium pressure was now just over 90 psi. I started the car, turned on the A/C set to it’s coldest with the fan at it’s highest and inserted a thermistor probe into the passenger side middle outlet. The thermistor settled at a temperature of 48 F and the air blowing out was definitely chilly.

For the next few days I will check morning equilibrium pressures, and evenings, after the car’s been running, I will check for leaks with soapy water.

Thanks again for everybody’s help
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