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Old 04-14-2006, 11:00 PM   #1
jayjr98
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Default A/C Low-Pressure Cut-off Switch... Where is it??

2000 Saturn SL1

Having A/C Problems... gauges read static pressures of 88psi on high and low sides, hence, system is still fully charged. The problem is the clutch is not engaging. I used a voltmeter and there was no voltage on the connector going to the compressor. I could just replace the relay, but I want to confirm that the low and high side pressure cut-off switches are working first.

I found the high-side pressure switch on the driver side, near the fuse box... but for the life of me I cannot find the low-pressure cut-off switch. Logic tells me it should be between the expansion valve on the firewall and the compressor... but I don't see anything.. am I just too tired to be doing this at midnight in my garage? It has to be there somewhere...

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Old 04-14-2006, 11:02 PM   #2
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Default Re: A/C Low-Pressure Cut-off Switch... Where is it??

Quote:
Originally Posted by jayjr98
2000 Saturn SL1

Having A/C Problems... gauges read static pressures of 88psi on high and low sides, hence, system is still fully charged. The problem is the clutch is not engaging. I used a voltmeter and there was no voltage on the connector going to the compressor. I could just replace the relay, but I want to confirm that the low and high side pressure cut-off switches are working first.

I found the high-side pressure switch on the driver side, near the fuse box... but for the life of me I cannot find the low-pressure cut-off switch. Logic tells me it should be between the expansion valve on the firewall and the compressor... but I don't see anything.. am I just too tired to be doing this at midnight in my garage? It has to be there somewhere...
It's a two way switch (high AND low pressure cutoff)

...
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Old 04-14-2006, 11:08 PM   #3
jayjr98
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Default Re: A/C Low-Pressure Cut-off Switch... Where is it??

After looking at the Haynes Manual, I believe you are right... but how does that even work.

Most A/C Systems will have a dedicated switch on the low side to prevent the compressor from being damaged if there is insufficient refrigerant... Hence, if the low-side pressure drops below approximately 20 PSI it will cut off, cycling the compressor.

The high side switch is essentially a safe-guard to keep your entire system from blowing up in case the high-side pressure gets too high (> 300 PSI I think).

The switch I checked was on a high-side line. I'm just curious how the low-side is monitored to ensure enough refrigerant with a switch that is on the high side?

Nevertheless, the switch tested good (no impedance) and there was 12v on the connector that plugs into the switch. So likely culprit is the relay, however... PCM controls turning the relay on and off. So either the relay is bad or the PCM is telling it to keep my compressor off.

This is frustrating for sure. System is fully charged and A/C worked great from day one... but for some reason or another, I am not getting any voltage to my compressor.

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Old 04-15-2006, 04:47 PM   #4
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2003 L-Series 3.0L Sedan
Default Re: A/C Low-Pressure Cut-off Switch... Where is it??

My L300 V6 manual describes my a/c pressure sensor as detecting the fluctuating high side pressures to let the ECM vary the fan speed when the fan switch is left in auto mode.The sensor is also used by the ECM to turn off the a/c relay if the refrigerant pressure isn't within 26-420psi. Mine is a transducer that outputs 0.2-4.9 volts corresponding with 0-459psi. I know that the old GM Frigidaire compressors had a blow-off mechanical valve attached to the end that was set to relieve excessive pressures from inadvertant overcharging by enthusiastic week-end mechanics. Back then it wasn't considered politically incorrect to discharge R-12.
See if your Haynes book tells/shows you where all the a/c components are located and ID'ed or try using the saturnparts.net parts diagram to locate. Some systems just had the lo-side pressure sensor on top of the accumulator(1-2 liter metal bottle), on the firewall, that you could short out to see if the a/c clutch would engage. BTW are you sure that the static pressure readings you stated indicate a full system charge? It can be deceiving w/o use of a/c gauges and monitoring the system while under operation.
Possible hint; no voltage to the compressor could mean a low charge from a leak that would require further inspection of the entire system before attempting recharging. A friend of mine has a 4-5 year old Honda and always had a/c until last year. Long story short, he didn't run potholes, go offroading, etc. possibly vibrating the a/c system to cause any leaks but said it didn't cool as before. Going on that information I was able to not find leaking refrigerant oil anywhere and simply topped off the system with R-134. It worked like a charm on just half a can. Worked all last year.That was/is an ideal situation but his a/c over a period of years leaked down and I'm guessing its from the front compressor seal. Still has the Honda and had to top it off again this year.

Last edited by fdryer; 04-15-2006 at 05:01 PM..

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