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do all compressors have a low presure switch?

pippo on Fri October 02, 2009 8:04 AM User is offline

I need to learn about how these switches work.

1) Do all comps have them (I have an 87 sunbird)
2) How do they work, I mean, I understand they shut down the comp if refrig gets too low. Then, how does one get refrig BACK inside the system, if the comp doesnt run? Shouldnt the fact that you pulld a vac "suck" it in by itself??


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pippo on Fri October 02, 2009 8:08 AM User is offline

Also, I forgot to mention, as this may be related: I noticed when observing my compresdor, apparently there is no "click" one usualy hears when a comp engages. I hear it on my other cars, but not on this sunbird comp. Seems like it never cliks off. Is that normal/possible?


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mk378 on Fri October 02, 2009 9:02 AM User is offline

Your car has a variable compressor system using a V5 compressor. The compressor has an internal mechanism that controls the rate it pumps according to low side pressure. When the low side psi gets into the low 20s, it stops pumping completely. In normal operation the clutch stays engaged constantly. The compressor controls itself internally.

The reason there must be some mechanism in place to keep the pressure from going too low (either variable compressor or cycling switch) is to prevent the evaporator from getting too cold. The minimum attainable evaporator temperature is the boiling point of the refrigerant in the evaporator, which is related to pressure. If the evaporator gets below the freezing point of water (32F), water will freeze on the air side and block the air flow.

The low-pressure switch on a variable system is only there to shut down the compressor in case of a near total leak-out of the refrigerant or very cold ambient temperature. In normal operation, the switch is always closed. The switches used on a variable compressor system are calibrated to about 10 psi instead of the usual 25 psi used in a cycling compressor system.

pippo on Fri October 02, 2009 9:22 AM User is offline

Thanks, MK. So, with my V5 type comp, I SHOULD hear that distinctive click when the clutch engages/disengages? Is it normal/possible for it to just run on and on?


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iceman2555 on Fri October 02, 2009 9:28 AM User is offlineView users profile

This questions lacks total information. There are two compressor used for this vehicle during this period. The turbo version is equipped with a H6 compressor. This system may include a Low Pressure switch, normally located in the rear head of the compressor. This is a protection device for low refrigerant charge. This vehicle will also be equipped with a Low Pressure Cycling Switch, or Low Pressure Cut Out (LPCO) located on the accumulator. This is a protection device to prevent evaporator freeze was never intended as a compressor protection device, although it has long been felt this was the design.
The non-turbo vehicles use a V5 compressor. This is a variable stroke compressor and does not use the LPCO switch. This compressors utilizes a pressure control switch located in the rear head of the compressor. The purpose of this switch is to monitor a pre set low pressure control thus allowing the work load of the compressor to maintain a low pressure/temperature suitable to prevent evaporator freeze up.
Providing the true problem with your vehicle is indeed a low pressure due to lack of refrigerant, the correct procedure to 'get refig BACK inside the system' or to recharge the system is to have the system refrigerant recovered, the system evacuated and then recharged (use of the correct recharge equipment) to OE spec's. Then a total system performance test may be conducted. Although it is a known is not an industry accepted procedure to use the compressor/vac to 'suck' refrigerant into a system. There are several factors concerning this procedure. During this procedure, the compressor may be actually operational without sufficient amounts of lubricant to maintain acceptable lubricant levels within the compressor. Secondly, it is almost impossible for a seasoned tech to properly recharge a system in this manner and for the DIY'er it maybe come a true nightmare. Lack of proper refrigerant recharge is a major cause of compressor failures, due to the lack of sufficient refrigerant to maintain migration of lubricant.
It was not stated and is not assumed that the problem is with a LPCO cycling the system due to lack of refrigerant or a possible lower low side pressure due to an undercharged retro fit. If the problem is in this area, then the repair is the same....recover...evac...recharge to specification....not pressures. The system must be recharged to a known amount of refrigerant prior to a performance test.
The 'clicking' sound is normally associated with clutch engagement and disengagement. The Low Pressure Switch located in this system, believed to be the 2.0 Non Turbo, is a compressor engagement control device. This a 'normally closed' switch. If pressures decrease below a certain level, the switch will 'open' and break the circuit, thus disengaging the compressor or not allowing for clutch engagement. Suggest to test the switch for a possible failure. Also back to square one....service the system...them perform a system analysis.
This vehicle should be equipped with a V5 compressor. This compressor does not have a 'cycling' operation. If the compressor clutch does not energize upon request and the amount of refrigerant in the system in known...the problem is probably electrical in nature. Acquire the correct wiring diagram for your system and precede with an evaluation. Keep in mind that this system also incorporates a HIGH PRESSURE CUT OUT switch also. This switch may also prevent compressor engagement. Both switches can be tested with a multi-meter for continuity.
If not mistaken this vehicle also incorporates a Power Steering and Wide Open Throttle Cut out switches. Failure of any of these may prevent clutch engagement. Obtaining the correct wiring diagram for your vehicle and a evaluation may be required.
To learn more about the system...why not simply acquire the proper resources and study the system operation and service techniques. Site sponsor has some fantastic resource material available.
Post a bit more concerning your vehicle type....and the failure conditions being experienced. Help if come.

The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
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GM Tech on Fri October 02, 2009 10:03 AM User is offline

I agree with all the above-
My charts show the 4 cylinder engine- 2.0L used the H-6 compressor while all other engines used the V-5 in 87 J-cars

The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

pippo on Fri October 02, 2009 11:56 AM User is offline

Thanks Iceman and GM. Yeah, it is a non turbo, 2.0L, so V5 should be it. . I am currently NOT having problems, I just wanted to learn more as when, not if, when the time comes, I want to be ready . I will search for this site's resources, as you suggest. Also, I will listen harder for that Click. I am getting ready though, to consider changing refrigerant, as not cooling v well (58-59 deg F at vent). Have some R12 at hand you may have noted another thread of mine on the can of R12).


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NickD on Fri October 02, 2009 3:14 PM User is offline

2) How do they work, I mean, I understand they shut down the comp if refrig gets too low. Then, how does one get refrig BACK inside the system, if the comp doesnt run? Shouldnt the fact that you pulld a vac "suck" it in by itself??

A pressure is strictly a monitoring device, has nothing to do with either the loss nor charging of the system. A mechanical pressure switch has a diaphragm under spring pressure, in either a LPCO or cycling pressure switch, reduction on the low set point causes a set of contacts to open. A rather ingenuous levered spring system is use so that the pull in pressure to reclose the contacts is much greater than drop out pressure. A typical R-12 cycling switch would drop out at 25 psi, but the pressure has to increase beyond 44 psi to close the contacts again. This is called hysteresis and is required or the compressor would cycle at ten times per second. Since the pressure must equalized, takes more like 45 seconds in particular on cold days.

There is a direct relationship between pressure and temperature with refrigerants, so the mechanical types are giving way to thermistors, the later being for more reliable as no diaphragms to leak, springs to break, or contacts to wear out. Hysteresis is taken care of by a analog to digital converter and software in a microcontroller.

If you are thinking about losing refrigerant and with an 1987, you don't have a pressure switch at the rear of your compressor, you have a high pressure relief valve. Those are crazy because if an extraneous source causing your high pressure relief valve to release, you will have lost a good share of your refrigerant. GM used them because they are cheap and can save a couple of wires, but were outlawed by the EPA in around 92-94. High pressure cut out switches are used after that, they simply break the clutch coil circuit at around 430 PSI, but pull in again at around 220 psi. No refrigerant is lost.

A pressure relief valve is suppose to close if it was popped, but one that old, wouldn't trust it or would leak test it first, better off to change it to a HPCO switch placed in series with your AC clutch relay solenoid.

Edited: Fri October 02, 2009 at 3:25 PM by NickD

pippo on Fri October 02, 2009 3:27 PM User is offline

OK, Nick, so my comp has a high pressure relief valve, and no cutout switch. Either way, I should hear it cycle, right? have you ever seen a comp that "clicks" so quietly you cant hear it?

Like I said, so far , working OK, other than that I am not happy with the coldness coming out (high 50's deg F)......

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mk378 on Fri October 02, 2009 3:46 PM User is offline

You will only hear one click when you first press the A/C button. After that it stays engaged constantly until you switch off the A/C or turn off the key.

If the accumulator is getting much colder than the vent air it could be a reheating issue. Clamp off the heater hoses to make sure the heater can't fight the A/C.

If it's not that, you need to hook up a manifold gauge and get the operating pressures in order to diagnose further.

NickD on Fri October 02, 2009 10:16 PM User is offline

That is correct, the V-5 variable displacement compressor does not in normal operation, cycle. Normal operation is above about 40*F and properly charged with no condenser nor evaporator air restrictions due to debris or a faulty fan. Typically your vent temperatures will be lower as the ambient temperature increases, really not much cooling when the ambient temperature is say 68*F outside, but then, you don't need much cooling anyway at those temperatures, you will get lower vent temperatures when it's 90*F outside.

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