Engine/Transmission (1994 - 1998) AC Pressure Switches

[KHelfrich]

I am having problems wit my AC high pressure switch and the low pressure cycleing switch IWhen I monitor the high and low pressure sides with 95 degree ambient temp The high does not go over 260 psi and the low cycles between 25 and 45 psi however when I rev to 2000 rpm the clutch cuts out and I loose AC I shorted out the high pressure and it no longer cuts out however it cycles So I shorted out the low pressure switch and the AC works great. Here are the questions:



1 Will shorting the switches cause any problems over pressure on the compressor is protected by a dump valve?



2 Do you have to d pressurize the system to change the switches?



3 Has anyone experienced this problem



Thanks

Ken

 

[R.ebel]

Sounds like you might be low on R-134a. I would evacuate, then refill with the correct amount of R-134a.

Shorting the switches can cause problems.

To replace the switches you will need to blow the charge.

 

[Andy Redmond]

I have a different spin. .



The low psi switch CAN BE REMOVED from the accumalator without losing the charge, just hurry and spin it off. The accum. has a schrater valve that the switch depresses...



I recommend changing out the o-ring (remember to lube it good with a/c oil) on the accum, if the switch is changed. I've changed a couple this season including the one on my own ride (one had a leaking switch and the others the oring wasn't sealing). As expensive as 134A is these days the switch and orings were changed on each. Also don't over tighten the switch, finger tight then 1/8 to 1/4 turn with a open end wrench should do the trick.



Be careful shorting the switch, although the compressor is equipped with a blow off/pop off should pressures above 400 or so ever be achieved.



Your high side is a tad high, but within range particularly if the compressor is the original and your truck has some miles.



Low side pressures are normal when the clutch is staying engaged, so sounds like the switch is doing its job. The lower we can get to 25 or 30 psi w/out freeze up and keep the hs in the 250 deg. F range will yield the coldest air temps at the center vent.



I do confer with rebel that the charge may be low, try adding a couple of oz at a time to see if the cycling stops. If so, then likely it will need to be evacuated, then recharged (it should hold 32oz). This way you can assure that you have found and repaired your leak.



I think this is a red herring but. . You can also do some tests on the compressor clutch coil to ck. for excessive amperage draw (current clamp), also ck. for voltage drop in the circuit (do you have 12 volts at the clutch), also look for resistance in the coil (s/b zero @ 70 deg. F). If any of the above the clutch maybe be disengaging although the switches are signalling it to stay engaged.



What is your center vent temp off idle on recirculate when the compressor switch is "jumpered. "? 40 is good with a mecury thermometer. Some of the infra-red's will show 20 deg. temps (likely registering cooled ducting vs the actual air temps).



You can also ck. for the condensor being blocked or a orifice tube problem. Does the center vent temps drop when compressed air or water is blown/sprayed on the condensor? (this trick can be deceiving too, as the temps should drop. . we are manually helping the heat exchange process). In a hot shop in the weather you describe even with a fan we can get soaring temps for the fan to pull thru the condensor at idle with the hood open etc. ; sometimes making the compressor work very hard to cool the interior down. This is why highway speed improve the heat exchange process.



Measure the inlet and outlet line temps on the condensor, ideally at least 30-50 (or more) deg. differential would provide great "superheat. " No differential temp means you likely have a plugged or inop orifice tube or condensor issues.



94-97 have an orifice in the condsensor where the luquid line attaches at the core support.



98-02's have the orifice tube well inside the liquid line necessitating replacement if an issue.



Good luck

Andy

 

[KHelfrich]

What about the high pressure switch does it also have a schrader valve? Also the system is cooling real good now with both switches shorted if I keep both shorted what problems would I encounter?

Thanks

Ken

 

[Andy Redmond]

Yes, Ken the hi psi switch works just like the low psi, just spin it off, you should hear a pop or hiss, while a minute refrig. loss occurs.



Should you remove either and the schrater leaks under the switch just retighten it and seek out a good a/c shop. A good shop may have a tool that will allow one to change a leaky valve core while the system is charged and not lose most of the charge.



You are adding stress to the compressor and extra load on the compressor clutch coil with the jumpers attached to overcome system deficiency.



Even on real hot days, (95+) once mine cools down (inside) the clutch will still cycle while its idling (refueling) or at road speed. I would estimate that it may run (clutch engaged) about one half of the time or less in some instances.



I would try to add some refrig. (with switches normally attached to the harness/connectors). If this helps, you have a leak/had a leak somewhere.



If you are uncomfortable adding refrigerant or further repair, seek out a good a/c shop for assistance. Often they will get it right the first time saving refrig. cost if your first guess isn't correct. Noticed that those 12oz cans are about $11-15, up from 6+/- since last season?



They will also have leak detection sniffer and uv lights to determine the source of the leak.



Also the importance of pulling a good vacuum (evacuation) after the system is opened before recharging is essential to purge air and moisture. A good evac. for45 min to an hour will pull 29+ inches of vac. which will boil off any moisture in the system.



Moisture left in the system will mix (or attempt to) with the refrig oil causing acid and sure compressor death. Not to mention poor performance.



Good luck

Andy