Air conditioner bad low pressure cut off

Hello,

The Air conditioning on my 93 xj is out so I took it to the shop.
They say the low pressure cut off switch is bad and when it is bypassed the air conditioner works fine.

How do I replace this switch? Where can I get one?

oh they also quote a new drier/accumulator and want to retrofit to r134a from r12. They say it will only cost $509. What a deal....

Thanks for any advice.

Connor

I take it you still have R-12 in it?

You need to drain the R-12 in order to replace the switch. I would look real hard for someone who still has an R-12 recovery system that can suck out and save the R-12 to refill the system after replacing the LP switch.

In the mean time i would be tempted to by pass the LP switch. My 85 never had one, and still does not have one and it runs fine. Mine are both converted to R-134a already. If the expansion valve plugs up, then the high pressure switch will protect the compressor, and if it runs low on freon it stops cooling which tells you it needs freon anyway (low suction pressure).

Never was too sure exactly why they used LP switches except to keep the system from icing up perhaps.

If you can not find an R-12 recovery system, you might shop Ebay for a used one, never know you might get lucky, and R-12 runs about 50 to $80 for the one time refill right now, and its only going to get harder to find.

Many AC shops no longer service R-12 units as they no longer have the recovery systems for R-12. They have all gone to R-134a.

ConnorMann said:

Hello,

The Air conditioning on my 93 xj is out so I took it to the shop.
They say the low pressure cut off switch is bad and when it is bypassed the air conditioner works fine.

How do I replace this switch? Where can I get one?

oh they also quote a new drier/accumulator and want to retrofit to r134a from r12. They say it will only cost $509. What a deal....

Thanks for any advice.

Connor

Click to expand...

How would a guy bypass the switch?

With a jumper wire. Just show a jumper wire into the two female ends of the connector that normally attaches to the swicth. Then wrap it up with some electrical tape and tie it down somewhere so its no hanging loose.

IF I were you.... I would get a second opinion on this !
Generally if your system is low on refrigerant the LPCO interrupts the control circuit for the compressor clutch. The system can have a slow leak which causes the LPCO to open.... This doesnt mean the LPCO is bad...
This is a safety feature so your compressor does not get a liquid flood back which will damage the compressor.
Attach a refrigerant gauge manifold to the low pressure and high pressure fittings... The static pressure (system not running) should be 100-120 psig and when running; the low pressure side should be 40 psig (no less than 35 psig).
The shop may be trying to sell you parts figuring you know nothing about the A/C system. Have to reclaim the R-12 to service the LPCO so that is a good way to convince you to also convert to R-134a at the same time....

You have my curiousity here. How is liquid refrigerant going to get to the compressor suction side of the compressor when the suction pressure gets too low? If anything it should less chance of a liquid slug.

Does not make any sense too me, at least not on a standard expansion valve system like the Jeeps have.

He said the first place he went to said the system worked fine with the LPCO bypassed, which if they are in the AC business tells me they put gauges on it, and determined the pressures to be normal, meaning he does not have a leak.

Of course they could be incompetent bafoons, which I have seen far too many times so I will agree that he should get a second opinion, hopefully at a shop that still supports R-12 service.

CharlesS said:

IF I were you.... I would get a second opinion on this !
Generally if your system is low on refrigerant the LPCO interrupts the control circuit for the compressor clutch. The system can have a slow leak which causes the LPCO to open.... This doesnt mean the LPCO is bad...
This is a safety feature so your compressor does not get a liquid flood back which will damage the compressor.
Attach a refrigerant gauge manifold to the low pressure and high pressure fittings... The static pressure (system not running) should be 100-120 psig and when running; the low pressure side should be 40 psig (no less than 35 psig).
The shop may be trying to sell you parts figuring you know nothing about the A/C system. Have to reclaim the R-12 to service the LPCO so that is a good way to convince you to also convert to R-134a at the same time....

Click to expand...

Hey, I just remembered working on my sons 96 ford tarus 2 weeks ago, I thought it was bad LPCO switch, or low fill due to a leak, only to discover that the female pins in the damn harness connector were no longer making contact. Wiggled the connector just the right way and the compressor would come on. Tightened the female pins with some needle nose pliers and that was the end of the problem. Been working perfectly ever since.

Check that before you go any further!:sunshine:

When compressor suction pressure is too low causing moisture that crosses the evaporator to collect as frost and freeze on the fins..... When enough builds up due to lack of heat load; there will be no thermal transfer....
The TXV is wide open allowing liquid thru and the liquid refrigerant does not flash to a gas while going thru the evaporator causing the liquid in the accumulator to overflow and slug the compressor.....
Granted.... this can require several problems in the system... , but on old systems where TXVs react slowly, and the accumulators are not functioning correctly.... this can occur.
Technicians at work have found damaged and destroyed compressors when this has occurred.

Thanks I had forgoten exactly how that worked as I don't do just this every day.

It takes a steady flow of freon, at low pressure on the suction side (caused by maybe a partially clogged expansion valve), plus a low very humid air flow rate across the evaporator for that to happen, but not likely right now as most of us have our ACs cranked up to max air flow rate to keep us from boiling over with no outside make up air assuming the dampers are working of course, but in spring and fall I can see it happening when the blower is set to low speed with 100% humidity outside air coming in, and a low freon charge or partially clogged expansion valve or both, then ice can build up on the evaporator, ice it over, choke the air flow off, and then liquid freon could get through the TXV and right into the suction side of the compressor and BOOM!

In my case I would notice the reduced air flow due to icing, etc, long before it got to that point on mine.

Accumulator not function properly? You mean the TXV and evaporator right?

EconoMike,

Accumulator/Drier is cylinder shaped (has an inlet and outlet with temperature sensor screwed in) which is on the return/suction line of the compressor.


ConnorMan,

The accumulator/drier replacement is recommended when doing the retrofit, but not necessary... unless it is identified to have an flow obstruction....
The LP cutout switch can be purchased at the dealership or an automotive A/C parts company http://www.aircoparts.com/shop/
To replace the LPCO switch; you will need to pump down the system since the threaded bung will open the system to the atmosphere when it is removed....

I have a problem with some of this. I have never seen a temperature switch on an automotive AC system under the hood (yet). What year did Jeep start doing this? My 85 & 87 do not have thermal AC switches under the hood nore do they have LPCO switches nore do they have LPCO switches. What year did Jeep add the LPCOs and Thermal switches to the low pressure side?

There is an electronic thermal sensor in the Evaporator core area on mine which I think is designed to cycle the compressor if the evaporator gets too cold, cold enough to ice up.

The accumulator / filter / dryer on my 85 an 87 are on the High Presure side are right after the condensor and just before the expansion valve. My 85 also has a high pressure releif valve on the FDA (Filter Dryer Accumulator).

I also disagree on the need for filter dryer replacement when switching to R-134a. When any R-134a containing refrigerant is used to replace R-12, the filter dryer must be replaced as the old style R-12 dryer desicant is not compatible with R-134a (or its oils, I forget which), as the new R-134a & its oil will disolve and or destroy the old style R-12 desicant such that it blocks the refrigerant flow, and can throw solids into the flowing refrigerant. As I recall it sort of party disolves and partly brakes up the beads so that sticky fine remains of the beads get picked up and circulate in the system, then they can stop up the expansion valve, or seal off the dryer exit pipe, or get into the compressor and then end of story.

The old R-12 desicant is not immediately destroyed, it is a slow gradual process, but typically happens over several months.

Also anytime you open a system it is recomended to replace the dryer, especially with the rinky dink hardware these shops use to pull a vaccum these days.

CharlesS said:

EconoMike,

Accumulator/Drier is cylinder shaped (has an inlet and outlet with temperature sensor screwed in) which is on the return/suction line of the compressor.


ConnorMan,

The accumulator/drier replacement is recommended when doing the retrofit, but not necessary... unless it is identified to have an flow obstruction....
The LP cutout switch can be purchased at the dealership or an automotive A/C parts company http://www.aircoparts.com/shop/
To replace the LPCO switch; you will need to pump down the system since the threaded bung will open the system to the atmosphere when it is removed....

Click to expand...

Ecomike said:

I have a problem with some of this. I have never seen a temperature switch on an automotive AC system under the hood (yet). What year did Jeep start doing this? My 85 & 87 do not have thermal AC switches under the hood nore do they have LPCO switches nore do they have LPCO switches. What year did Jeep add the LPCOs and Thermal switches to the low pressure side?

There is an electronic thermal sensor in the Evaporator core area on mine which I think is designed to cycle the compressor if the evaporator gets too cold, cold enough to ice up.

The accumulator / filter / dryer on my 85 an 87 are on the High Presure side are right after the condensor and just before the expansion valve. My 85 also has a high pressure releif valve on the FDA (Filter Dryer Accumulator).

I also disagree on the need for filter dryer replacement when switching to R-134a. When any R-134a containing refrigerant is used to replace R-12, the filter dryer must be replaced as the old style R-12 dryer desicant is not compatible with R-134a (or its oils, I forget which), as the new R-134a & its oil will disolve and or destroy the old style R-12 desicant such that it blocks the refrigerant flow, and can throw solids into the flowing refrigerant. As I recall it sort of party disolves and partly brakes up the beads so that sticky fine remains of the beads get picked up and circulate in the system, then they can stop up the expansion valve, or seal off the dryer exit pipe, or get into the compressor and then end of story.

The old R-12 desicant is not immediately destroyed, it is a slow gradual process, but typically happens over several months.

Also anytime you open a system it is recomended to replace the dryer, especially with the rinky dink hardware these shops use to pull a vaccum these days.

Click to expand...

My XJ is a '98 and even on the < '91 XJs there were Low Pressure CutOut switches installed....
Their is a temperature sensor attached to the drier which controls the compressors clutch when the evaporator temperature drops below 32*+/- (will have to look up exact temperature in FSM) in cold weather climates...
To clarify my statement about replacing the Drier while doing a conversion from R-12 to R-134a. It is recommended; due to the build up of contaminants over time (I understand the reason and agree with your explanation), but some people who do the conversions go the cheap route and feel that it is not necessary....
Most backyard mechanics try to get away with the bare minimums; refrigerant, oil and drawing system under a vacuum.
Not disagreeing with your explanation... just clarifying mine....
The overall requirements for doing the conversion.... The service connections need to be replaced with the newer style fittings for using only R-134a, the old mineral oil needs to be chemically flushed from the system/compressor and replaced with the new PAG oil, the filter/drier needs to be replaced, the systems O rings need to be replaced (the new oil molecules are smaller and tend to seep thru the older O rings) since they have lost some of their elasticity after constant exposure to engine compartment heat, and it is not a bad idea to replace the suction and discharge lines that have flexible hoses incorporated...
The new PAG oil used is hygroscopic (absorbs moisture) and care needs to be taken to minimize its exposure to outside air.

"My XJ is a '98 and even on the < '91 XJs there were Low Pressure CutOut switches installed...."

My 85 & 87 appeared to still be OEM hardware, R-12 when I bouth them used in 2002 & 2004. Neither had an LPCO.

You wouldn't have access to an AC FSM for those years by any chance? THe AC FSM data seems to be more difficult to aquire.

"Their is a temperature sensor attached to the drier which controls the compressors clutch when the evaporator temperature drops below 32*+/- (will have to look up exact temperature in FSM) in cold weather climates..."

Only one sensor/switch on my dryers, and I am pretty sure it is a HPCO switch.

"To clarify my statement about replacing the Drier while doing a conversion from R-12 to R-134a. It is recommended; due to the build up of contaminants over time (I understand the reason and agree with your explanation), but some people who do the conversions go the cheap route and feel that it is not necessary...."

You mean cheap route like Me? LOL. Yes, I tried the R-134a quicky kit,it lasted three months before the Compressor front seal decided it was just way past it's prime. I did get a three month repreive during July August...in Houstons 100% humidity, 100 degree days. Then I shopped for new compressor and did the major overhaul, only thing I reused on my 85 was the evaporator and expansion valve. Everything else is brand spankin new.


"Most backyard mechanics try to get away with the bare minimums; refrigerant, oil and drawing system under a vacuum."

Oh, Is oil a bare minimum! :roflmao:

"Not disagreeing with your explanation... just clarifying mine....
The overall requirements for doing the conversion.... The service connections need to be replaced with the newer style fittings for using only R-134a, the old mineral oil needs to be chemically flushed from the system/compressor and replaced with the new PAG oil, the filter/drier needs to be replaced, the systems O rings need to be replaced (the new oil molecules are smaller and tend to seep thru the older O rings) since they have lost some of their elasticity after constant exposure to engine compartment heat, and it is not a bad idea to replace the suction and discharge lines that have flexible hoses incorporated..."

I still have some mineral oil in both of mine. I went with the PAG (?) oil instead of the ester oil, since the PAG oil is compatible with the old mineral oils. All the shops here in Houston have gone with the PAO oil for that reason (compatabiltiy). I just did not want to do all that draining and flushing. So far it has not been a probblem with my system.

Oh, you forgot to mention replacing the old R-12 hoses with the new barrier lined hoses for R-134a. Seems I recall that the old hoses deteriorate internally when exposed to the new oil/refrigerant causing problems and the new hoses have a barrier liner that helps keep the R-134a from leaking out through the hose material.

"The new PAG oil used is hygroscopic (absorbs moisture) and care needs to be taken to minimize its exposure to outside air."

I have heard and understand the hygroscopic (I am a Chem Eng.) but it just dawned on me that the opened bottle I used last year (and saved the unused remains of, is probalby worthless now for use in any future retrofits!!!!! Been in the house, with AC and cap on, but it if it as hygroscopic as I suspect, it sucked moisture into the oil since last year as it is not what I would called SEALED anymore.

Guess I could boil it on a hot plate for a short while before I used it.

Didnt forget about the old R-12 hoses..... I did say " It is not a bad idea to replace the suction and discharge lines that have flexible hoses incorporated...
YUP.... Not a good idea to reuse Hygroscopic oil or brake fluid that has been laying around even if the cap is tight; it will absorb moisture....
Boil it on a hot plate for a short while... Now that is funny.... Can I watch ??? 8)

CharlesS said:

Didnt forget about the old R-12 hoses..... I did say " It is not a bad idea to replace the suction and discharge lines that have flexible hoses incorporated...
YUP.... Not a good idea to reuse Hygroscopic oil or brake fluid that has been laying around even if the cap is tight; it will absorb moisture....
Boil it on a hot plate for a short while... Now that is funny.... Can I watch ??? 8)

Click to expand...

Yep, you did mention the hoses. Sorry.

What's wrong with boiling (or heating) the oil to flash off any tyraces of water? Not sure what PAG's boiling point it is, but it should be pretty high. Much higher than water. Of course if it forms an azeatrope like alcohol it won't help.

Brake fluid??? Hmmm.

how bout going to an a/c supply house and getting a strap on the line tstat designed to open at a preselected temp probably around 35 degrees some are temp adjustable strap it to the lo side insulate it and wire it in to the old lo press cut out

kennzz05 said:

how bout going to an a/c supply house and getting a strap on the line tstat designed to open at a preselected temp probably around 35 degrees some are temp adjustable strap it to the lo side insulate it and wire it in to the old lo press cut out

Click to expand...

That would make way too much sense, besides it takes all the fun out of smoking the compressor and replacing it later.:laugh3:

Always some smart as* out there trying to ruin our fun!:rattle:

Actualy, that is an extreemly excellent suggestion!!!!!!

almost 30 years in residential ac service and workin on all kinds of p.o.s. units oversized units in computer rooms thats what i did to keep it from freezeing up somebody put 5 tons of a/c in a 15 x15 room with about 10 computers actually i think they were servers and no humans so freezeing was a real prob and yes im sure this is too much info sorry for ramblin...ken