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A/C compressor continues to run, bad pressure switch?

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So last week I decided to tackle the non-functioning A/C system in my '89. It has been converted to R134 per the fittings and official looking sticker on the fan housing. I may have two separate issues.


I used a product from Advanced called Interdynamics EZ Chill R134a with a gauge and hose. It was 18oz. As I began to fill, the compressor kicked on and stayed on, even after I emptied the 18oz. I saw the little bubbles swirling around through the sight glass in the accumulator as well. However, per the cheap gauge, the low pressure line still showed zero pressure (it shows ~30-50psi while filling but drops to zero when I let off the button). I read that the system takes 24oz so I bought another bottle, 12oz this time. I nearly emptied this bottle as well, but the gauge still showed no pressure.


Given no evidence of backpressure, I assumed I must have a leak in the expansion valve or evap and I have refrigerant filling up a body panel somewhere. However, when I reached in to turn the truck back off, I felt icebox cold air blowing in through the vents. So the system works.


1st problem: no pressure reported: Bad gauge?


Second problem: the compressor will not cycle off. I have to pull the plug out of the low pressure switch to cut power to the compressor.


Possible third problem? I'm not sure the pressure switch plug is wired properly. I have a wire running from the stud on the starter relay, straight into the pressure switch plug, then back out straight to the compressor. I thought there might be a clutch relay involved somewhere between the power and the pressure switch.


For now, I'm leaving the A/C disconnected for fear of burning up something, plus it makes the belts squeal like a pig when I start it up with the compressor clutch engaged.


I'm not done poking around but thought I would tap into the CC mothership for any wisdom while I ponder.


If the pressure is indeed too low, it could be causing the compressor to keep spinning.

If I put in too much refrigerant (due to bad gauge) it might also be forcing the compressor to keep spinning.

Maybe the low pressure switch (clutch cycling switch) is stuck open (closed circuit) and needs to be replaced. - Even with the truck off, as soon as I connect the low pressure switch plug to the switch the clutch coil magnetizes, almost like it is stuck open (or electrically closed).


I think my first move might be to try a different gauge, but I would like to get some feedback on the wiring to make sure it is indeed wired properly.



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referring to the cheap gauge: As you are filling the ac (pressing the trigger) the gauge will drop to zero, when you let off it will (or should) show the estimated pressure..


There should be an a/c clutch relay.  


do you know if your MJ was non a/c converted to have a/c?


also click on the link in my signature and poke around my manuals you might find something helpful.




Edit:  Give this a shot


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thanks blue88. gauge shows pressure while filling but drops to zero when I release button. I need to get both low and high read with some actual manifold gauges.


Don't know if A/C was added after the fact.


I read the diagnostic page (thanks) and the only item relevant is that my sight glass has a lot of bubbles, indicating moisture. However, since the outlet air is ice cold, the system is generally working. I just need to figure out how to get the compressor to cycle off. I'm staring at the wiring diagram trying to figure out how the low pressure switch gets power when it decides to come on. it looks like the in-cabin selector switch is supposed to go to the clutch relay to tell it to send power to the low pressure switch which then decides to send power to the compressor if it needs to come on but it looks like I'm shortcutting all of it by just sending power to the switch without going through a relay circuit.

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Quick Update. I took a break from wrestling with my brake upgrade to play with the A/C. I read only Jeepforum that the low pressure cycling switch is attached to the accumulator via a schrader valve and that I should be able to unscrew it and see if the switch is stuck 'closed'. Ignorantly, I unscrewed it. For me, there was no schrader valve. The force of the discharging accumulator blew the switch out of my hand and the howling sound briefly knocked out my hearing. Luckily, I was able to grab the switch and screw it back in against the force of the still discharging accumulator. Does this mean that the schrader valve might be stuck open on the accumulator (I didn't take the opportunity to stick my head in there and look while all the refrigerant was blowing out) or does my '89 system maybe work differently than what they were talking about on JF? If my switch is bad I don't see how I can replace it without completely evacuating my system. I may have already screwed the pooch by letting too much juice out anyway.


My current plan of action is to try and recharge the system and waste another ~$30, then rent some manifold gauges to see what pressures I'm actually seeing. the problem with that plan is that it may tell me that I indeed have a bad low pressure cycling switch and apparently the only way to change it is to discharge the system.


Any thoughts from the CC wizards, apart from 'take it to an A/C repair shop'?

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might as well replace the switch (can't find a price on rock auto) my AC was leaking through the switch it self;

Accumulator ($11-$24) when i replaced my accumulator it did not have that valve, just an input, output, and a port for the switch;

O-Rings ($4-$10) they are cheap, if you have the system depressurized might as well replace them.


have a shop pull a vacuum if you can't, then recharge the system.

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Thanks. Although, it's not 100% depressurized. It was pushing pretty hard against the plug as I screwed it back in. I'm sure the pressure is very low but there's a chance it could have pushed all the free air out that was swirling around and causing the air bubbles in the sight glass. This still doesn't solve my problem if indeed the switch is bad.

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