Well I have to say that fixing my A/C system has been a real journey of frustration which I plan to share when I hopefully get this thing wrapped up.
One of the problems turned out to be a leaking low pressure switch on the back of the A/C compressor. The part number is JLM 1260 but NOBODY has one. Both FCP Euro and Cardit advertise them but they are out of stock. So I purchased a low pressure switch for a Bentley/Rolls on EBay that looks exactly alike and it should be here tomorrow. Fingers crossed it fits.
My compressor is a Sanden 510 and my question is does the low pressure switch act solely as a safety measure shutting the compressor clutch off to save the compressor due to lack of lubrication if the refrigerant pressure drops too low … or … does it also have anything to do with the clutch cycling during normal operation.
I found a post somewhere about the XJ40 that said the cycling is done through the climate control microprocessor based on the evaporator temperature sensor. Which kind of make sense since none of the replacement compressors have a low pressure switch. Anyone have any idea if that’s correct.
Well I just received the low pressure switch for my a/c compressor. It looks perfect, just like the original. Correct, size and threads. So what’s the problem ?
Just out of curiosity I hooked it up to my multimeter to determine what PSI the switch opens at (and therefore shuts off the compressor clutch). By using my air compressor to blow air into it It turns out to be 40 psi. That PSI seems way too high for a “low pressure safety switch”
I previously purchased a universal low pressure switch (unfortunately the threads are slightly different) and it doesn’t shut off the clutch until the pressure drops to 20 PSI which seems much more reasonable.
According to the chart you optimally want to get the low pressure (suction) side of the system down to
around 38 PSI and with the switch shutting off the compressor at 40 PSI it’s never going to get there.
The real BUMMER is that before I butchered my original Low pressure switch that was leaking I never ran this test to see what pressure it was set at.
Try and get another (used) switch, test that then either install it or buy a generic aftermarket one?
I can’t remember if you run R134A or Duracool R12 replacement or whether your compressor is a Sanden, but I think you might look at a similar sedan of the same year and check out rockauto, e.g.
Hi . I had the same low pressure switch problem. It was leaking as a fountain… I have a sanden compressor. As i decided to change to r134 i decided to replace the compressor with a new one. There are new compressors that are drop in replacement. They are made of 2 parts: the compressor itself and the head that changes from different model years xj40. The One that was fitted by my mechanic has no hole for the pressure switch . I asked how It possibile It does not need the switch and i was told It has It inside the compressor body . I see the clutch engages and disengages as It was before and all works as usual. As you can see there Is a cable from the inside…
Groove, I think the switch that you are concerned about is actually a High Side Low Pressure switch. (HSLP) If the overall pressure in the system falls below a safe level, the compressor shuts off.
It has nothing to do with the low side pressure. I think it should work.
My original compressor was a Sanden 510. I replaced it with a new one about seven years ago. I’m still running R-12 in my system, fortunately I have a nice supply of virgin Dupont Freon that I’ve hoarded over the years.
I’ve fixed all the leaks in my system and it’s held a deep vacuum for two days now with no change, so knock on wood it’s ready to be gassed up.
Well that would be nice if it actually is a “system low pressure switch” and not just reading the suction (low pressure) side. I could kick myself for not testing the PSI setting on my old leaking switch before I butchered it up to use just a plug.
I installed the new switch anyway but didn’t connect the two wires that go to it. I then jumped those wire together which tells the microprocessor that the switch is closed. So everything will work normally just no low pressure protection. I can always plug it back in later to see whether or not it will actually affect the cooling because it’s shutting off the clutch early.
Believe me I tried to find a new or used Sanden low pressure switch but good luck on that. I purchased the one I have because of the size (it has to fit into a slight recess) and the thread pattern (and don’t get me started on that)
Hmm, would the compressor on a 1988 model, theoretically at least, have been the same as 1989? A local u-pull-it yard still has a 1988 XJ40. Now whether they would sell just the switch or want to charge for the entire compressor ($49 + 9 core charge) I do not know. And of course how does one know whether it is any good?
Thanks for that great information. I never thought to look at my electrical schematic (not the one in the Haynes manual, it’s so incomplete that it’s almost useless).
Several things were leading me down the rabbit hole on this switch.
Sanden refers to it as a low pressure switch
The “universal” low pressure switch I picked up for a few bucks was set at 20 psi which seemed more reasonable for the low pressure side than 40 psi.
The biggest thing that led me astray was the back of my compressor. My low pressure switch is located by the suction (low pressure) port and I have a high pressure relief valve by the discharge (high pressure) port. Now that also seems reasonable to me
I’m glad now that I went ahead and installed the 40 psi switch anyway. I’ll plug the wires back in and see if makes any difference, which based on your info I’m rather positive it won’t. "HIGH side LOW pressure switch’ … seems a bit of an oxymoron.
Great question. I should have put “location” after the two labels. The arrows point to two blank screw-in plugs.
If you’ve ever tried looking at the back of your a/c compressor you’ll find that it’s totally hidden by a/c pipes from the top and the car’s front end cross beam from below. You have to use an inspection mirror from underneath the car to get any kind of look at it. No way could I get a picture of it. Any work back there has to be done by feel.
As an aside, I once rebuilt my A/C compressor thinking it was making the horrible screech but of course the compressor was fine (DOH!) and the culprit was the delaminated harmonic balancer …but a least I now know how to do it if I ever have to deal with it again …have all the special tools too - oh well just another day in Jagland LOL
Good news… After three days I don’t believe there are any leaks
Bad news … I’m not getting anywhere near the the vent temperatures that I was expecting. The system is struggling and takes about 10 minutes to get down to about 50 F and that’s it. Before this all started 40-42 deg was the norm.
A LOT of time and effort spent for disappointing results.
Oh no, say it ain’t so! So…what has changed from before to after? New evaporator, of course…R12 refrigerant as before…same expansion valve? Condenser? Hoses? System pressures? Hmm…same compressor, but new confusingly-named High Side Low Pressure Switch…is the performance the same with said switch operating and with the switch bypassed?
Well not really disappointed, more like disgusted with the whole thing !!!
The only things I ended up replacing were …
The low pressure switch … which was leaking and I believe in retrospect was the sole cause of all the problems
The receiver/dryer … always needs to be replaced when opening up the system
The expansion valve … I modified the original into a testing attachment so I could pressure test the evaporator
40 oz of R-12 Freon
So all my worry about the evaporator leaking was false. The really good news was I didn’t have to remove it (the bad news was the effort and $ spent on purchasing a replacement that I ended up not needing).
When everything was done I buttoned up the system and it held a deep vacuum for 2 days, I then pressurized the system with dry nitrogen just to be totally sure and again it held that pressure for two more days. So I was absolutely sure I had no leaks.
I was very careful to use the exact amount of Freon when I charged the system but I’ll hook up the gauges today and double check the operating pressures.
Another interesting thing is the sight window on the three receiver/dryer’s I’ve purchased lately. Unlike the old ones I’m used to where the flow of refrigerant (and any bubbles) is clearly visible, the new ones are virtually useless in trying to see the refrigerant flow at all. I suspect whoever you purchase one from today they all come from the same manufacturer in China.
… After three days I don’t believe there are any leaks
… I’m not getting anywhere near the the vent temperatures that I was expecting. The system is struggling and takes about 10 minutes to get down to about 50 F and that’s it. Before this all started 40-42 deg was the norm.