A/C System Troubleshooting - Blows Cold then Hot. Pressure Sensors, Comp Clutch etc

Hey Guys,

I'm troubleshooting my A/C and have read quite a few threads on it and I'm just looking for clarification.

I've got a 2000 XJ Sport with a 4.0.

Anyway, My symptoms are quite similar to some other threads... When I turn the car on, the AC blows cold. After a little while, say 10-15 minutes and the car the warmed up, the clutch disengages and it blows warm air. It seems to stay that way until I shut the car and come back once the engine has cooled down a bit.

So, I hooked up some gauges and when the clutch is running when I first start the engine I get: (Note: I jumpered the "Low pressure clutch cycling switch" or low pressure sensor that attached to to the Accumulator )

Ambient temp appx 70-75deg F 88% Humidity.

Case 1: Start up/Warming up before clutch disengages. No Jumper // Electric FAN OFF
LP: 30 psi
HP: 260 psi
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Case 2: Clutch Disengages and I pull the Low Pressure Sensor and Jump the Connector - Clutch Engages(Yay!) // Electric FAN ON
LP: 22 psi
HP: 190psi
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Case 3: Jumper Wire Still Implemented and Clutch Engaged: Electric FAN OFF
LP: 30 psi
HP: 280 psi


I would think this eliminates the "compressor shim fix" as my clutch still engages. I just want some community feedback on the pressures I'm measuring and wanted to to know what you all think. According to the FSM, the HP is a little on the high side ish, but maybe not...it's pretty humid here.

Is the sensor bad? Is there a part number/ Best place to buy it from? Accumulator bad Any other things to look at?? Overcharged System? Anything else to try before evacuating and recharging?

Thanks in advance!

It sounds like what you're describing is Evaporator Freeze Up. Ice forms on the evaporator which prevents air flow, shutting down the vehicle allows the ice to melt and the system works again until it freezes again.

My '97 FSM states that the compressor should engage/disengage no more than ten times per minute (every six seconds). If the compressor cycles more than that it's usually because of low pressure. The compressor cycling helps to prevent the freeze up.

If the compressor fails to engage, it could be either the low pressure is too low or the high pressure is too high (among other possibilities). Your numbers look OK, except for Case #2.

When I test my AC I always have a large fan blowing air into the grill to simulate driving. Maybe you should do this and report back with the results without the low pressure switch jumped. If you end up doing the evac/recharge you should probably replace the accumulator and don't forget the oil.

It must be nice to AC problems. I've got fourteen inches of snow on my driveway with 11 degrees temperature.

Here's some data that may help.

2000/2001 AC switch functions:



Schematic shows switch positions under various discharge and suction pressures. The black arrows within the switches are from the FSM diagram, the red arrows show the alternate positions based on pressures.



AC Temperature/Pressure chart:

Thanks!
And thanks for posting that info...I was reading those diagrams from the manuals as well, but great to have here for discussion.

I ran a test today..It's a little warmer..around 80F. Same thing, AC blows cold on warm up. After the engine warms up, compressor kicks off. This time, I tried to jump the low pressure sensor and the compressor clutch didn't engage.. I can't get any readings with the compressor not on. When the clutch is disengages the LP jumps to 70psi (Fluid Hammer I think). Now I'm a little nervous about the clutch. I'm leaning towards evacuating the whole system and adding it back in by correct weight. (FSM: 1.25lbs)

What would necessitate changing the accumulator? I found one at a local parts store for $29.00. Seems cheap enough, but just wondering. FMS says 3fl oz to add if I replace the accumulator.

If you want to find out what the system is really capable of, do this:

Connect a known good gauge set to the high and low sides.
Disconnect the positive lead at the compressor clutch.
Connect a jumper lead to the clutch wire.
Position a digital thermometer in the center vent duct with the display viewable through the windshield.
Place a large fan blowing on the condenser / radiator.
Start the heep, put the a/c selector to the first position (recirculate), fan on high, now engage the clutch (connect the jumper to the battery) observe the pressure readings and vent outlet temp at 1500 to 2000 rpm.

Also, an easy mod to make the electric fan run when the clutch in engaged (like it should be) is to jumper the extra set of wires at the high pressure switch connector.

My 98 doesn't have that "feature", my son in laws 2000 does, I jumpered his and it did help at lot with the pressures.

Remember too that more is not better when it comes to refrigerant.
http://www.parker.com/literature/Refrigerating%20Specialties%20Division/Ref_Temp_Chart.pdf
If you run the low side pressure slightly less than recommended, it will actually cool better.

The ONLY way to get it 100% correct is to weigh in the correct amount, anything else is just guessing and tweaking.

Momentarily jumper the high pressure switch and see if it turns on. You may simply have a faulty high pressure switch.

Watch your pressure when you do this.

Also when the ac stops working, you should check to see if the clutch still functions. When a clutch goes bad, it can do this. Check to see if you're getting power to the clutch when the ac quits.

Just a little update... Before digging deeper into the AC system and recharging etc..I thought I'd give the clutch-shim-fix a shot. I pulled the clutch, removed the one and only shim in there (maybe someone already was in the before?) and it did the trick! AC blows colder than before and seems to function properly. The clutch seems to cycle appropriately and before it was not really right.

Anyway, I fab'd up a little clutch holder so I could loosen the nut:


I wonder how long it will last? I'll keep everyone in the loop.