Hi all. The AC on my XJ quit working over a few days about 3 months ago. I took it to a local AC repair shop and was told that it had a bad compressor. They quoted me $2500 for the repair, which I figured was about half what the car was worth. Anyway, I finally got around to installing a rebuilt compressor (not much fun) and replaced the dessicant pack too. Now it won't take a charge, and is behaving exactly like the old compressor. That is, the high and low side pressures are basically the same (around 120 psi). Also of note the radiator cooling fan is not coming on either. I'm thinking I have an electrical problem rather than a pump problem. I'm looking for ideas on how to trouble shoot the AC pressure sensor, and also the locations of the relays for the radiator fan/AC compressor. All the fuses that seemed to be related checked out OK. I love the car, but summertime in Florida with no AC makes my affection wane....

 

Is the clutch engaging and allowing the compressor to turn?

 

Im assuming you did a vac down on the system

How long did you pull a vacum for and how long did you wait while hold the vacuum?
Did you put the charge in via the low port?

The system will not begin to cycle if the charge is low, check the low pressure switch and if needed manually power the compressor

Also let us know if you swapped the receiver/dryer and or the tx valve

Cheers
34by151

 

The compressor and dessicant pack were what I replaced. The system was vacuumed down for 30 minutes, and held the vacuum for 30 minutes. The "clutch" on these compressors doesn't work like a conventional system, ie it is engaged all the time, per the online manuals. Its my understanding the compressor can vary its output by a solenoid valve internally, hence why the electrical connections go into the rear side of the compressor rather than to the front "clutch" area. That makes for a tough diagnosis. So, to answer your question about the clutch engaging, the inner part of the pulley is spinning, but no output from the compressor. Thanks for your reply!

 

Do the pinpoint tests on the LP sensor (its a restive sensor not a switch)
Yes the Compressor is run via a PWM signal from the CCM but that should not matter unless the compressor is bad or you have no signal
Its worth putting a meter (multimeter with PWM or CRO) on the signal and check its ok

It'd normally vac down for a min of 45mins and the same for the leak test.
You did not say if you charged via the low port or pushed liquid via the high

If you pushed liquid there is a good change you did damage to the compressor

Assuming you changed via the low and did not push liquid the first point to check is the LP sensor.
You should have seen the volts rise as you changed the system.
Assuming this to be the case the next thing is to replace the TX valve as it is most likely blocked.

Assuming you replace the TX and you see the volts on the pressure sensor rise during charging. If you still do not have a working system the issue will be either the compressor or the CCM.

You can bypass the CCM by supplying the compressor a 5v PWN signal

My bet is the issue all along was a blocked TX, it remains to be seen if the old/new compressors are also faulty

Cheers
34by151

 

Thanks for the ideas. I probed the pressure switch and had 4 volts coming out the signal line. I then unplugged the connector to the compressor and applied 4 volts to it with the engine running. I could hear a change in the compressor that sounded like it was engaged. However there is still no differential between low side and high side, both reading about 120 PSI (90 degree F ambient) When I reconnected the compressor electrical to the input from the car I could again hear the compressor engage, or make a slight noise like its working. This noise happens whether the AC is turned on or not. I was only able to get about 6 ounces of R134 into the system and it won't take any more. I also added two one ounce PAG oil charges prior to adding any R134 (one for the compressor, one for the dessicant pack.) These both went into the low side port and were pulled in by the vacuum. I suppose there is a voltage range on the PCM signal to the compressor. Any ideas what the top of the range is?

The TX valve looks like a project beyond my capabilities but if it was clogged wouldn't I see a differential pressure at the compressor?

 

You wont get a pressure rise if the TX is fully open all the time.
You really need to change the TX before moving. Its a simple swap but you do need to pull the evaporator to get to it.

A PWM signal is not voltage based although it will vary in volts. PWM is switched from 0 to 5 volts (in this case). So if it 0 all the time the PWM = 0, If it is 5V all the time its 100%. 4 Volts does not mean to 80% though, you need a meter that reads the width of the pulses (not volts) or a CRO
If you want to simulate the signal you need a square wave generator.

You can read the LP signal with a meter however as its a resistive sensor

Look at it this way. The TX controls the flow. If it is closed you develop pressure on the high side and by doing that converts the liquid into gas

The more the TX is open the lower the pressure differences will be. If the TX is jammed closed the pressure changes will be increased.
If the TX is blocked it will have either normal pressure or higher depend on the size of the restriction. The system may in this case look OK but perform poorly due to the low flow

Cheers
34by151

 

The car is 9 years old and has almost 100,000 miles. Think a drier and TX valve must be in order. Plus a vacuum and recharge.
The AC blows cold air. Just takes a few minutes. Plus it is low 90's. I expect to see a couple 100 degree days soon. The performance has been stable, it was not super cold three months ago when I purchased the car.
Looking at Rock Auto I see the drier. Which part is the TX valve?

 

The TX Valve is listed in the JEPC as "Expansion Valve" in the "Air Conditioning and Heater unit" section

Part number C2Z6539

RRP is around 175Usd but check out AC supplies because you should get one for around half that

Btw TX or TX valve is short for Thermal Expansion Valve

As I indicated before the Receiver/Dryer and TX should be replaced every time you charge the AC.
The TX is often not done because of the time and effort removing the heater assembly and mostly you get away with this as the system performance will just go down a bit. Also from a service agents point of view you just get to do it latter for a much bigger price!!
The Receiver/dryer is a cheep part and is the filter for the system. Not changing it given its cheep and quick to do is the same as changing the engine oil and not replacing the oil filter. You might get away with it once or even twice but sooner or later it will bite you.

I have no issue at all on a 40+ DegC (104F) day running the AC at 19-20. Even If the car is in the sun (no window tint) it would be under 20 very quickly < a minute
Id have to time it next summer but my guess is the fan on auto blows full for about 20 seconds. Then would back off to silent in another 20 seconds

Cheers
34by151

 

Thanks for all the ideas. As I was disconnecting my gauges from the car something didnt seem right. Depressing the low side schrader valve briefly the system had very little pressure. I tried charging directly from one of those short hose with a gauge setups and it immediately started working. Now everything seems to be nominal

 

On the face of it, Sounds like you have a gauge issue.
Now you have a working system, It is worth reconnecting the gauges to check they are ok

A few things come to mind other than faulty gauges
1. Gauge manifold ports set incorrectly
2. Quick connects for AC lines had the valves closed

Cheers
34by151