AirCon Problems L320 4.4 petrol 2005

Even though I have had the aircon compressor replaced twice, as we assumed that the first one must have been faulty.......the system can be charged and successfully holds a vacuum, when the aircon is run it starts to pressurize and then cuts off. The garage that fitted and charged the system were beaten as to where to go next.
I have recently discovered that this has aircon control module, switching the pump on and off as there is no clutch on these, but cannot find out where it is located. Also I assume there is some kind of pressure switch or sensor which I also cannot find any mention of.
Help with the location of the module and pressure switch/sensor would be appreciated along with any info regarding what else could be causing the fault.
The only code showing on the aircon system is for low pressure but the gas has been remove and it is just in a vacuum state at the moment to be recharged once the problem is solved.

No clutch, you sure about that? No clutch would mean the compressor runs all the time unless it's electric or a bypass system is used. There has to be a pressure switch or maybe more than one.

The climate control module is in the cabin, the one with the climate control settings on the front. The refrigerant pressure sensor is next to the battery box with a black three wire plug in it (also adjacent to the refrigerant filling ports).
What you have been told is sort-of correct, there isn't a conventional 'all or nothing' clutch on the compressor, it's a PWM controlled variable displacement compressor.
Rather than faffing around and guessing (or paying a garage to guess on your behalf) you need to read the fault codes in the ATCM and check actual pressure value. The system could be overcharged, causing it to shut down due to overpressure, or the pressure sensor could be defective and faulting once pressure starts to build - or something else completely. You really need to read the DTC's as a starting point. Also check the ECM for engine cooling fan errors and check the live values of the fan to make sure the indicated rpm & lockup % (PWM signal) is plausible.

ETA:
Refrigerant pressure sensor (note sensor, not switch) between the refrigerant ports, arrowed.

Anyone know If Sayers M R Auto Repair (London road, Feltham) Is still in business? I rang both numbers and they are both dead

Many thanks for the knowledgeable reply.
I currently have the two fault codes below.

Body Control
B1C57-14 Passive Start Ignition Relay Coil Circuit General Electrical Failure Short to ground or open (2B)

Ventilation
System Pressure Bus Signal/Message Failure Signal is Blow Allowable Range (2F)

The body code fault it seems is apparently common on RRs and nobody seems to know quite what is signifies from what I have read.

Dopey said:

Anyone know If Sayers M R Auto Repair (London road, Feltham) Is still in business? I rang both numbers and they are both dead

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I believe they shut down about five years ago could be more.

I'm a bit confused reading this as the AC won't kick in without having the correct pressure so you can't really test it with a negative pressure or vacuum. It might be that you're saying the only code you have is for low pressure and that's because it has no gas currently?

Not wanting to labour the above point, I would suggest the the compressor does have a clutch as that is how it's engaged and disengaged, I'm no expert but I've never seen a car with anything else and not at this age.

All that aside, from my (limited) experience with AC I can offer this anecdote from an issue I had with a Granada Scorpio some years back, my issue was the cooling fan, I bought the car with non working AC and found that the condenser had a hole in it, then once I'd replaced that and the drier and had it gassed, it would not come on at all. If I jumped the relay it would work so I knew it was electrical. The issue turned out to be the secondary electric fan (on the front of the condenser on said "Scorchio"; it was totally seized and so the AC would come on, and with the fan inoperable the pressure would increase way too quickly and it would shut down. Once I got the fan working everything else was fine.

Not sure if you can get a pressure reading when your AC is running to see if it is going too high and that might give you a clue. Of course, we'll need someone that knows far more than I to be able to give the specs of what it should be before it kicks the compressor out again. Is your electric fan working?

Not familiar with the petrol v8, my diesel has an electric viscous fan so it runs with the engine but when it needs a bit more it's driven electrically, perhaps yours is the same or it may have a secondary fan? If it holds pressure you may as well have it gassed so that you are in a position to test it properly.

Can you jump the clutch on the compressor (aware of the notes in your original post here) to see if you can make it work without the control unit.

Finally, did you replace the pulley when the compressor was replaced or are you reusing the old one? The issue could be there...

Blob said:

I'm a bit confused reading this as the AC won't kick in without having the correct pressure so you can't really test it with a negative pressure or vacuum. It might be that you're saying the only code you have is for low pressure and that's because it has no gas currently?

Not wanting to labour the above point, I would suggest the the compressor does have a clutch as that is how it's engaged and disengaged, I'm no expert but I've never seen a car with anything else and not at this age.

All that aside, from my (limited) experience with AC I can offer this anecdote from an issue I had with a Granada Scorpio some years back, my issue was the cooling fan, I bought the car with non working AC and found that the condenser had a hole in it, then once I'd replaced that and the drier and had it gassed, it would not come on at all. If I jumped the relay it would work so I knew it was electrical. The issue turned out to be the secondary electric fan (on the front of the condenser on said "Scorchio"; it was totally seized and so the AC would come on, and with the fan inoperable the pressure would increase way too quickly and it would shut down. Once I got the fan working everything else was fine.

Not sure if you can get a pressure reading when your AC is running to see if it is going too high and that might give you a clue. Of course, we'll need someone that knows far more than I to be able to give the specs of what it should be before it kicks the compressor out again. Is your electric fan working?

Not familiar with the petrol v8, my diesel has an electric viscous fan so it runs with the engine but when it needs a bit more it's driven electrically, perhaps yours is the same or it may have a secondary fan? If it holds pressure you may as well have it gassed so that you are in a position to test it properly.

Can you jump the clutch on the compressor (aware of the notes in your original post here) to see if you can make it work without the control unit.

Finally, did you replace the pulley when the compressor was replaced or are you reusing the old one? The issue could be there...

Click to expand...

Read Graculus's reply above:- there isn't a conventional 'all or nothing' clutch on the compressor, it's a PWM controlled variable displacement compressor.
A clutch is far too simple and reliable for a modern car.

Datatek said:

...
A clutch is far too simple and reliable for a modern car.

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My L405 (and most other modern cars) have both a electric clutch and variable displacement... The valves are well known for sticking, never known it on the L319/L320 though.

In the OP's case, the AC system needs the correct weight of refrigerant in it before any diagnosis can be meaningful.

brianp38dse said:

I believe they shut down about five years ago could be more.

Click to expand...

ok thanks m8

Datatek said:

...
A clutch is far too simple and reliable for a modern car.

Click to expand...

Like this one (of many)?


Up until recently I worked for a M-B dealer, we were getting the same defect (intermediate bearing failure) on the Renault engined A176 & A177's on a monthly, sometimes weekly frequency. We tried to keep a couple in stock but they were seemingly on perpetual backorder

Graculus said:

Like this one (of many)?
View attachment 321294

Up until recently I worked for a M-B dealer, we were getting the same defect (intermediate bearing failure) on the Renault engined A176 & A177's on a monthly, sometimes weekly frequency. We tried to keep a couple in stock but they were seemingly on perpetual backorder

Click to expand...

Poor design and quality affects many marques, the aircon clutches on my P38's have done over 140K miles on one and 270K kilometres on the other. I doubt the electronics or the variable vane compressors in modern cars will do any better. Renault and Mercedes, what a combination

My old L319 is approaching 180k still on the original AC compressor, the variable displacement compressor valve is just a simple solenoid working against spring pressure, they rarely fail electrically (neither does the ATCM that sends the PWM signal), it's usually the brass variable venturi that sticks due to wear - probably caused by the CNC tools or head registration not being replaced & reset often enough.
But yes, M-B & Renault are a combination that will keep vehicle tech's busy for years to come...