A/C pressure readings?

[Ramp Rat]

What should be the actual (meaning using old school manifold pressure testing) readings be for a 2006 Jetta TDI BRM? When testing with the VCDS it is showing about 186 psi (translated of course). But, I’m old school and like to see the actual readings using a physical testing with a manifold gauge.

The temperature discharge readings at the dash ducts read about right but, I’d like to know what the low and high pressures are supposed to be.

Thanks for any input.

Steve.

 

[Uwe]

Aren't A/C system pressures highly dependent on ambient temperatures?

-Uwe-

 

[Ramp Rat]

Aren't A/C system pressures highly dependent on ambient temperatures?

-Uwe-

Yes, to a point. It’s the high and low system readings that mean something. In a system that is close to the correct amount of refrigerant. The low pressures may drop to 30psi and the high side could be as much as 230 psi or even more. But, a system that is low on refrigerant the low side will not get low enough and the high side will show a low pressure.

Most vehicles have a chart showing the ambient air temperature pressure values for that temperature. Knowing these values can give you a clue as to if and when the system might need a little refrigerant added.

I’ve looked in the manuals that I have and they only state the amount of refrigerant needed at first service. Nothing is stated about what are acceptable pressures.

Thanks for any help.

Steve.

 

[dieseldub]

That's where this gets tricky too is these are a variable displacement compressor, and the part that likes to fail on them is the component that controls the variable displacement.

That being said, with these variable displacement compressors, so long as the charge level is correct, the low side is generally pretty consistent right at 25-35 PSI when the compressor is on, whereas the high side can vary much more wildly depending on ambient temperature, air flow over the condenser and charge level (anything from 140-250 PSI).

The pressure sensor the HVAC module is looking at is high pressure side only.

What is common with these is as the refrigerant control valve stops working correctly, you get either minimal pressure differential created when the valve is commanded to make displacement or none at all (high and low sides basically are equal, as it would be when the compressor is still off).

With a properly charged an efficiently working system, the high side reading in VCDS is typically anywhere from 13-15 BAR when switched on. 6-7 BAR might be the reading when it's switched off and given enough time for both sides to equalize.

 

[Trade Member]

Can you not just compare your temperature and pressures via a chart?

r134a refrigerant pressure temperature chart - Google Search

 

[Ramp Rat]

Interesting!

Let me see if I have this right? I understand that while the engine is running the A/C compressor is also running all the time. Because these compressors don’t have a compressor clutch that cycles on and off as the low pressure side drops there is a control valve that controls the refrigerant flow?

If I put an old world A/C refrigerant manifold gauge set on the system I should see what I would consider normal low pressure side readings. (25-35 psi) taking into account the outside air temp. The high side will very but that’s not unusual so long it’s within expected range.

If I see the low side not dropping to a pressure expected and/or the high pressure side also being low than, I either have low refrigerant level or a control valve starting to fail? A simple check would be to add a small amount of refrigerant and see what happens.

Does this make sense?

Thanks for the information, Steve

 

[dieseldub]

Interesting!

Let me see if I have this right? I understand that while the engine is running the A/C compressor is also running all the time. Because these compressors don’t have a compressor clutch that cycles on and off as the low pressure side drops there is a control valve that controls the refrigerant flow?

If I put an old world A/C refrigerant manifold gauge set on the system I should see what I would consider normal low pressure side readings. (25-35 psi) taking into account the outside air temp. The high side will very but that’s not unusual so long it’s within expected range.

If I see the low side not dropping to a pressure expected and/or the high pressure side also being low than, I either have low refrigerant level or a control valve starting to fail? A simple check would be to add a small amount of refrigerant and see what happens.

Does this make sense?

Thanks for the information, Steve

The compressor internals are spinning full time, yes, but it's not "on" all the time. The pressures will still act a lot like a conventional compressor when switched off. The swash plate moves to a position where the displacement is effectively nothing and therefore refrigerant is not being pumped and a pressure differential not being created.

When switched on, the refrigerant control valve moves some refrigerant pressure around enough to effect an angle change of the swash plate to greatly increase the displacement of the pump, refrigerant starts to be moved and a pressure differential is created.

If, when you switch the compressor on the pressures stay where they are or the low side just doesn't get as low as it should and the high side doesn't get as high, it's almost certainly a bad refrigerant control valve.

 

[Ramp Rat]

Thanks dieseldub, that explanation makes everything fall into place. I seem to remember when reading thru my service manual that some of the A/C components can be replaced without draining the system. Is this true with the control valve? I do have a nice RobinAir vacuum pump just in case.
Again, thank you very much.
Steve.

 

[Ramp Rat]

OK, got a couple more dumb questions.

After doing several test runs and checking actual pressures and comparing them to what the VCDS reads, it looks like the Refrigerant Control Valve is working when it wants to.

I’m guessing that the Refrigerant Control Valve is the same as what the Bentley Service Manual calls “Compressor Regulator Valve”? Seems to be an integral part located on the lower rear of the compressor.

Question 1:
I’m assuming that this valve cannot be replaced but is part of the compressor requiring a complete compressor replacement?

Question 2:
Having some experience with US A/C systems, it looks like the system will need to be evacuated, vacuumed down and recharged. Is this correct?

Reason I’m asking is because the Bentley manual says that the compressor can be removed without discharging the refrigerant. ?????
Who’s right? My gut feeling or Bentley?

Thanks, Steve.