Thread: D4 A/C Compressor issue

Originally Posted by imaz

First two were free as no regas was not done.

That's good it didn't cost you a cent then mate, great result

Originally Posted by DiscoJeffster

As for the fan you’re saying to check actually rpm versus commanded rpm presumably?

Yep. The DTC was a bit of a giveaway though.

Originally Posted by DiscoJeffster

How does one distinguish between the pressure limit being met and the compressor throttling back versus normal operation? What does normal operation look like? I’m interested to take a look at mine.

IMG_1856.jpg

The 3 bits of information you want are :
"Air conditioning system pressure" - The high side pressure measured in the liquid line between the condenser and evap.
"Evaporator sensor" - Temperature sensor on/in/by the evaporator.
"Compressor/motor current" - Not current at all, but some form of measure which relates to the RCV position. I see 0 when it's switched off, ~240 when it's flat out and it idles somewhere around 132.

If the evap temperature is above > 5.5 degrees, the ECU will generally open up the RCV to increase cooling. The evap temp seems to sit between 4.5-5.5 degrees. That can vary as the unit manages the cabin humidity also, but that's "generally normal" for my D3.

The system pressure directly correlates to the condensing temperature using the r134a PT curve. 7.7 bar is roughly 30C.
The manual says ~33Bar is the safety limit which is ~90C at which point it will try and set the compressor to minimum displacement.

So from a diagnostic point of view, you want to see the evap temp low and steady and the compressor not commanded flat out.

Cheers Brad.

So I went for a pootle around the streets and generally saw 1 deg on the front condenser. Got the rear down once I engaged the rear ac (obviously), and saw max 200psi pressure with around 170psi being fairly average.

Outside temp a mild 23 so it wasn’t having to do anything today.

Mine seems to show a very different view of “motor current” with it never going above 0.6 whatever it is.

It was cold in here. Looking forward to trying when it’s playing up. Sometimes I find I have to turn off the AC and back on again for it to start cooling again. Ot would be interesting to see the stats when it’s doing that.

Those head pressures at 23 degrees ambient seem a tad high for 134a,but maybe OK.
They depend on quite a few variables.
That’s liquid line pressure,so the actual head pressure will be slightly higher,depending on condenser pressure drop.

It would be good to revisit the pressures at ambients of 30 to 35 degrees,and see what they are,while driving.At idle they are often higher due to no ram air,although with an electric override on the viscous radiator fan,it may cut in and reduce the HP.

Example of it just not working. Ambient coming through the vents. Was about 5-10 minutes after starting after being parked in the cool underground.

Effectively my interpretation is the compressor was commanded to max output (0.75 is the highest compressor value I’ve seen) but the pressure was the lowest indicating the compressor wasn’t actually able to action what was being requested.

No cooling occurring as seen by the evap temp.

I had the compressor valve changed (well I provided it and asked for it to me changed) two years ago, but effectively this random not enabling issue persists.

I should add “commanded 20deg C” on the HVAC

[

Yeah, condensing temperature of ~27C. The compressor is at minimum displacement. If it's not the valve, then it *has* to be the compressor. Even if a TXV was jammed open I'd expect to see more head pressure than that.

If it was electrical I *assume* (dangerous, I know) the HVAC ECU would be bitching about the valve circuit being open, to ground or high.

Originally Posted by josh.huber

That's good it didn't cost you a cent then mate, great result

Just ordered a new viscous fan from UK. Should be here in a week. $500 landed and taxes. Another disco fella purchased one last week from the same online seller and we both inspected it, looks to be identically genuine with the serial scrubbed out? Anyway, should be good.

My Aircon is still cold, down to 8.2c at the vent from what I could measure with the test viscous fan I have.

Originally Posted by scarry

Those head pressures at 23 degrees ambient seem a tad high for 134a,but maybe OK.
They depend on quite a few variables.
That’s liquid line pressure,so the actual head pressure will be slightly higher,depending on condenser pressure drop.

It would be good to revisit the pressures at ambients of 30 to 35 degrees,and see what they are,while driving.At idle they are often higher due to no ram air,although with an electric override on the viscous radiator fan,it may cut in and reduce the HP.

Where is the resisters on the viscous fan circuit? These can fail if the disco is reliant on one.