Intermittent AC - Pressure Switch?

[dizzysn]

First things first - I searched, just didn't see anyone with my exact symptoms and wanted to verify before buying a switch.

Car AC has been working perfect for my entire ownership of the car. However out of nowhere, today, the AC just didn't work. Blowing lukewarm air. Hooked a pressure gauge up to the low side pressure with the car running and the AC turned on, and it read over 100PSI. AC off read the same thing. I didn't not hear the compressor kicking on.

After doing what I needed to do, I got back in the car and turned the AC on for shits and giggles, and it worked perfectly. Blowing freezing cold air out just as it should. Got to my next stop, got out, did my shopping, got back in, warm air again.

There are no check engine lights, but I don't have VCDS (stupid I know, I'll be getting it soon) to check if there are any soft codes.

This sound like the pressure switch to anyone else?

[Robstar]

I had those symptoms on my A4 a few years ago, I used VCDS to try and see if I could get the switch to turn on and off. Ended up blowing the switch clear off the condenser lol.

A new switch fixed the issue.

[dizzysn]

Ended up blowing the switch clear off the condenser lol.

Damn! Did VCDS show any codes you for? I'm going to try to get it scanned tomorrow.

[Robstar]

It was back in 2014 before i had my S4, and when I was new to VCDS, I didn't do a scan prior to me messing around with turning switches on and off manually via the software. No OBD2 codes were present, but I didn't do a system scan for soft codes. IIRC i was messing with the compressor valve or someting along those lines. I remember turning something on in the software, cold air started to blast, then a loud POP and a PSSSSSSST sound (lol I was an idiot for messing with this but it was kinda funny)

Im pretty sure my b8 A4 has very similar AC system to my 2015 S4, and I did experience the identical symptoms as you describe. I hope its at least some insight for you.

[dizzysn]

Ok so just to bump this. New pressure sensor was installed and worked perfectly for exactly one week to the day, and then AC died again this afternoon while it was 100 degrees with no cloud cover. Once again the compressor is not kicking on, and I'm seeing 140PSI in the low pressure side (it's not this high when the AC is working). AC was completely ice cold this morning, and then it just didn't work this afternoon. Pulled these codes:

DTC 00457 - Vehicle Electrical System Control Module – J519- Please check DTC memory

and

DTC 00256 A/C Pressure/temperature Sensor –G395- Incorrect Signal

There *IS* a TSB about this, but they include two scenarios, both with another DTC code that wasn't pulling up.

DTC Scenario 1
DTC 00457 - Vehicle Electrical System Control Module – J519- Please check DTC memory
DTC 00256 - A/C Pressure/temperature Sensor –G395- Incorrect Signal
-AND- 09 - Vehicle Electrical System Control Module – J519: DTC 00256 - A/C Pressure/temperature Sensor –G395- No signal/communication


DTC Scenario 2
DTC 00457 - Vehicle Electrical System Control Module – J519- Please check DTC memory
DTC 00256 - A/C Pressure/temperature Sensor –G395- Incorrect Signal
-AND- 09 - Vehicle Electrical System Control Module – J519:
-AND-DTC 01592 - Air Quality Sensor –G238- No signal/communication DTC 00256 - A/C Pressure/temperature Sensor –G395- No signal/communication

My car isn't pulling the other codes though so I'm a bit lost at the moment.

[Smac770]

Low side and high side should be similar if the circuit has been sitting overnight. This pressure varies with ambient temp. With engine running and compressor not running (AC off) minimum 4 bar at 68°F, 5 bar at 77°F, 6 bar at 86°F, 7 bar at 95°F. Turning on the compressor should establish low side at 1.0-1.2 bar and high side at some value approaching 14 bar when N280 activation current is at it's turn on max of .65 A.

But your DTCs seem to point to a comms issue with the G395. So I presume you cannot read a refrigerant pressure value in group/block 001 of the J255 (climate control, address 08)? If there's no measurement from the G395, the J255 is likely to not attempt enabling the A/C when you turn it on. You'll know it's not trying to invoke the compressor if you do not see the N280 current go from 0 A (off) to near .65 A (turn on peak). The N280 backs off the current as less compression is needed.

The G395 is on a LIN bus. You might have a wiring corrosion issue, that is what the TSB is about. But if there's a failure of the LIN bus, more than just the G395 is impacted typically. The G238 air quality sensor and J530 HomeLink transceiver and E592 driver's seat memory buttons are all on that same LIN bus. There's no assurance that the wiring failure is in a section common to the G395 and G238 though. It might be in the G395 final leg.

You may also need to check the 12v and ground at the G395 plug. G395 is supplied by the fuse 12 in the first column (black) of the fuse panel C (driver's end of dash).

[dizzysn]

Low side and high side should be similar if the circuit has been sitting overnight. This pressure varies with ambient temp. With engine running and compressor not running (AC off) minimum 4 bar at 68°F, 5 bar at 77°F, 6 bar at 86°F, 7 bar at 95°F. Turning on the compressor should establish low side at 1.0-1.2 bar and high side at some value approaching 14 bar when N280 activation current is at it's turn on max of .65 A.

But your DTCs seem to point to a comms issue with the G395. So I presume you cannot read a refrigerant pressure value in group/block 001 of the J255 (climate control, address 08)? If there's no measurement from the G395, the J255 is likely to not attempt enabling the A/C when you turn it on. You'll know it's not trying to invoke the compressor if you do not see the N280 current go from 0 A (off) to near .65 A (turn on peak). The N280 backs off the current as less compression is needed.

The G395 is on a LIN bus. You might have a wiring corrosion issue, that is what the TSB is about. But if there's a failure of the LIN bus, more than just the G395 is impacted typically. The G238 air quality sensor and J530 HomeLink transceiver and E592 driver's seat memory buttons are all on that same LIN bus. There's no assurance that the wiring failure is in a section common to the G395 and G238 though. It might be in the G395 final leg.

You may also need to check the 12v and ground at the G395 plug. G395 is supplied by the fuse 12 in the first column (black) of the fuse panel C (driver's end of dash).

Really appreciate all that info.

I gave up and brought it to a shop I trust. They did a continuity test on the wire from the module connector to the pressure switch and it failed, so there’s a definitely a broken cable.