Bad AC pressure switch?

[mtroxel]

I know, I hate it too. Guy gets an idiot light or a code and the first thing he wants to blame is the sensor. But here's my story.

Had the front of the car apart. Sprayed wather through the condenser to clean it out. Started the motor with the plug disconnected from the condenser.

1. Made an immediate "implausible signal" - no surprise. Cleared that.
2. Next day I get a G395 Upper Limit Exceeded - Intermittent 29.3 bar
3. Later that day I get a G395 Lower Limit Exceeded 0.0 bar
4. I've had a couple other lower limit exceeded codes, and of course the AC goes out when I get these.
5. Often, if I turn off the car and restart, the AC runs fine.
6. In fact when it does run, I see evaporator output temps at 2° - 3° C. So I find it hard to believe I am suddenly low on R134a.

Would anything else make the pressure be this erratic? Or did I just bollix up that switch when I had things apart?

[bluecan]

Could be the sensor lin wire? If the lin wire insulation is breaking and touch the chassis from time to time, it could result that. That happened to mine. Measure the resistance of lin against chassis when ac not run.


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[mtroxel]

If the lin wire insulation is breaking and touch the chassis from time to time, it could result that. That happened to mine. Measure the resistance of lin against chassis when ac not run.

Good advice. I assume the lin is the signal wire? I'm reading the TSB on this.

http://10104.x.rootbsd.net/~jyoung/vwvortex/AC_TSB.pdf

Here's what I do know about the plug to the pressure switch:

1. I get continuity to ground on the ground wire.
2. With the car running, I get 13+ volts on the Terminal 75 power pin.
3. I get 9+ volts on the signal wire (lin?)
4. I get all of those readings, no matter how much I wiggle the harness.

I now have no AC no matter what I do. When I was intermittent, some of my G395 errors were too much pressure. I got one that said pressure was low at 0.0 bar. Since 0.0 bar is a complete vacuum, I have to believe the sensor was just wacky for a day or two, and now its completely dead. Hope the dealer has one to pick up tomorrow. Its warm/humid this week.

[ahu]

Hello,
I have problems with AC on my a4 b8 2009. It just wont work. I have refilled gas but it still doesent blow cold air. I tried to check it with vcds cable and got this (i would appreciate if anyone can help me to get it working again)

Address 08: Auto HVAC (J255) Labels:. 8Tx-820-043-3Z.clb
Part No SW: 8T1 820 043 AB HW: 8T1 820 043 AB
Component: KLIMA 3 ZONEN H06 0150
Revision: C0000000 Serial number: 01211120080275
Coding: 64000000
Shop #: WSC 06335 000 00000
VCID: 71E9ADAEA848E9CCE7-8024

1 Fault Found:
00003 - Control Module
014 - Defective
Freeze Frame:
Fault Status: 01101110
Fault Priority: 2
Fault Frequency: 1
Reset counter: 111
Mileage: 152206 km
Time Indication: 0
Date: 2018.08.06
Time: 10:46:30

Freeze Frame:
Temperature: 36.0°C
Voltage: 12.10 V

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[bluecan]

Checking the lin wire voltage does not give any helpful information. Instead. Tsb asks to check the integrity of the lin wire. I think you should check continuity and resistance to chassis. It could be a broken/cut wire that does not give signal to the control module. If you got a new sensor, no brained to give that a try first.


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[ahu]

Now someting else is showing on vcds.. is this helpful?:

03023 - Running-in Requirement
005 - No or Incorrect Basic Setting / Adaptation
Freeze Frame:
Fault Status: 01100101
Fault Priority: 3
Fault Frequency: 1
Reset counter: 113
Mileage: 152212 km
Time Indication: 0

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[mtroxel]

Well, mine was the G395 sensor. It makes sense when you get an over-pressure error of 29.3 bar, and later in the day a "lower limit exceeded" of 0.0 absolute bar. Your car is never going to produce an absolute vacuum in that system. So for guys searching on this, I found this discussion at Ross-Tech helpful:

https://forums.ross-tech.com/showthr...he-HVAC-Season

Blucan, I don't know what I'm doing. I do know that signal wire is a wave signal. But I assumed a 9.X volt reading was coming from the module under the dash so that at least meant the harness was good for that pin? Pin 1 is the ground and I had good continuity to any ground from there, and pin 3 gave me battery voltage with the motor running.



After I swapped for the new G395 (updated part is 4H0 959 126 B), my VCDS showed about 12.9 bar instead of the "ERROR" reading I'd gotten the last 2 days. But the AC button still wouldn't turn on, and I had ahu's error, 03023 Running-in Requirement 005 - No or Incorrect Basic Setting / Adaptation. The good news is that I had a pressure reading.

I went into 08-Auto/HVAC>Basic Settings>Compressor Run-In/Cooling check. Then click ON/Run. Heard the compressor spin right up and the radiator fans spooled up...cold air from the dash. Took a minute of that before it said "Performed" and the HVAC shut off on the dash. Turned it back on at the dashboard and have had cold air now.

Just took a look, I'm back to an evaporator output of 2° - 3° C when the car is cool and the blower isn’t running too fast.

[jfo]

Great! I hate AC problems...too many possibilities and not always easy to diagnose.

[NYDCB]

Been a hot summer for me too! When I got it serviced it got worse. Now the AC light won't stay on. C'est la vie!

[mtroxel]

Great! I hate AC problems...too many possibilities and not always easy to diagnose.

Yea a new pressure switch was an educated hail Mary. If that hadn't done the trick, I was going to call the dealer. These modern systems have several more variables than my dad's 66 Mercury's AC system.

[ahu]

...
I went into 08-Auto/HVAC>Basic Settings>Compressor Run-In/Cooling check. Then click ON/Run. Heard the compressor spin right up and the radiator fans spooled up...cold air from the dash. Took a minute of that before it said "Performed" and the HVAC shut off on the dash. Turned it back on at the dashboard and have had cold air now...

I have done everything like you did, but in my car cold air comes only when Compressor Run-In/Cooling check is running. when ''performed'' it stops blowing cold air. What can i do?


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[mtroxel]

I don't know ahu. After I ran the Compressor Run-in/Cooling check, it turned off the AC at the end. All I had to do was push the AC button and I was good. Do you have codes left in 08-Auto/HVAC? My drama also left codes in 09-Central Electric but I had already cleared those.

The good news is, your compressor works and you have R134a. So its down to sensors and controls. If yours won't run, there have to be some codes in one of those two modules.

[ahu]

I just want to say that my AC now works normal. I just run basic settings again, and now everything is fine. Thanks

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[mtroxel]

OK, this is how ditzy I am. Last weekend I removed the bumper to fix something, and thought I'd be smart and spray water through the condensor to clean out all the cottonwood seeds. Unplugged the G395 and sprayed, put it back together: no AC. VCDS says G395 failed. I searched.....and found my own damn thread from almost exactly a year ago.

So after a quick trip to AutoZone, I've got AC again. So I cut the old one to see what's going on. As you can see, a drop of water (that's all it takes) got in there and screwed up the contacts. If you flip the black part around, there is even a hole there at the bottom of the receptacle. Flip the PC board over, and you can see a processor on there. Might have fried that too.

When it's plugged in, the plug completely covers the receptacle. So if you spray out your condensor, put a baggy over that G395 sensor.

[CONative]

Resurrecting an older thread...

After reading this and a few other threads, I’m planning to replace my G395 sensor as I have not had A/C since last fall. My A4 was in a minor wreck and shortly after getting it back I lost A/C. Don’t think it’s anything the shop did, just bad timing. I checked my pressure as I had a left over can of Freon from recharging my S4, all good. I reset my fault codes and the compressor does kick on, but barely any cold air.

My question though is - after resetting the fault codes and getting the A/C button to work, I get a hissing sound from the dash. Anyone else experience this when their G395 went bad? Only getting a 00295 fault code. Just want to make sure there isn’t something else going that I should be worried about. TIA.


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[mtroxel]

I reset my fault codes and the compressor does kick on...

Then don't waste your money on a G395. When that switch is bad, the compressor won't kick in. It thinks there is no pressure.

My question though is - after resetting the fault codes and getting the A/C button to work, I get a hissing sound from the dash. Anyone else experience this when their G395 went bad?

No hissing when the G395 is bad...cause there is no compressor running to make a hiss sound. I'm afraid, my friend, have a leak. Your compressor is turning on and the hiss is the pressure escaping. I think you need a good AC shop to diagnose this.

[Ejr]

Well, mine was the G395 sensor. It makes sense when you get an over-pressure error of 29.3 bar, and later in the day a "lower limit exceeded" of 0.0 absolute bar. Your car is never going to produce an absolute vacuum in that system. So for guys searching on this, I found this discussion at Ross-Tech helpful:

https://forums.ross-tech.com/showthr...he-HVAC-Season

Blucan, I don't know what I'm doing. I do know that signal wire is a wave signal. But I assumed a 9.X volt reading was coming from the module under the dash so that at least meant the harness was good for that pin? Pin 1 is the ground and I had good continuity to any ground from there, and pin 3 gave me battery voltage with the motor running.



After I swapped for the new G395 (updated part is 4H0 959 126 B), my VCDS showed about 12.9 bar instead of the "ERROR" reading I'd gotten the last 2 days. But the AC button still wouldn't turn on, and I had ahu's error, 03023 Running-in Requirement 005 - No or Incorrect Basic Setting / Adaptation. The good news is that I had a pressure reading.

I went into 08-Auto/HVAC>Basic Settings>Compressor Run-In/Cooling check. Then click ON/Run. Heard the compressor spin right up and the radiator fans spooled up...cold air from the dash. Took a minute of that before it said "Performed" and the HVAC shut off on the dash. Turned it back on at the dashboard and have had cold air now.

Just took a look, I'm back to an evaporator output of 2° - 3° C when the car is cool and the blower isn’t running too fast.

So I’m getting;

Address 08: Auto HVAC Labels: 8Tx-820-043-3Z.clb
Control Module Part Number: 8T1 820 043 AC HW: 8T1 820 043 AC
Component and/or Version: KLIMA 3 ZONEN H06 0140
Software Coding: 64002000
Work Shop Code: WSC 02399 823 56573
VCID: 72E0D080ADBFCA1404-8026

1 Fault Found:
00256 - A/C Pressure/Temperature Sensor (G395)
002 - Lower Limit Exceeded - Intermittent

Freeze Frame:
Fault Status: 00100010
Fault Priority: 2
Fault Frequency: 1
Reset counter: 243
Time Indication: 0
Date: 2021.04.22
Time: 20:00:02

Freeze Frame:
Temperature: 27.0°C
Temperature: 100.0°C
Load: 0.0 %
Absolute Pres.: 1.2 bar
RPM: 680 /min
Speed: 0.0 km/h
Current: 0.000 A
Temperature: 19.0°C

But the AC is blowing cold. An odd issue I have is the outer (left and right vents) always blow slightly either less cold or less heated air IYKWIM. Like say I set the temp to low, the middle vents blow cold for sure, but the outer ones do still blow cold but not as cold as the middle ones, and vice versa. So I’m thinking maybe this is a sign of it about to go out? Plus the code on VCDS. I went and ordered a new temp/pressure sensor (G395 sensor), I’ll see if that’ll get fix the imbalanced temp output. Also is Absolute Pressure: 1.2 bar ok with the motor off, cuz I took those readings with the motor off.

[mtroxel]

Don't know if you're still chasing this. I've been out of town. Could be the G395. But it would also be that your G395 is telling the truth and you've just lost a little134a, or even a weak compressor. If the new sensor doesn't do it, you might have to take it to an AC shop for that one.

[Ejr]

Damn, it gets here today, I guess I’ll find out. I recently just had refrigerant taken out and new put in, but it wasn’t cuz of this issue, it was a cylinder head job that required a lot of tear down, however, issue still here, it wasn’t until I scanned it that I found the code.

[Theiceman]

that switch has a fairly high failure rate .. given AC can be so tough to troubleshoot i would not hesitate on replacing that sensor and consider it money well spent even if it didnt fix it.

[Ejr]

Gotchu, thanks for the help, will update.

[mtroxel]

Gotchu, thanks for the help, will update.

Yea Theiceman is right. It's not the most expensive part you'll ever buy for a German car. And it might take you 5 minutes to swap it out.

[Smac770]

One TSB refers to the G395 leaking refrigerant and delivering an incorrect value. But none of the TSBs seem to refer to G395 lower limit exceeded. Instead it's upper limit exceeded or lower limit not reached (along with no or incorrect signal). And as your DTC notes, the issue is intermittent.

As to the refrigerant pressures - https://www.audizine.com/forum/showt...1#post14214555

It could simply be you have low refrigerant level. But then the question is where did it leak out? The G395, the compressor, hosing, ...?

[mtroxel]

But then the question is where did it leak out? The G395, the compressor, hosing, ...?

It was very tactful of you not to mention his evaporator...

[Zuzuwu]

Those sensors go bad frequently. My car is on its third one at 140k and it’s going bad again


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[Ejr]

A lot of new information is being shoved at me, thanks nonetheless. I don’t think if it’s low refrigerant, cuz it’s been like this for sometime, and even after getting new refrigerant put in professionally so I know they checked levels. I will have the sensor replaced tmw and see how it responds, I’ll also run it for codes and see what comes back.