AC troubleshooting - high pressure sensor bypass for testing

[vtraudt]

Does not blow cold.
Compressor not kicking in.
No pressure on high side.
18 psi on low side.

VCDS code:

Address 08: Auto HVAC Labels: 8E0-820-043.lbl
Part No: 8E0 820 043 AM
Component: A4 Klimaautomat 2831
Coding: 00100
Shop #: WSC 00672
VCID: 6CD649571C85462F5D-5122

1 Fault Found:
00819 - High Pressure Sensor (G65)
30-00 - Open or Short to Plus

On older cars, bypassing the lower pressure switch activated the compressor clutch. But on these '3 pin' systems, the compressor may not have a clutch anymore, but rather is duty cycled by the module.
So not sure if or how the compressor can be 'forced' to run for test purposes.
Is there a way to bypass the high pressure sensor?
Or can it be tested?
Or is the only way 'trial and error' (buying a new sensor, install)? Only to find out 'its not the sensor'.

[Audibot]

18psi is WAAAY too low when the system is at rest/off. You likely have a leak somewhere. Pressure should be closer to 5-6 bars (73-90 psi), if I remember right. So your switch would not turn on, and if any components in the a/c system don't work, the whole system doesn't work.

Check the G65 switch on the condenser. The o-rings are known to blow out and leak out refrigerant. If this happened, it would be dirty/oily in that area. This has happened to me.

If, for some reason, you replace the o-ring/sensor and it doesn't fix the issue, you may want to consider getting refrigerant with UV dye and a UV lamp (I'd suggest evening/night to do this) and see if you can find where it's leaking.

You should be able to test the compressor itself by jumping it to the battery and hearing the clutch kick in. But pressure is your first issue right now.


If that all fails, consider having a professional shop look at it.

[vtraudt]

[QUOTE=Audibot;13741815

Check the G65 switch on the condenser. The o-rings are known to blow out and leak out refrigerant. If this happened, it would be dirty/oily in that area. This has happened to me.

If, for some reason, you replace the o-ring/sensor and it doesn't fix the issue, you may want to consider getting refrigerant with UV dye and a UV lamp (I'd suggest evening/night to do this) and see if you can find where it's leaking.

You should be able to test the compressor itself by jumping it to the battery and hearing the clutch kick in. But pressure is your first issue right now.

[/QUOTE]

Thanks for the suggestions/tips.

a) G65 leak: I will check the part number, may have another one sitting on the shelf. If system is so low as you indicate, not much lost swapping it out.
b ) alternative: check how much the switch is, get one.
c) Are leaks 'visible' (soap water, bubbles?)? Or are they so small that they cannot be detected?
d) I have UV dye here (and light/glasses). Never had much luck (used for oil leaks also) with those dyes.
e) force the compressor to run: how? This is a 3 pin switch (G65). How to bypass to force the compressor to kick in? Or "jumping it to battery" (clutch).

[vtraudt]

Anybody have the part number for that switch?
It is a 2006 A4 3.0 V6 CONVERTIBLE (so still a B6).

[jasc23]

Hi,
I have a 2008 A4 Q manual Avant with 60K miles.
Car is in fantastic condition and I get compliments from Audi/VW fans everywhere.
I am a very avid DIY and have worked on Audis / VW for years now.

Last year, the car started having a very strange A/C problem. It would stop working randomly for a few minutes, then cool just fine 90% of the time.
Since I was really busy with work around the house and other problems, and I know that in general, when you have A/C issues, it's best to leave it to the "PROS".
So, I brough the car into my local dealer.
They diagnosed bad compressor. So I went ahead and replaced Compressor, Condenser, Drier, Orifice tube, G65 sensor and re-charge.
After they replacement, the A/C worked great, for like 20 minutes, until the fans kicked in.. then the exact same problem as before showed up.
Went back and complained, they said they could not replicate.

Now, the issue became more frequent (since it's hotter outside) and cannot drive the car without A/C.
So, I started troubleshooting the issue with VCDS and found I have a code 00819:0 16 for the G65 sensor (High Pressure switch) cuts on/off intermittently only when the fans are running. When the car is in ACC, the switch shows a steady 6 bar pressure. I have confirmed that the refrigerant pressures are fine (75 static) and do not fluctuate when the fans are running.
Once the fans start, G65 jumps to 33 bar (Maximum value) and triggers the code - Value too high (Outside Spec) which in turn turns off the A/C Compressor.
Once the fans turn off, everything works great and A/C cools just fine.

I am stumped, I have tried so far:
-New G65 sensor: Same outcome
-Used HVAC panel/module: same outcome
-Unplug fans connector: A/C runs but since there are no fans, the pressure rises steadily and eventually (after 20 min) the system shuts down the A/C because it doesn't see the fans and the pressure reaches 28 bar (verified with manifold).

Here's a video of the issue:
https://www.youtube.com/watch?v=7VKLI5DSUVQ


Any help is greatly appreciated.
Thanks

[Audibot]

I'm not sure how your a/c worked for even 20 minutes.

Keep in mind the fans are supposed to turn on when the a/c is on. The 6 bar (87 psig) pressure when the system is off is correct. However, the 33 bar pressure is too high. You're seeing the shutoff at 28 bars. You should be at ~20 bar (290 psig) when the system is on, but up to 28 should work. I suspect your system is overfilled and the switch is protecting you.

FWIW, I'm looking at the Audi repair manual (other laptop), and it says that the switch is closed between 1.6 bars and 28 bars. Regarding the fans, the second speed coolant fan kicks in at 16 bars, and stops below 12.5 bars. The compressor has a relief pressure valve at 38 bar, which you haven't hit yet.

Another reason the a/c can kick off is if the engine coolant temperature gets too high.

I recommend going to a professional a/c shop to see if they can bleed it down some, or evacuate and refill the system appropriately.

Edit:

You may also be in this scenario:

Condition 3: High and low pressures normal. After a period of time, high pressures rise above specified value, low pressure drops below specified value
Moisture in system, restrictor or expansion valve icing.
• Recover and recycle refrigerant.
• Flush with compressed air and Nitrogen.
• Replace accumulator and restrictor, or
receiver dryer and expansion valve.
• Recharge system.

[shurur9]

As said, it sounds like the system is overpressurized or maxed out. Hot weather will make it shutdown because the pressure increases with temperature.

You can bleed some off a bit at a time, on a hot day, with a nail and a rag until it starts running again...or so a friend told me. But you are suppose to bring it to a shop so refrigerant can be recovered..

[Turbo510]

I have confirmed that the refrigerant pressures are fine (75 static) and do not fluctuate when the fans are running.
Once the fans start, G65 jumps to 33 bar (Maximum value) and triggers the code - Value too high (Outside Spec) which in turn turns off the A/C Compressor.
Once the fans turn off, everything works great and A/C cools just fine.

Totally speculating here, but this sounds like a possible circuit-resistance problem. The highest current draw by the fans is when they are accelerating, and if the G65 has a ground or supply that it shares with the fan module, that could account for the off-scale reading.

[jasc23]

I appreciate the responses from Audibot and Shurur9, however, you guys are not understanding the issue.
The 33 bar is not the real pressure inside the A/C system. 33 bar is simply the value you get when the sensor loses signal.
I have confirmed this with a set of gauges and the static pressure is 6 bar, then when the system decides to work it goes up to 22.. but before it can work steady, the sensor shows "33 bar" and turns off the compressor.
It is not a mechanical problem, it is an electrical one... the system has been drained and refilled twice.
The reason why it works for a few minutes is because the car is moving, so no need for fans. And for some reason it works for a bit longer.
However, unplugging the fan module does nothing to the problem. It just kills the fans.

If it was a mechanical problem, the system wouldn't cool in the first place, am I missing something?

Here's a link to a video of the issue, which is intermittent:
https://www.youtube.com/watch?v=7VKLI5DSUVQ

[jasc23]

Turbo510, I think you may be onto something here.. let me check diagrams and see if the ref voltage wire or signal wire for that sensor share anything with the fans.

[jasc23]

UPDATE:
So, I looked in my Bentley manual:
From the G65 sensor: The blue/brown wire (signal) and the red/gray wire (Ref voltage) Both go straight into the HVAC panel.
The problem continues to happen when I unplug the fan module. I even swapped the fan module and get the same result.
I just ordered a "cheap" sensor instead of the MAHLE units I have been trying... we'll see what happens then.

[jasc23]

UPDATE: This afternoon I did some more troubleshooting and here is what I got so far:
I traced all wires, connectors, etc.. everything seems well.
Being a bit stumped, I decided to run the car with the main alternator wire unplugged.
Result: Car started just fine, A/C ran perfect, fans kick ON, everything works as supposed. Kept the car running for approx 20 minutes and the A/C kept blowing without hiccup.

I am even more confused as to how can this be possible. Does this mean that the sensor doesn't like 14V when the alternator is charging and the sensor is fed 14V as opposed to 12V when in ACC power?
Is the alternator somehow faulty or bad? Like I said before, the car runs perfect otherwise, no codes, no issues, no dead batteries, etc..
SOOO WEIRDD??

[EuroxS4]

I would not be surprised if you had a bad G65 swith. They are known to fail and also known to piss freon straight through the switch emptying the system. However I think VCDS would be helpful in this case so you can look at the pressure in the system with the ac system off. Depending on temp will tell you everything. With ambient temp of 65 degrees low side should be between 25-35psi. Anything lower would indicated that the charge is low. The higher the temp the higher the low side pressure. My money is on the switch or compressor. I would try the switch first as they are known for failure and well all know audi cant make any sensor to save their fucking life. Hell they still havent figured out how to make a ac switch that doesnt fail even on the newest latest and greatest switches still fail at a high ratem along with evaporators and compressors.

If you want to jump the compressor I would do so by applying either 12v to the N280 but I would not do it for long periods of time you want to be between 0.4-0.8amps max at 12v. The way N280 is setup is like a solenoid its open when not on and closed when engaged. I think both wires from N280 are color coded. I think best bet is with a power probe if you want to have the ac compressor run. However also note forcing the compressor to run wit ha low charge can damage the unit. The fail safe built into the system is that when the system is under 200grams it will be disabled. It needs a minimum of at least 200-220grams of freon. However with such low value you will have some vents that are somewhat cold and the other in succession with be a little warmer.

It looks like pin 1 is your ground and pin2 is your 12v coming from ecm.

[EuroxS4]

UPDATE: This afternoon I did some more troubleshooting and here is what I got so far:
I traced all wires, connectors, etc.. everything seems well.
Being a bit stumped, I decided to run the car with the main alternator wire unplugged.
Result: Car started just fine, A/C ran perfect, fans kick ON, everything works as supposed. Kept the car running for approx 20 minutes and the A/C kept blowing without hiccup.

I am even more confused as to how can this be possible. Does this mean that the sensor doesn't like 14V when the alternator is charging and the sensor is fed 14V as opposed to 12V when in ACC power?
Is the alternator somehow faulty or bad? Like I said before, the car runs perfect otherwise, no codes, no issues, no dead batteries, etc..
SOOO WEIRDD??

You will have an abnormally high pressure if the system is overcharged which is also not good for compressor. Verify system charge is accurate should be between 410-550grams

[jasc23]

UPDATE: Last night I spent time tracing all the wires that lead into that sensor and everything seems well, followed them into the HVAC module (E87), power supply and ground. I was wrong in my previous statement, the power wire (RED/GRAY) does not go into HVAC head, it joins a power supply at some point under the dash.
The signal output of the sensor continues to be 33bar (only with engine running), even though, I checked with gauges: I am at 6 bar static and about 15 bar when running (only for less than a minute).
Then, I was able to momentarily swap a G65 sensor from a friends car (Exact same B7) to test it out. The result was the same.
I then stopped by a junkyard picking up a used HVAC module to test out, coded it with VCDS, same result.
Then, in a moment of desperation, I ran the car with the main alternator cable unplugged- The car ran fine, to my surprise, the A/C cooled without hiccup, pressure in VCDS never exceeded 20bar, fans turned ON, everything worked the way its supposed to. Ran it like that for 30 minutes to really test.
After that time, the battery voltage (understandably) dropped to 11.5V and called it a day.

What do you think? defective alternator? bad sensor? But how?? I measured it and it's outputting 14V when running. Diode test checks out fine.

[Turbo510]

....you can look at the pressure in the system with the ac system off. Depending on temp will tell you everything. With ambient temp of 65 degrees low side should be between 25-35psi. Anything lower would indicated that the charge is low. The higher the temp the higher the low side pressure...

Don't you mean with the AC On? When Off, the entire system's pressure equalizes.

[1highfly]

Hi

I have had the same problem with my A4 b7

I have basically replaced everything apart from pipes and A/c heat exchanger

new pump, orifice, drier, today yet another g65 sensor (not one from eBay as most of them don't work when you get them)

after reading the post about the alternator may be the issue I start looking at testing the g65 wiring

all seemed to be work good ground and vcc, signal was rapping up and down but 33 bar then 5bar ( with gauges on 75 psi)


a few tests later on wires run from another battery to g65 sensor still the same
I then powered the g65 sensor direct from the car battery + and - all working no issue with 33bar vag com reads g65 5bar to 14bar
took the live off and reconnect to the sensor wire + still worked

so bad Gnd easy fix so I put the Gnd on the mounting bolt for the horn and the fault comes back 33 bar again

so my fix is the sensor Gnd is now fastened to the Gnd terminal on the bulk head in front of the battery NICE COLD AIR

I Hope This helps

[EuroxS4]

Don't you mean with the AC On? When Off, the entire system's pressure equalizes.

No ac off that system always has a pressure in it. But normally between 4-8bar without compressor running.

[Turbo510]

No ac off that system always has a pressure in it. But normally between 4-8bar without compressor running.

I wrote "equalizes" meaning the pressure becomes the same throughout the system, not that it doesn't have pressure at all.

[jasc23]

Highfly, why did you relocated the sensor ground wire to the bulkhead when that ground is literally 12" from the sensor under the power steering reservoir?
If that ground was bad, wouldn't the other stuff connected in the same ground not work as well? In my case, I detached the ground, confirmed continuity, sanded down the terminals and plugged it back in.
Maybe there is value in testing a wire from that ground to another one?
Perhaps you can share a picture of how you did it.
Also, what G65 sensor are you using? is it the OEM VW part? or the MAHLE one?

[jasc23]

FINAL UPDATE: I purchased the OEM sensor from Europa Parts. Which, btw were amazing and got the sensor out that same day, arrived the next day! Highly recommend them.
So, after weeks of frustration and 3 sensors, I was very hesitant to test this one.
Threw it on the condenser with the car mostly apart for troubleshooting, hooked up VCDS and fired it up.
Immediately, read 5bar, then turned ON the A/C and everything worked great. Readings got to 9 bar (perfectly normal) and all was good with the world.
This was an amazing and humbling lesson!

[EuroxS4]

FINAL UPDATE: I purchased the OEM sensor from Europa Parts. Which, btw were amazing and got the sensor out that same day, arrived the next day! Highly recommend them.
So, after weeks of frustration and 3 sensors, I was very hesitant to test this one.
Threw it on the condenser with the car mostly apart for troubleshooting, hooked up VCDS and fired it up.
Immediately, read 5bar, then turned ON the A/C and everything worked great. Readings got to 9 bar (perfectly normal) and all was good with the world.
This was an amazing and humbling lesson!

Wait you mean to tell me you were surprised that chinesium fucked you??? Rule#1 don't ever get fucked by chinesium!

[jasc23]

Well.. it was not totally chinesium... 2 sensors were MAHLE units from FCP, accordding to the catalog, it was a perfect fit.
The other sensor was a more "generic" unit from ebay that had reviews saying it worked on their B7 cars. This one was total junk!
And, yes apparently this sensor is specific for AUDI and says so right on the label.. it's not shared with any other Audi platform (B5, B6, B8, etc..) and fortunately did not cost a fortune. about $60 vs. $45 the FCP unit cost.
But, yes, lesson learned, whenever there are funky issues, first blame cheap sensors, then bad connectors, then crapped out modules, then chewed up/corroded wires/bad grounds.

[EuroxS4]

Well.. it was not totally chinesium... 2 sensors were MAHLE units from FCP, accordding to the catalog, it was a perfect fit.
The other sensor was a more "generic" unit from ebay that had reviews saying it worked on their B7 cars. This one was total junk!
And, yes apparently this sensor is specific for AUDI and says so right on the label.. it's not shared with any other Audi platform (B5, B6, B8, etc..) and fortunately did not cost a fortune. about $60 vs. $45 the FCP unit cost.
But, yes, lesson learned, whenever there are funky issues, first blame cheap sensors, then bad connectors, then crapped out modules, then chewed up/corroded wires/bad grounds.

The lesson to be learned is if you need something specific do the right thing save yourself time, money and headaches and buy a oem part. Granted this should apply to most crucial parts that you don't want to fail or last a good amount of time. There are plenty of mahle and oem vendor counterfeit parts everywhere. If the price is too good to be true it normally is or doesnt fit the car. Always go by factory part numbers.

If your in Northern NJ it would be foolish to wait for FCP when you can have the part the same day from a local vendor. Northside, IMC, Worldpac all have huge warehouses near northern NJ.