Diagnosing A/C Problems + Electrical

[Lukeearl44]

Hello all

The A/C stopped working one day out of the blue, straight to ambient+ temp air. I did some diagnostics, and the high and low side pressures were just about the same, around 80 psi (in 75F ambient temp if that matters). No codes, no blown fuses anywhere in the car (even though I never found a fuse designated for A/C). Knowing more about electricity than car A/C systems, I unplugged the 2 pin connector on the condenser and measured the voltage across it. Toggling the A/C in the car shows battery voltage with the A/C on, 0V when off, telling me that the issue wasn't a sensor - the computer wanted the A/C on too. Knowing this and that there wasn't a difference in pressure, I thought it was either a bad compressor or clutch. I purchased a remanufactured compressor/clutch combo from R&Y and installed it, following the 30 mins of internet knowledge I'd obtained. The system maintained a vacuum for a couple hours, I filled the system, and voila, cold air. I filled the system to achieve the correct pressures (high and low) for the given ambient temperature, using a bit less than 2 12oz cans.

Then problems. I toggled on/off the A/C a couple times, and then it stopped being cold. No more pressure drop, no more cold air. I turned back to the connector for the clutch. Backprobing the 2 pin plug, only with it connected, showed a 7 volt drop across the compressor, not a 13V drop. I found the other 6 volts between the negative on the pin and chassis ground. Internet scouring for a few hours didn't present anything as to the terminal point of those two wires. Using an entirely seperate car battery and some leads, applying 12V across the compressor (bypass car issues and just engage the clutch) results in the sound of the clutch engaging, but the A/C still doesn't work. Doing this to engage the clutch, the low side drops about 10 psi slowly, then up 5. High side does not change, and the steady state still looks like the high/low sides 5-10 psi different.

Hoping to figure out where the 2 wires leading to the compressor clutch come from so I can either fix or replace. Also wondering what other component of the A/C system may be preventing the system from working. Wisdom regarding either would be very appreciated, as would an applicable wiring diagram.


(First post. Please take it easy on me.)

[maksym]

In most cars the compressor would not activate if the refrigerant pressure is too low or too high. It is unlikely yours is too high - so follow the low pressure pipe to find the low pressure switch - basically, two wires sticking from the pipe. Unplug the connector and try to short it with a paper clip. If this engages the compressor (while ac is on) you are probably low on refrigerant yet again snd need to find a leak- valve cores and what not (replace them next time you evacuate the system just in case).


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[Smac770]

The J519 manages the G17 ambient temp sensor, the G395 refrigerant pressure sensor, and the N280 AC compressor regulator valve. "no codes", according to what? VCDS? Do you see values for the G17 temp, G395 pressure, and N280 current in the measuring values of the J519 (09-central electrics) or the J255 (08-auto hvac)? As for voltage management of the N280, it's about current flow management more than voltage management. AC on should see the N280 current go to 650 mA or so and then back off as cooling is established.

https://www.audizine.com/forum/showt...1#post15072949

https://www.audizine.com/forum/showt...1#post14941163

etc.

[Lukeearl44]

I'll try bridging the low pressure switch. I didn't get any codes from OBD11, but haven't looked for specific values as I'd have nothing to compare em to. I did get a code after vacuuming out the system after installing the compressor, for a low refrigerant pressure, but that was expected. As soon as I cleared that, AC worked, until I toggled it a few times and it stopped working. No codes on OBD11 when it first stopped working or now. Maybe I didn't fill it enough? Pressures looked fine for ambient temp and I used about 600g, but I'm a novice when it comes to A/C.

VCDS is in the mail right now, not the biggest fan of OBD11 for this kind of stuff. I'll look into those values and how they change when toggling A/C.

I did measure the current running to the compressor at about 1 amp, which drops down to .6 after a few seconds. Cooling was obviously never established, and it stays pretty constant at .6A after the initial few seconds.

[Lukeearl44]

Got a VCDS cable and ran a diagnostic. Not including the whole file because there are a lot of other codes unrelated to this one. Two codes in HVAC -

00808 Control Circuit A/C compressor 008 - Implausible Signal - Intermittent
and
00457 - Central Electronics Control Module / BCM (J519) 013 - Check DTC Memory - Intermittent

I did get values for the listed sensors (G17 temp, G395 pressure, and N280 current) and a few others, and all look about as expected. Pressure was 9.2 bar (130ish psi), N280 current was consistent at 650 mA when the AC was on. Pulling the N280 valve out of the old compressor, I seem to understand now how the system works now (please correct me if I'm wrong) - the compressor/pulley are always connected, with no clutch, and the N280 valve bypasses the compressor by default. As current across the solenoid increases, the valve closes, and the compressor can build pressure. If this understanding of the system is right, I'm still not sure why the AC doesn't work when the bypass valve is directly powered by a 12V battery. That should push more than enough current to fully close the bypass, build pressure, and make air cold.

Hoping someone can make me feel dumb and help me in fixing this.


Address 08: Auto HVAC (J255) Labels:. 8Kx-820-043-3Z.clb
Part No SW: 8K1 820 043 T HW: 8K1 820 043 T
Component: KLIMA 3 ZONEN H04 0121
Revision: C1000000 Serial number: 01161120120428
Coding: 6600228000
Shop #: WSC 06325 000 00000
VCID: 3928F5EAF95FCC0258D-806C

2 Faults Found:
00898 - Control Circuit A/C compressor
008 - Implausible Signal - Intermittent
Freeze Frame:
Fault Status: 00101000
Fault Priority: 6
Fault Frequency: 26
Reset counter: 149
Mileage: 228842 km
Time Indication: 0
Date: 2024.08.15
Time: 00:08:11

Freeze Frame:
Current: 0.625 A
Current: 0.000 A
Voltage: 12.50 V

00457 - Central Electronics Control Module / BCM (J519)
013 - Check DTC Memory - Intermittent
Freeze Frame:
Fault Status: 00101101
Fault Priority: 6
Fault Frequency: 7
Reset counter: 149
Mileage: 228842 km
Time Indication: 0
Date: 2024.08.14
Time: 18:19:41

[Smac770]

00457 - Central Electronics Control Module / BCM (J519)

Should have included more of the log. Because the J255 is saying there's issues in the J519 (09-central electrics, which happens to manage pretty much all the relevant stuff, including the N280 AC compressor switch) which bothers it.

9.2 bar is low for 650 mA. Yeah, the only engine with the magnetic clutch compressor is the 2.0 TDI, iirc. But back to the bar, could be the compressor is not working well, or could be the refrigerant volume/weight is low.

In a log I have from some time back, I was running 8.2 bar and 000 mA normal driving, then I turned on the AC. N280 current shot to 645 mA, G395 pressure jumped to 12.0 within ~5 seconds, then leveled at 11.2 after another 3 seconds. Ran for some time at that level before backing off the regulating current and therefore also the refrigerant pressure, once the G263 temp (air temp after the evap coil) had cut in half from 30°C to 14°C. As G263 temp approached 3°C, the current was near 500 mA and pressure was mid 10s.

"Voltage: 12.50 V", that's with the engine running?

In your case, it's sustaining that 650 mA because it's still waiting for the pressure to built to a level where it'll do some actual cooling. Was the G263 temp dropping at all from ambient?

[Lukeearl44]

Central Electrics is throwing a code for intermittent loss of connection to N280. Believe it was triggered when I was messing with the connection back when I thought I could just connect a car battery across the two wires.

12.5V was with the car running. Battery voltage at that time was higher, somewhere in the high 13s. Not sure if it should have matched battery voltage, but I’d imagine 12.5 should be sufficient, even if it may suggest an issue if it is supposed to match battery voltage.

9-9.1 bar before turning on AC, 9.2 bar after turning on AC. N280 current 0 before and 635 ma after. Best guess is that I’m not necessarily low on refrigerant, just that the condenser isn’t doing anything. G263 temp is not changing after turning AC on and seeing N280 current jump up to 635 mA. Believe it was 27ish C.

On chance, looked at the oil level in the vacuum pump used to draw a vacuum on the system, and it was low out of sight on the gauge glass. Not entirely sure where it went, but decided to take the compressor back off, dump/blow out as much oil as possible, then blew out the AC lines, in the high side out the low. I got a lot more oil/wet crap out than expected. Could it be possible that fumbling the order of operations on the AC gauges set when drawing the vacuum, and turning off the pump before shutting the valves, drew vacuum pump oil back into the AC system? I know the exhaust from the pump comes out the oil fill cap, and I’m pretty sure a lot more than the oil spec came out of the lines.

Would having too much oil (and the wrong kind) cause the compressor to clog/not function? The way I’m seeing it, 635 mA is being sent to the N280 with little to no pressure buildup from the compressor. Looks like hardware, not electrical. Right now the compressor is back in, compressor oil is in, going to fill the system tomorrow with refrigerant and try again, being careful with the vacuum pump.


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[Smac770]

I don't know anything at that level about the refrigerant / refrigerant oil side. Yeah, you're not getting any electrical error from the J519 about the N280, and the measurement looks fine. It's simply the compressor is not doing anything. Or the N280 is shot and not responding to the current flow to do whatever it does. Might also be the pulley? These have those breakaway pulleys, so if the compressor locks up, the belt will keep spinning. If you spin the pulley, is it obvious that it's spinning the compressor? Or just it spin without resistance? Needing a new compressor comes through in the threads often enough that such a conclusion would not be a surprise.

[Lukeearl44]

Filled with refrigerant, no luck. N280 current 645 mA, no pressure building, G263 temp 3deg C above ambient. Pulled the N280 valve out and tested it with a battery, and it does seal/close when it’s supposed to. Spinning the pulley definitely spins the compressor, as that was needed to fill the compressor with oil. The compressor seems to work on the bench, but not in the car. I did buy it remanufactured, but I’d like to give it the benefit of the doubt that it’s not bad from the start. Could be a bad compressor, hope not.

Is there any way that there’s a clog somewhere else, or some other failed component that’s preventing the pump from building pressure (or giving that impression)?


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[Smac770]

It could be something with the refrigerant circuit. That's where I just don't know enough about refrigerant circuits to say if the scenario could be another component in the route, such as a failed expansion valve or whatever else can go bad that would prevent pressure build up. Or how you would problem isolate that. Another search hit suggested the circuit might have a blockage, such as before the compressor (can't compress nothing), etc. Don't know if one can "blow out" the hard lines to confirm volume flow is possible or such.

[Lukeearl44]

I only know AC systems on a very basic level, but I did experience a repeatable, definite flow restriction when blowing out the lines high to low. Would make sense to be the expansion valve, and the restriction increased as pressure did. Pushing about 150psi air in on the high side had much lower pressure air coming out - from a gut feeling/basic logic standpoint, if the compressor was indeed outputting high pressure refrigerant, it stands to reason it would make it through back to the low side connection. Far from an expert, but the basic mechanics make me think bad compressor. Ordered a second one, will update next weekend. Too bad warrantees only apply if installed by licensed techs.

As another note, take caution with the pulleys on the compressors. They’re cast aluminum, and I chipped a pretty good chunk off mine just releasing tension when putting the drive belt back on. Caught the corner of the serpentine belt tool and cracked off.


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[Lukeearl44]

Looks like a bad J519. I traced the wires back from the engine bay to a harness under the dashboard, found the correct two pins for power and ground. Using some needle probes, I measured the voltage across those two pins (output to the AC compressor/N280 regulating valve). It bounces between 1-3 volts, where I’m pretty sure it’s supposed to be 12V. The conclusion I draw from that is that it’s a bad module. I measured the resistance of each of the two wires to be about half an ohm. The AC system now works when I apply 12V directly across the N280 valve, bypassing the module and the wiring to the N280.

Does anyone know the best place to purchase a new module? I see a few on Ebay, is that worth the risk or do I buy from Audi? Also, does this have component protection/will Audi need to code this in or can I do it from VCDS?

[Smac770]

N280 should be something like 7.5V at 650mA. I've already gone through the measurements with Allowencer when he was having N119 and N280 issues. That's in my first link back in post 3. For Allowencer and I, the N280 circuit measured 12±1Ω across pins 8 to 10 on J519 black 17 (this is with the plug unconnected).

Yes, the J519 implements component protection, so you'll need ODIS to reset that. I don't know if anyone has determined if the J519 has a simple 8-pin CP chip you can just transplant, like you might see in some CP workaround videos. But I don't know if CP active disables the AC; I know it disables the interior lighting and heated seats.