If everything is in place and the AC is not cold, it could be the outside (ambient) temperature sensor is faulty. The sensor is clipped in front of the radiator, which the bumper must be removed to replace.
– Audi A8 D3 AC system: As I understand while having this experience, the AC system blows cold or hot air whenever it feels like last few months and it’s totally out, the ECON settings was forced to ON = AC OFF. G65 High Pressure Sensor 8E0959126 (old part#) 3R0959126 (new part #) sensed the pressure and tells the N280 pressure regulator NOT to turn on. According to ElsaWin, if the waveform on the G65 is over 87% duty cycle, the Freon pressure is too low and under 12% duty cycle, the Freon pressure is too high. G65 is just a Pressure PWM (pulse width modulator) sensor, so after market ones should do $39 vs $89-105 from dealer.
– VCDS will show 00819-High Pressure Sensor (G65) – sometimes “under limit” and sometimes “over limit” whichever the case, the AC won’t kick on.
– If you don’t have a VCDS, buy an AC manifold set from harbor freight or amazon for $44-55 and you can check without any problems:
a. If the AC has Freon and the N280 is turned OFF by G65 sensor: low side pressure is high (60-80) but the high side (RED) is steady at the same pressure as low side (60-80).
b. If the AC has Freon and the N280 is turned ON (system is working) by G65 sensor: low side pressure will decrease (30-40) while high side (RED) is increasing above 100-200.
– If you have a leaking G65 like mine, when you hook up the AC manifold, the low side and high side will have 0 pressure – the system is empty. While it’s empty, you can get an air vacuum from harbor freight for $19 to check for leaks, providing you have a 120PSI compressor at home. If NOT, you can go to the AC shop and they can check it for you – $80 minimum. It will take more than an hour to verify if the system is leaking.
– The best is to build a simple 50hz 50% duty cycle circuit to check the AC compressor and the electronic system – by bypassing the sensor, simulate the correct pressure signals to the ECU for it to turn on the N280 because sometimes, the new pressure sensor can be DEFECTIVE (since it’s electronic circuit) and won’t turn on the N280 even you replace it.
To me, Audi has bad reputation for reliability because of their complex control system – People who work on them don’t understand the system and start to shotgun (replace without educated information or guess) parts until they get the right one on customer’s expense. The myth of blowing up the AC compressor due to overcharging the Freon – Yes, it’s good to have gauges and correct information of the AC Freon capacity – I couldn’t find any sticker on my car that shows how much Freon is needed for my 4.2L – TSHong who owns a shop and the specs he quoted was 21.2 oz +/- 1.8 oz, so maximum we can charge is 2-12oz cans. To me, even with 1-12oz can from empty system, it’s already cold.
I like to do things at home, I hate going to the shop and wait and worry someone will screw up my car (every time I open something, something is missing because the people before didn’t do it right), it’s not the money… Really.
If the AC is working properly and the Freon are charged – we should see the following: Engine OFF – Low side pressure = 80 and High side pressure = 70-80.
Engine ON – AC ON – ECON OFF – Low side pressure 30-40 and High side 200-250.
In the winter, we don’t use AC often and we tend to turn off the AC until the Spring came and it blows hot air, my setting for AC ECON OFF doesn’t even stay because the system turns it ON automatically due to insufficient AC freon pressure measured by G65 High Pressure Sensor (8E0-959-126 now 3R0-959-126).
Normally, we just charge the system with some Freon 134a and we should be on our way, but not this time. The High Pressure Sensor is leaking from the connector.