AC won't turn on? Troubleshooting guide

Passat 35i, manufactured in October 1996, 1.9 TDI, 90 hp, engine code 1Z.

'My manual air conditioning system hasn't been working since winter (it was refilled last fall by ATU), meaning the compressor clutch isn't receiving any power! I've already checked the A/C switch --> it's working. Next, I checked the fuse for the A/C fan control unit (located in the engine compartment, front right), and that's okay too. The cable from this control relay to the compressor is also fine.'

The temperature sensor has continuity (low resistance), and approximately 14V is present at the sensor when the engine is running.
The pressure sensor for high and low pressure is also receiving voltage, but I couldn't test for continuity because it's difficult to access. Instead, I bypassed the wires of the four-pin connector (connecting each 12V wire to the ground wire) --> the air conditioning still doesn't work.

I once disconnected the connector at the relay to check if there was any continuity or voltage between the wires and the ground. It didn't make any difference whether the engine was running (fan on setting 1) or whether the air conditioning was switched on or off, so the possible cause of the problem seems to be a specific activation condition. I don't necessarily think there's a cable defect. Can anyone help me?

Possibly, the refrigerant has already leaked out. I experienced the same thing. However, the refrigerant leaked out within two months. I suspect that the people at ATU might have also added a contrast dye. If you shine a UV light with a lamp into the engine compartment, you might be able to see a leak.

Best regards, WarLord.

You were right. After searching aimlessly in the electrical components, I just took a quick look at the filling valve in the high-pressure circuit. The indicator fluid was leaking out, which meant the precious refrigerant was escaping!

I had also wondered why the hoses to the compressor could be so easily squeezed.

So, I went back to ATU and told them that the last filling must have caused the valve to leak, and I got a new one free of charge. The only thing I paid for was a new valve, which cost 7 Euros.

'Has anyone had experience with dryer cartridges? Should they really be replaced with every climate check? They are supposedly only able to absorb 8% water from the system, and after that, they become ineffective. At lower temperatures, ice crystals could potentially form and endanger the system (expansion, breakage of components due to overpressure).'
Is there any truth to that? }

The dryer should be replaced after the fourth use or after any maintenance work performed on the open circuit.

Best regards, WarLord.

The dryer should be replaced after the fourth fill-up or after operation in an open-loop system.

Greetings, WarLord.

Hello,
Where did you get this information from?
I'm standing in front of a climate control unit, and I've been giving a lot of thought to the dryer. There are many different opinions on whether it should be included or not.

Regards,
Andy

Are you looking for technical articles - general?

It contains some information about air conditioning systems.

btw: a dryer like that costs around 250 euros.
The compressor costs 500 euros or more.

CU Gremlin.

It costs around 50€ (less on eBay), but I think it's overpriced each time. The general consensus is that it's okay to change the refrigerant three to four times. However, in my case, the cooling performance already deteriorated after three changes, but I don't know if that was due to the dryer being full.

Best regards, WarLord.

hm...

In the catalog, it says €21 plus VAT for Hella dryers. I just quickly checked for a Golf 4...

CU Gremlin.

I was told back then that the dryer for my 35i TDI would cost around 50 euros.

Best regards, WarLord.