Manual AC Not Working? Troubleshooting Guide

Hello.
Here's a problem I'm having with my recently purchased 1999 Passat.
The air conditioning system won't turn on. The magnetic clutch isn't making any noise, and the light on the switch isn't illuminating. Of course, VAG-COM doesn't report anything, as it's not designed to. The previous owner took it to a dealership, and they said the magnetic clutch is broken, which would cost around 500 euros. Since I generally don't believe everything that other workshops say, I connected the control wire of the magnetic clutch to 12V, and sure enough, the magnetic clutch engaged and worked, and the system was cooling. I only ran the air conditioning briefly to test if everything was working, and it was cooling. Suddenly, I could also turn the air conditioning on and off again using the switch, and everything was working perfectly. However, after the car was off for 20 minutes, the air conditioning wouldn't turn on or off again using the switch. Now, my suspicion is that there's simply too little refrigerant in the system, and (or?) the pressure sensor (pressure switch?) is telling the relay 'don't turn on.'
Does a certain amount of pressure need to be present in standby mode in order to be able to start the system?
Okay, I need to have the system refilled, but shouldn't I replace the pressure switch first? Does anyone know where that part is located? I couldn't find it quickly. I don't want to have the system refilled and then it still doesn't work. Maybe someone else has some advice.

Thank you and best regards.
Dear Caro,

Hello,

Regarding pressure switches:
Remove the headlight on the right side.
Unscrew the screw (located under the cover) between the latch housing and the cover.
Disconnect the connector.
To prevent damage during loosening and tightening, it is necessary to use a suitable tool to support the hexagon.
Remove the pressure switch for the air conditioner - F129 -.

1. Refrigerant level in the system is too low!
2. Relay for the compressor magnetic coupling is defective.
You can easily check it by disconnecting the refrigerant pressure switch and bridging it. There are 4 wires connected to it. The first two wires are for the magnetic clutch cutoff, which activates when the refrigerant pressure is too low or too high. The other two wires are for fan control: "Fast" (loud). Normally, when the AC is on, the fan runs slowly (quietly).

"If the refrigerant level is too low, it hardly cools at all, and you only get lukewarm air. I've experienced this myself. Therefore, I suspect there might be an electrical problem."

Hello,
'The air conditioning is working again. Without performing any further tests, I replaced the pressure switch and then had the system refilled. Now the system can be turned on and off again, and it's cooling optimally. Unfortunately, the question of whether the problem was actually caused by the pressure switch, or simply a lack of refrigerant in the system, remains unanswered. However, since the pressure switch only cost 40 euros, I actually think the approach was reasonable. I had almost ruled out a defect in the relay, because when I bypassed it and connected 12V directly to the solenoid valve, the system could be turned on and off again for a short time using the on/off switch.'

Thank you and best regards.
expensive