Air Conditioner Pressure Switch F129

Hi,
'For a few days now, my Climatronic system has been malfunctioning in my Golf III. Reading the error codes narrowed the problem down to the pressure sensor F129, which is screwed into the dryer. According to my information, this F129 consists of two pressure sensors:'
One system increases the speed of the radiator fan by one level when the pressure reaches 16 bar, controlled by the 'Radiator Fan' control unit. The other system is a combination switch that activates when the pressure is either too high (greater than 24 bar) or too low (less than 2 bar), and it shuts off the compressor via the Climatronic control unit.
The latter is also responsible for my problem. According to the wiring diagram, it is closed in its default state (i.e., under normal operating pressure).
The pressure sensor in my Golf, however, has a somewhat undefined switching state (R ~ 100 Ohms).
The question is whether it's possible to replace this pressure switch without first draining the coolant. While I don't think so, it's possible that a valve in the fitting could be closed when the pressure switch is removed, similar to the connection points on the air conditioning system for filling/draining.
Or does this peculiar switching state even indicate a pressure that is too low?
What do people with experience say about it?

Best, Andy.

Hi,

I assume that your system is already operating with insufficient pressure due to refrigerant leakage. After 4 years, my CT unit had so little refrigerant left that it shut down with a 15-second flashing error code.

However, even with refills, the effect lasted for only about 1 year each time.

You can easily replace the pressure switch without draining the coolant.

Hi,
A little feedback:

Replacing the pressure switch is really straightforward and, believe it or not, possible without having to drain the entire system. It's worth remembering to get a new sealing ring beforehand; mine was unusable after removing the old pressure switch.
The used, new pressure switch now exhibits switch-like resistance values.

This, however, was not the solution to the problem.

I then found the solution in the missing power supply to the pressure switch.
A defective SMD transistor in the Climatronic control unit was preventing a SMD FET from switching, which was necessary to provide +12V to the pressure switch. Because the pressure switch was not receiving voltage, the subsequent logic circuit could not detect that the system was operating at the correct pressure, and therefore the compressor was not being activated.

I replaced the transistor with a standard BC 547 that I had lying around, and now the FET is being controlled correctly again.

The alternative to this annoying noise is simply a new control unit from the dealer, costing 319 euros plus VAT.
Hella, as the manufacturer of the control unit, is unfortunately unable to supply spare parts for it.

Now it's pleasantly cool again...

Best, Andy.

Hello Andy,

Thank you for your comment. I'm experiencing the same problem with a Passat, and I was already planning to change the coolant and have a pressure test done.
Okay, I'm going to check the function of the pressure switch and its power supply first.

Best regards!
Dirk.


Hi,
A little feedback:

Replacing the pressure switch is really straightforward and, believe it or not, possible without having to drain the entire system. It's worth remembering to get a new sealing ring beforehand; mine was unusable after removing the old pressure switch.
The used, new pressure switch now exhibits switch-like resistance values.

This, however, was not the solution to the problem.

I then found the solution in the missing power supply to the pressure switch.
A surface-mount transistor in the Climatronic control unit was defective, and this transistor switches a surface-mount FET to provide +12V to the pressure switch. Because the voltage was missing at the pressure switch, the subsequent logic circuit could not detect that the system was operating at the correct pressure --> the compressor was not activated.

I replaced the transistor with a standard BC 547 that I had lying around, and now the FET is being controlled correctly again.

The alternative to this annoying noise is simply a new control unit from the dealer, costing 319 euros plus VAT.
Hella, as the manufacturer of the control unit, is unfortunately unable to supply spare parts for it.

Now it's pleasantly cool again...

Best regards, Andy