I've been reading this forum for a while, and thought that it was about time I contributed something, so here goes...

My aircon was not working, so I set about trying to fix it.

This information may not apply to your car, mine is a 54 plate Alhambra 130 tdi with climate control.

The compressor clutch was not engaging, there was no voltage at the compressor clutch connector and no error codes on my VAG-COM.

There appears to be a lot of mis-information about the G65 air conditioning pressure sensor around, so hopefully this will clarify things a little.

This sensor is fitted to vehicles with the climate control module and is located behind the front bumper on the drivers side of the car.

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It can be replaced without removing the bumper, but is easier if you do. If you jack up the front of the car on the drivers side and look up behind the bumper, you can see the sensor attached to the aircon pipe. Itââ,¬â,,¢s behind the horn.

I've had to replace mine in the past, so I can 100% confirm that you can do this without having to evacuate the gas from the system.

There is a Schrader valve in the union where the sensor connects to the air con pipe, and an o-ring around the valve to prevent leaks.

Disconnect the wire and unscrew it from the aircon pipe (17mm spanner) and there will be a small burst of gas as the valve closes when the sensor is removed. Replacement is reverse of removal

QuoteI have found a description in a VW technical document about air con that describes how the sensor works.

G65 is an electronic pressure sensor. Refrigerant pressure is applied to a silicon crystal inside the sensor and a microprocessor converts the pressure into a pulse width modulated signal.

There are 3 wires on the connector (T3 in Elsawin) to provide power, ground and the output signal.

Connector T3/c1, earth, Brown wire
Connector T3/c2, signal, Yellow/Red wire
Connector T3/c3, power, Black wire

The pulse frequency is 50Hz, so the pulses repeat every 20 ms (milliseconds).

The width of the pulse is proportional to the refrigerant pressure, so the sensor can detect low and high pressure.

At a low pressure (1.4 bar, 20 psi) the signal width is 2.6 ms (of the total 20ms repeating signal), at high pressure (37 bar, 536 psi) the signal width is 18ms.

I have a Velleman HPS140i handheld oscilloscope, and can confirm that the output on the scope looks like a pulse width modulated square wave signal.

In other words, trying to measure this signal with a regular DC voltmeter doesn't tell you what is really going on. Even on AC it will likely give you a misleading voltage.

Unless you have an oscilloscope, you will have to trust the diagnostic codes on VAG-COM.

The G65 sensor is directly connected to the J293 "Relay 454", so I thought that the fault may be there.

I spent ages trying to find my J293 relay, and finally found that it is located on the relay panel above the fusebox behind the dash on the drivers side.

I thought that my J293 "relay" was faulty, so took it apart to see if it could be repaired.

To my surprise I found that this part (7E0919506) a PIC based microcontroller, so a repair is not trivial.

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Fortunately mine was not faulty, and replacing the G65 got my air con working again.

Rick