DTC 00792

Vehicles with A/C Pressure Switch -F129-

DTC 00792, Part 1:

NOTE:
- As a running change in production, pressure switch -F129 is replaced by high pressure sensor -G65-.
- The function of Coolant Fan Control (FC) Module -J293 and vehicle wiring layout will vary depending on whether vehicle is equipped with A/C pressure switch -F129 or high pressure sensor -G65-.
- A/C pressure switch -F129- is comprised of 2 switches: the compressor is deactivated via terminals 1 and 2 if the pressure in the refrigerant circuit is too high or too low (switch opens), and the coolant fan is switched to second speed via terminals 3 and 4 (switch closes).
- If the switch between terminals 1 and 2 in A/C pressure switch -F129- opens, the A/C Control Head -E87- initially assumes there is excess pressure in the refrigerant circuit and stores this condition as a malfunction. If the measured ambient temperature is between 0 degree C (32 degree F) and 50 degree C (122 degree F) and the switch remains open for longer than 30 seconds, the system switches from excess pressure to low pressure (low pressure switch function), refer Read Measuring Value Block, (display group 001, compressor cut-off conditions). Evaluating Display Group 001
- If A/C pressure switch -F129- registers as "high pressure switch open" 30 times during a driving cycle (e.g. because of a loose connection), the compressor is switched off. The compressor can be switched on again by pressing the "Compressor on" button, or by switching the ignition off and on again. If this malfunction occurs over several driving cycles, the compressor cannot be switched on again unless DTC memory has been erased.

Vehicles With High Pressure Sensor - G65-

DTC 00792, Part 2:

DTC 00792, Part 3:

NOTE:
- As a running change in production, pressure switch -F129 is replaced by high pressure sensor -G65-.
- The function of radiator fan control unit -J293 and vehicle wiring layout will vary depending on whether vehicle is equipped with A/C pressure switch -F129 or high pressure sensor -G65-.
- High pressure sensor -G65- transmits a square wave signal at a rate which varies according to the refrigerant system pressure. High Pressure Sensor -G65- Signal, Checking
- Coolant Fan Control (FC) Module -J293 reacts to the pulse rate of the signal from high pressure sensor -G65 as follows: 1) It informs the A/C Control Head -E87- that the pressure in the refrigerant system is OK and that A/C clutch -N25- can be switched on (compressor is switched on via -J293-. 2) If the refrigerant system pressure rises, the control unit switches the radiator fan to speed 2.
- The signal generated by high pressure sensor -G65- is also provided as an input to the Motronic Engine Control Module (ECM). As the amount of torque needed to drive the A/C compressor varies according to the refrigerant system pressure, the ECM processes this signal in order to enhance engine performance. Depending on the type of ECM, this signal may or may not appear as a duty cycle on the read measuring value block display (function 08).
- If excessive refrigerant system pressure is detected 30 times during a driving cycle and A/C Control Head -E87 switches off the A/C clutch -N25, the compressor will not come on again automatically. The compressor can be switched on again by pressing the "Compressor on" button or by switching the ignition off and on again. If this occurs over several driving cycles, it will not be possible to switch the compressor on again until the DTC memory has been erased.
- Various versions of Coolant Fan Control (FC) Module -J293-, used in conjunction with high pressure sensor -G65- may be installed. At the time of publishing however, All TT equipped with -J293- / -G65- have version where coolant fan(s) -V7- speed 1 is always switched on automatically whenever the A/C clutch -N25- is energized (regardless of refrigerant system pressure).