Volvo VNL A/C fail always ON

Some weeks ago i found the problem on one Volvo VNL.

The Air Conditioning compressor was always ON even if the AC-OFF button on the control climate unit was pressed and led ON



It is possible to check with a multimeter direct on the AC compressor connector.
After chech the Volvo VNL wiring diagram i see this truck don't have relays for activate the compressor. This use a direct 12V signal from the "Control climate unit" to the compressor.
On the "control climate unit" pin number A12 you can check the voltage, with the AC-OFF button presssed and led ON, the vontage need to be 0V.



The pin A12 is located on the "Climate control unit" rear small connector.



The solution is to replace the "Climate control unit" for a new one. This is a electronic failure.

The biggest problem with this is that even though the AC-OFF LED is ON in the control panel, the compressor is always connected and running, even when not in use. (more fuel expense and the compressor will burn out sooner).

This problem can be avoided by installing a 12v led direct from the output of pin A12 to know if the compressor is really OFF or ON.

Or if you don't want to buy a new control unit, you can put a dash board switch for cut or send the 12V to the pin A12 manually.

 

This is because two differetn AC units are mounted on Volvo trucks.

ECC Electronic climate control.
MCC Manual climate control.

Electronic ones has the OFF button (Led on when AC compressor is OFF)
Manual ones has the ON button (Led on wnen AC compressor is ON)

I think the difference between both is:

The ECC will still try to maintain the selected temperature in the cab
without the compressor.

The Manual not.