As i said in my first post on this subject, i do not know about the wiring on the 3.9 classic. My DSE has a thermostat fitted in the top hose thru a relay to operate the aircon fans to assist the engine cooling whilst towing. Without that rig there is no connection to the engine temp, the aircon fans cannot be switched by the engine temp.
Not so sure about that Wammers.
As said above on the 3.9 there is a simple thermoswitch in the coolant system to bring on the fans when additional cooling is required by the engine. It doesn't make sense to remove that feature for the P38
On the P38 I'll quote from Rave ETM
Condensor Fans:
The HEVAC panel provides a signal normally at 12 volts for "off" and 0 volts for the "on" condition, which in the case of Petrol vehicles is fed into the ECM and diesel vehicles directly onto the coil of relay 18. In line is the Air-Con pressure switch 2, which turns the fans on/off.
When Relay 18 is energised, it provides connection between relays 13 and 14 (fan 1 and 2) in series or parallel with each other. This is dependant upon the state of the air-con pressur switch 1.
For the petrol option, when the signal enters the ECM, the ECM will switch the output on/off to the fusebox.
The ECM will switch the output on sometimes for engine cooling reasons even though the engine has been turned off.
So on petrol models there is definitely the capacity for the fans to come on independantly of the aircon requirements. In the Circuit diagram there is an input from the engine coolant sensor into the HEVAC Ecu.
I can't make out if its the same for the diesel. The circuit diagram shows an input from the diesel ECM to the HEVAC same as the Petrol but the output does not go through the ECM. I would suspect that what this means is that the fans can only provide additional cooling while the engine is turned on in the case of the diesel model.
This might be down to the heavier starting requiremnts of the diesel.