Hi all,
I recently purchased a 4.0 96 XJ. I am puzzled by the cooling fan arrangement. My radiator fans continually run at approx. half speed, once ignition is on. Is this normal?
Looking at the circuit diagram there should be a ‘single pressure’ switch in the air con. On my car the connector is looped (and this appears to be factory) and as far as can determine, is the reason for the half speed.
I changed the fan module and it is still the same. Disconnecting the triple pressure and radiator sensor make no difference, but breaking either fan connection stops the fans.
So working out the circuit attached, this is how I believe the system works.
One fan has permanent earth, one has permanent 12v (73)
The lower relay has 12v from (74) and when closed the single contact links the two fan circuits together (effectively in series) and they then run at a reduced speed (approx. 50%).
The ‘single contact’ relay in the fan module can be called/activated by either the 86C radiator sensor or the single pressure switch, by earthing the coil.
On mine the single pressure switch is bypassed with a link, so the earth select for the single contact relay is always on. Is this like this on other cars? It is the 2 wire connector, by the 4 wire aircon pressure switch, at the back right of the engine bay.
For anyone who has read this far, and may be interested, when the radiator sensor is at 100C the upper, two contact relay is called putting both fans to full power. The lower fan has its own earth, and the upper fan its own 12v feed (76) and therefore the fans are now independent and run at full power. This does work on mine when I link the 100c connection.
Thanks
Steve

 

check the attached

 

Steve:
You got it! And yes, the AC switch modification was a TSB that allows the fans to run a low (series) speed all of the time. It was done to prevent the fans starting in high without running series first because that turned out to blow fuses due to starting current.

 

Thanks Ross,

Glad I am not losing the plot. Much appreciated.
At least I understand the fans now if anyone else asks.
Regards
Steve