Trouble shooting the cooling fan on the Jeep Grand Cherokee Diesel. The cooling fan will remain on maximum speed if there is an issue with the coolant temperature sensor on the front of the water pump or the wiring to it.
A common cause for this wiring to become damaged is when the cover for the high pressure diesel pump is removed and reinstalled. There is a small channel in the cover for the wiring harness to run, if you get this slightly out of place, when you tighten the cover boots it can cut through the wiring inside the insulating harness.
Watch the short video to see this exact fault.