The fan on my truck engages automatically when first starting for about 15-20 seconds, then disengages. With the air system pressurized, engine off, key on, when engaging the clutch manually, the fan physically moves in towards the engine. However, when sending air manually with the engine running, fan will not engage. Truck has yet to get over about 190 when running, but I have never heard the fan engage at any other time other than when first starting. Any ideas?