M1078 fan clutch question

[twrecks]

So I located an M1078 and the gentleman I purchased it from told me it had a bad fan clutch and even included a new one in the deal. My question is before I go through the labor of installing it I want to make sure everything that makes it work is functioning. I've had a few old military trucks and gone through a lot of labor to find a stuck valve that was cleared in a few minutes. I have power at the circuit breaker and at the switch that turns it off for high water fording. With the switch in the normal position the fan is free wheeling and there is air pressure on the line going to the clutch. When I flip the switch to the off position there is still air pressure coming through the line going to the clutch. With the switch removed, there is air pressure going to the clutch. With the line disconnected it still free wheels. Does the air pressure cause it to engage or does lack of air pressure cause it to engage? Also, it ran for 35 minutes and the temp never went above 165 degrees according to the gauge, I read where these have a temp sending unit that will over ride the high water turn off switch if it starts getting hot. Is there a seperste temp sending unit that over rides? Any help appreciated.

 

[NDT]

With no air the clutch is engaged. He was right. Double check you have the right fan to go with the clutch you have. The fording switch overrides everything.