Cummins running ruff

[the missing link]

2 things with the fan
1 the fan clutch is engaged with no air presser , until you hit 90 psi it will not put air to the fan to shut it off. When air is sent to the fan it kicks out the clutch
2 your a/c will turn your fan on / off ,when you start you truck or stop. If you shut off your heater controls this should tell if it is you a/c . Your heater will also use your a/ c to take humidly out of the air
At least on my trucks this is how it works

Hope this helps.