1998 Peterbilt clutch fan won't engage on auto

Ok?

 

You could just buy the switch with the air lines going to it, but 1st find if no air is in any of tanks under truck, then see if fan will turn, now if it wont turn you know that the truck has a air off fan on type of switch, if it turns you have the air on fan on type of switch? thats just so you know what type of switch to buy

 

Every time mine quit it was sensor on the front of thermostat housing! Than one time it was 20 year old wiring harness!

 

Off topic, but your thumbnail is pure gold

 

Some of them had a high pressure A/C on the top left of the radiator. If it was unplugged the fan would come on. Cats had a temperature switch close to the thermostat. Does it have power to one side? What happens if you cross it. Does it have power to both? What happens it you turn it off, unhook one and start it back up.
If you list the last 6 of the vin some others here may help you a lot more than me. Pictures send pictures.
Does it not come on for temperature. A/C head pressure or both.

 

Are you sure it's getting hot enough to kick on automatically? A Cat engine doesn't kick fan on till 217 degrees