I have a 2005 379X Pete C-15. The cooling fan comes on and off randomly, the temp gauge reads good temp. it will come on and run 2,3,4,5, min or stay on longer or all day. I can be idle it will come on while its 0 deg out side. when it comes on it will cool the engine to below 150 deg going down the road.When I shut the truck off you can hear the fan solinoid release air. I done a diag. from the dash and got a coolant temp sensor fault. I had it into Peterbilt Mississauga, they said it was the ECM and had moisture in it. , I had it removed and tested and it seemed fine other than coolant temp sensor faults. it was reinstalled and the coolant temp sensor replaced. it seemed ok when I picked the truck up but now its doing the same thing. When its not on , I can run it off and on from the dash fine. I have checked the A\C switches are off and temp controls are on heat. coolant is full no other warnings or faults. Thank you,
Could you have a bad switch in the heating cooling system that is making it think it is on defrost, which triggers the AC compressor, which will make the fan cycle? When it happens can you watch for the AC compressor engaging?
Is your fan clutch leaking air when the fan is disengaged? If it does you need a rebuild to your fan clutch. If your fan doesn't leake air try replacing that selenoid valve in your fire wall that has the air lines connected to it. That should solve your problem. Im no mechanic but have gone through these problems in my own trucks through the years. That shop just got you for your money. Hope i helped.
