C15 fan on continuous

I've searched the forum and read everything I could find as to what could and can cause this problem. Replaced A/C hi-pressure switch, jumped the harness. Fan still on. Changed fan solenoid on firewall, didn't fix it either. Changed IAT sensor, fan still on. When I check voltage going to the fan solenoid I had 12.22volts while engine was running, it would spike to 13.45volts when engine would shut off and briefly open the solenoid which would then disengage the fan briefly. This started as an intermittent problem and has now become continuous. Could it be a bad coolant temp sensor??? How would I diagnose the sensor with my ohm meter??? Checked the fan override switch in my dash and seems to work fine. Any help and ideas appreciated. Engine is a NXS C15. Thanks

 

Yes, the fan clutch disengages with shop air. No air leak at the fan clutch. The harness going to the fan solenoid has 12.22 volts when engine is running. At 12.22 volts the fan stay's engaged. When I turn the engine off, voltage at the harness spikes to 13.45 volts which energizes the solenoid to disengage the fan briefly. The fan's been on since I left Sheridan WY, drove across IH-90 all the way to Seattle WA, gross weight 130000lbs and not once did coolant temp go higher than 185deg's. I tried everything I could think of to create heat but the gauge never budged. I should also mention that the fan solenoid has air at the supply side. Thanks for your reply Heavyd.

 

Nope, nobody's been in the ecm...

 

I'm not very good with diagnosing wiring and sensors. Would you mind explain in detail how to do this??? Thanks

 

Stopped in at Peterbilt in Surrey BC yesterday. I asked them if they could check with CAT ET. Nope, they were to busy. Asked if they could print the diagnostic tree for me so I could check everything. Nope, they couldn't do that either. Come back Tuesday they said, nope say's I, gotta be in Calgary by Monday. Both service managers had never worked on the floor. I'm thinkin it's got something to do with the A/C as well. Forgot to mention in prev post's that when my A/C condenser blew in December the problem went away. It was intermittent then but with no Freon in the system the fan worked normal. Got the A/C fixed in January and the fan worked fine for a coupla day's but then started acting up again. It just gradually became more frequent that the fan would cycle for no reason and now it's just on contiuous. Appreciate all the help guy's. Thanks