help. a/c on, engine fan wont shut off

Whenever I turn the a/c on or use the defrost my engine fan comes on and wont go back off even if i turn the a/c off. A mechanic told me where to unplug the a/c fan harness from a schrader valved sensor then put a jumper wire in the harness. I did and it worked. This just happened last night. I had barely enough time to grab a new sensor at a kenworth dealer slap it on, and make my delivery appointment. So new sensor on there and i have the same problem fan will not go off. So is that brand new sensor defective or is my trouble something else? Rite now i have the harness jumpered so the fan will stay off. Will i be ok to roll like that a few days? I can turn the fan on/offf manually i guess if i idle with ac on just flip the fan on??

 

Thanks for your input guys! I should have posted more info but it's a pain posting when I have to do it from my cell phone like I did yesterday. Ok here's some more info..

It's a C15 bridge motor unbridged 550hp1850tq 6NZ flash 2003 T600. First off I watc h my gauges like a hawk and even when it was stock MBN ecm always turned the engine fan on manually when the water temps hit 201* on the digital Cat driver information display and pull it bac k down to 188* where I shut it off. The highest I ever see it go even with the fan on is 204* the factory has the fan set up to come on automatically at 217* which I always thought was way too high and late... So don't worry I'm not going to overheat my motor watch the gauges religiously and use the fan as needed.... .....I didnt just graduate from truck driving school lol...

The a/c system was completely redone this past spring and works fine. New compressor, new drier, two new expansion valves and filled with freon to factory specs. Worked fine all summer causing no problems.

My engine fan automatically comes on to pull air through the a/c condensor when I'm parked idling or when I'm slowly driving through a parking lot in 2nd or 3rd gear. When I go down an on ramp and pick up speed to 65-70mph it will stay on for a couple of minutes then goes off and never comes back on again as long as I'm at highway speeds. I say "never" ocassionally on a hot summer day pulling a mountain grade it will automatically come on but again that is very rare and my engine water temps never go over 201-204* so you might as well say it never comes on.

This all happened to me wednesday night going south on I-81 in VA. It was nighttime and the outside ambient temp was about 50*. It was raining. I'm pulling a hill loaded heavy and my water temp was at 192* ( my motor normally runs 188-192* all the time) with the defrost on (the a/c is on when defrost is selected) and my fan comes on. It ran continuously and would not stop even after going to vent with the a/c off for about 50 miles. I stop and shut her down to grab a burger at the 101 exit and check things over. I get back in the truck and make sure the a/c is off before I crank it back up. It starts and the fan is not running. I go about 10 miles down the road and turn the deforst on to de-fog my windshield and almost immediately the fan comes on and again will not shut off for anything. I drive on down to knoxville with the fan on the whole time and stop at MHC Kenworth...

A mechanic there told me to unplug the a/c harness from the switch fan override sensor and to stick a paperclip in the harness to jumper it and see if that made the fan go off. Well, it worked... So the sensor was bad and I bought a new one and swapped it out right there in the parking lot. I was pressed for time to make mny delivery that morning and left but it was still doing the same thing as if changing the sensors out helped nothing..... So when I get to my delivery I put the jumper paperclip back in there so the fan would stay off ( I got 4.8mpg!!! because the fan was running) and stopped at nashville later that night to get ANOTHER $45 fan switch sensor... I'm going to put that on in the morning and HOPE it fixes it if not I dont know what to do...

I was really wondering if it would damage my a/c system to run with that jumpered wire for a few days or weeks if needed??? If I'm sitting around idling and the fan is not manually switched on will it damage my a/c system? I can turn it on manually if so... I just hope this second brand new sensor fixes it and the first one was faultly... It was made in mexico... Only concerned about POSSIBLYdamaging the a/c not worried at all about overheating the motor trust me that AINT going to happen the way I watch gauges and use the fan....

 

I replaced the harness last spring when we re-did the whole a/c system but anyways I was worried over nothing. Put another new $45 fan switch sensor in it today and the problem is solved. So that first new sensor was faulty. That's made in Mexico quality for you... Thanks for all your input drivers maybe this thread will help somebody else out some day...