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

Depends on what that sensor was for, a guess is thats the high pressure sensor. So it is usually a Normally Closed switch that opens at a certain pressure to complete the circuit. The paper clip closed the circuit so the ECM read the system as having normal pressure.

NC switches are ones that complete the circuit until pressure is applied, then the switch opens and no power goes through. NO (Normally Open) interrupt the circuit until pressure is applied. then the circuit is closed and power goes through the switch.

An example of this a brake light switch. These are usually NO until you step on the pedal and air pressure closes the switch and the brake lights come on.
Hanadarko had a problem with the parking brake switch on his DD-60. That switch is a NC switch until you push the yellow knob in and release the brakes. Pressure gets applied to that circuit and the switch gets opened.

You can test these by using a jumper wire or disconnecting them to replicate what the switch is supposed to do and not do, IE either on or off

 

does your truck have a APADS to control your ac. These units do go bad, and cause the engine fan to stay on. Just a thought, you might wanna take a look at that.

 

I JUST went through the same thing on my '04 W900- Only after second sensor, SM at South LA Kenworth informed me the problem was not sensor, but the harness (near the AC drier, on radiator). I cleaned all the connections, problem solved, for a while. Now and again, I have to unplug/plug to gain contact. Stop at KW is in tomorrows plans.

 

Sometimes we see the actual pins in the connector at the high pressure switch are spread open. The paper clip trick can sometimes make it worse. The paper clip can be thicker than the terminal that is suppose to go in there and bend the little terminals too wide. This will cause goofy operations too.

 

I have same problem. Where is this sensor?

 

Sometimes plugs get worn out with vibration from the engine and no matter what you do you just have to get new plugs.
I had plugs go bad on a 96 Peterbilt 357 Dump Truck. Both hi and low switches on that truck were sent a ground from the control panel.
With AC switched to on that system sends a ground strait to the low side switch 1st, then through the high side switch and then to pin 85 in the AC relay located in the fuse box.
On that AC relay pins 30 and 86 are hot with the key on but no ground to pull in the relay is found until it passes through both hi and low switches.

And you know the dealer ain't gonna put 5 inches of wire on a ready to solder plug, you have to put the entire plug together, crimp on the new pins, install them into the connector, shove in the little water tight seals, snap on the cap, then solder and heat shrink the wires that you had to buy elsewhere.
Preferably with glue lined heat shrink.

 

I am having this same problem on my 2006 western star with a c15 accert. I will go inspect my plug to the sensor. I had the system redone, new compressor and drier. New pressure switch. unplugging the compressor is the only way I have turned off the ac compressor. Engine fan is stuck on. Using jumper wire I verified, fan selinoid is good. Unplugged the pressure switch no change. Plugged the old switch in, but not screwed onto the refrigerant pipe. No change. All ac switches dash and sleeper and defrost turned off. Wanted to try the paperclip truck, but this sensor has 4 connections in the plug. Not sure what all those connections do, and did not want to short out anything. Any suggestions? I do believe the engine fan and ac compressor problem is related