Fan override switch??

The ECM only supplies ground for that circuit when fan should be off. Any time the circuit is open the fan is on. All you would need to do is wire your override switch into the fan circuit.

 

Ok, I pulled out my wire diagrams. Cummins actually supplies power to the fan solenoid. Detroit is the one that supplies ground. The circuit is still the same though. Whether it is Cat or Cummins they all have the override switch between the ECM and fan solenoid. The switch doesn't route power back anywhere, it just opens the circuit to kill power to the fan. The ECM doesn't monitor anything. It just turns on the 12v when the fan is to be off and turns off the 12v when it wants the fan to be on. When you install your switch in between the ECM and fan, the ECM won't detect anything. As far as which wire at the fan solenoid, again it won't matter as long as the circuit is open the fan will come on. If you install your wiring before the solenoid, the solenoid will not get power when the switch is toggled and the fan will come on, if you install your wiring after the solenoid, the solenoid will not get a ground when the switch is toggled, and the fan will come on. 9100's are base models with no accessories. They meant to be cheap truck with no bells and whistles. That is why it doesn't have an override switch. The wiring will also be different compared to a 9400 slightly. I check these post a couple times a day. Don't hesitate to ask more questions if you need to.

 

Good info Heavyd......... common sense would want the override switch...

 

to simplify it

when you have juice going to the fan it allows the fan to freewheel or be off

when the power is turned off the fan will lock on which is a failsafe so if power is lost the fan will run continuously to prevent overheating

 

Exactly! You are correct!

 

all you have to do is run a wire from pin 23 on the oem connector on the ecm to a switch then from the switch then to ground

 

Umm, pin 23/wire 97AU OEM connector for his truck is idle speed increase/decrease! I wouldn't start cutting wires at the ECM and grounding them through a switch!

 

Leave all 12v positive wires alone and put a switch in earth circuit between solinoid and ground. This way its 2 less power wires randomly run to cab and back. run earth wire to cab through switch then to any earth in cab. worst that can happen is wire shorts to ground and fan will come on.