MBE4000 Cooling Fan Problem

Ok, here's what I did for anyone else interested...

The solenoid has two wires, marked on the plug itself, A and B. Wire A is the positive wire and wire B is the ground. With it connected to the solenoid, wire A shows the running voltage of my truck, 14.2 volts. Wire B shows 2.6 volts, I don't understand why but it does. If you disconnect the plug and check it again, wire B shows nothing. This is checking the wires coming from the truck, not the actual solenoid.

I took wire B and cut it. I ran the wire from the truck wiring to a switch with 3 poles and two positions. The wire from the solenoid, I ran to the switch also. The wire going to the solenoid I put on the center pin of the switch and put the trucks wire (from the ECM I assume) to one of the outside poles. The other outside pole, I ran a wire from it to a chassis ground.

Like this, in one position, the solenoid is basically as wired from the factory and flipping the switch changes it to a grounded state and disconnects the wire from the ECM. I wanted to make sure I wasn't grounding that wire from the ECM since I was seeing that 2.6 volts on it for some reason. The fan works off the ECM in one position and manually ones on in the other. The only issue is when manually engaged, the check engine light comes on. I'm assuming this is because there is no feedback to the ECM and maybe this could be resolved with a capacitor of some sort, not sure. Soon as I flip the switch back to normal mode, the check engine light goes off.

So, so far so good. Thank you guys for answering and giving me the information I needed. Maybe this will help someone else too.