Engine fan running constantly

The module will be located on the firewall, the switches are located under the cab.

 

The ECM controls the fan. Power is at the solenoid all the time the key is on and the ECM controls the ground side to turn off the fan. If everything is ok with the wiring as soon as you start the engine there should be split second with the fan actually off, then you hear the solenoid chirp and then the fan comes on and stays on. This means the ECM is seeing some kind of condition to need the fan on all the time. This could be high water or oil temp, or a/c high pressure input. Since you have no engine codes and you have played with a/c pressure switches I am guessing there may be wire problem. When you start the engine if the fan is just on all the time no matter what you could have an open wire between the fan solenoid and the ECM. If I have the correct diagram, your solenoid ground wire will be black to the connector, then it goes to a tan coloured wire numbered 23D. This wire goes directly to the ECM, no other connectors except right at the ECM. You can try to trace this wire to see if it is rubbed through anywhere or maybe no good right the connector at the solenoid. These wires get brittle after time and can break inside the insulation. You can also try to wiggle the harness to find the bad spot and see if the fan releases. 23D goes to the OEM connector cavity A2, you may need to replace the wire. Don't overlook the connector at the ECM too, you may have bad pins in there. Keep us posted!

 

before i have the same problem ,was the thermostat