1996 Kenworth T400B with Cat Engine 3306.

The engine fan is always on. There is power to the fan selinoid all the time. does anybody have a wiring diagram

You should have a switch on your dash for the engine fan. You can put it on or on automatic. If the switch is in the "on" position it will stay on all the time. I would check that first. If it is in the "on" position all you need to do is flip the switch to "auto."

the switch is in auto

ECM programming maybe.

Does your engine temperature guage properly?

Have you disconnected your engine temperature sensor and tried it?

As far as we know the temp guage is working correct. Yes we have unplugged the sensor and switch.

Is air pressure getting to the fan clutch fitting?

yes it is