Radiator fan won't kick in

My son's car is getting hot, so I have gone through the cooling system part by part. Replaced water pump, temperature sensor and thermostat, flushed the system and started back up, and found that the fan is not kicking in.

Replaced the fan relay and still nothing. Replaced the fan motor. Still nothing. The lead wire from the relay to the fan motor ohms out ok, and the black wire has continuity to ground. The only time the fan turns is right when you start the car, and it comes on for about 3 seconds then stops.

If I didn't have a crew cut, I'd be pulling my hair out on this one.

Any suggestions?

How hot do you let the car get before you decide it isnt coming on? I thought mine was doing the same thing and i had to wait until the car got really hot. Almost until the end of the normal range on the dial.

let it get to the upper 1/4 of the normal range. I temporarily ran a switch from the battery to under the dash and back to the fan so my son can use it for work and school. He just has to remember to cut it off to keep from draining the battery.

Still would like a permanent solution.

I thought that my fan wasn't coming on either but it comes on when it's over halfway almost to hot... If you unplug the sensor on the motor does the fan come on? Does it run when the a/c is on?