Why doesn't my truck fan kick up on high when the tempature goes up

SOURCE: Extremely high idle speeds develped overnight

check idle speed control make your throttle valves not stuck.check for vacuum leak around the throttle body area look for broke gasket.

SOURCE: Cooling fans will not kick in.

The fans won't kick on then you need to replace the coolant temp sensor this runs your fans.

SOURCE: Here's a wierd cooling fan issue

Thats not too weird as the car would kick on the fans at full speed when it detects a problem with the sensor. Your ECU is not reading the sensor properly and needs to be checked by Ford. I hope this helps you.

SOURCE: Fan kicking in too often 1998 f150 4x4 4.6 tow

If the engine is not over heating then it is doing its job. The constant cycling is based on the temp of the air traveling through the radiator and the a/c condenser not the outside temp so much. The start up high rev is just how these clutches work in ford fans. It sounds like it is operating correctly.

SOURCE: COOLING FAN WONT KICK ON. DID A JUMP TO THE

The cooling fan relay is located in the engine compartment just behing the left headlamp...not under the dash. There are cooling fan test connectors built into the wiring harness near the left strut tower. The cooling fan relay is controlled by the cooling fan temperature switch located on the rear of the engine. There are 3 temperature sending units on your car if it is equipped with a temperature gauge. If not, there are 2 sending units, One is for the computer and the other is for the fan relay.

The one for the cooling fan will have 2 wires, one Tan and Orange and the other Brown and White.

The cooling fans and cooling fan relay get their power from fusible links built into the wiring harness near the starter relay.