Below is a summary from the Ford Service Manual for testing the fan clutch ...and as you can see when the engine compartment temperature reaches 205*F the fan clutch fully engages and provides a maximum fan speed
Fan Clutch Test ...Maximum Speed Requirement...
1) Block off areas on each side of the radiator in the engine compartment and the front of the radiator grille. This will raise the temperature of the air striking the fan clutch and should cause the fan blade to operate at maximum speed.
2) Start the engine and run it at approximately 1,500 rpm until the normal operating temperature has been achieved.
3) Place the climate control function selector switch in the MAX A/C position and the blower motor switch in the HI position.
4) Adjust the strobe to 3,600 rpm for 7.3L diesel engines and adjust the engine speed until the strobe light flash and the water pump pulley mark are synchronized.
5) NOTE: The temperature of the air hitting the fan clutch should be above 96°C (205°F) for maximum fan speed.
6) If the fan blade speed is less than 2,850 rpm on 7.3L engines, install a new fan clutch.
Here's the complete version...
Originally Posted by
danskool
...This help? found this out of my Service Manual ...Fan Clutch Test
Spin the fan blade (8600) by hand. A light resistance should be felt. If there is no resistance or very high resistance, the minimum and maximum fan speeds must be checked as follows:
Fan Clutch Test—Minimum Speed Requirement
Use a suitable marker to mark the water pump pulley (8509), one of the fan blade retaining bolts and the crankshaft pulley (6312).
Connect a tachometer to the engine.
Install a throttle adjusting tool.
Connect the Digital Photoelectric Tachometer.
WARNING: To avoid the possibility of personal injury or damage to the vehicle, do not operate the engine until the fan blade has been first examined for possible cracks and separation.
Start the engine and run it at approximately 1,500 rpm until the normal operating temperature has been achieved.
Operate the strobe light at 4,320 rpm for 5.4L and 6.8L engines and 3600 rpm for 7.3L diesel engines, and aim it at the water pump pulley. Adjust the engine speed until the light flash and the water pump pulley mark are synchronized.
Aim the strobe light at the fan blade bolts. Adjust the strobe light until the light flash is synchronized with the marked fan blade bolt (the fan blade appears to stand still).
The fan blade speed must not be greater than 1,900 rpm on 5.4L engines and 2,000 rpm on 6.8L and 7.3L engines.
Turn the engine off.
If the fan blade speed was greater than 1,900 rpm (on 5.4L) (on diesel engines and 6.8L, 2,000 rpm), install a new fan clutch (8A616).
Fan Clutch Test—Maximum Speed Requirement
Perform Steps 1 through 5 of the Fan Clutch Test—Minimum Speed Requirement.
NOTE: The temperature of the air hitting the fan clutch should be above 96°C (205°F) for maximum fan speed.
Block off areas on each side of the radiator in the engine compartment and the front of the radiator grille (8200). This will raise the temperature of the air striking the fan clutch and should cause the fan blade to operate at maximum speed.
Place the climate control function selector switch in the MAX A/C position and the blower motor switch in the HI position.
Adjust the strobe to 4,320 rpm for 5.4L and 6.8L engines and 3,600 rpm for 7.3L diesel engines.
WARNING: To avoid the possibility of personal injury or damage to the vehicle, do not operate the engine until the fan blade has been first examined for possible cracks and separation.
Start the engine and adjust the engine speed until the strobe light flash and the water pump pulley mark are synchronized.
Aim the strobe light at the fan blade retaining bolts. Adjust the strobe light until the light flash is synchronized with the marked fan blade bolt (the fan blade appears to stand still).
If the fan blade speed is less than 3,600 rpm on 5.4L engines and 3,500 rpm on 6.8L engines, 2,850 rpm on 7.3L engines, install a new fan clutch.