Here are the testing notes from the manual:
ELECTRONICALLY CONTROLLED VISCOUS DRIVE – 5.9L DIESEL
If the fan assembly does not free-wheel and a metallic
grinding sound exists, replace the electronically controlled
fan drive (Refer to 7 - COOLING/ENGINE/RADIATOR
FAN - REMOVAL).
NOTE: The following test may take up to 15 minutes
to perform. The engine should be at normal operating temperature.
1. Set the parking brake and verify the transmission
is in park or neutral.
2. Set air conditioner (if equipped) and blower fan to
3. Start and allow engine to reach normal operating
4. Stop engine, connect the scan tool and select
appropriate model year and engine option.
5. Check for and correct existing DTC’s
6. Using Tool 6801, backprobe pin 1 of the electronically controlled viscous fan drive connector, with the harness
connected located at the lower fan shroud to battery ground.
NOTE: The fan drive control coil is energized to 12 volts at this time.
WARNING: A spark may occur when connecting pin to ground. Be sure that no combustible material is in
7. Using the scan tool, verify that DTC 0480 set. If it is not set, verify that a good ground is available.
8. Start the engine.
9. Actuate engine speed to 2000 RPM.
10. Go to the SENSOR screen and observe the fan speed.
11. Run the engine at 2000 RPM until the fan speed increases to 1850 RPM or more for 30 seconds.
NOTE: Fan RPM may ramp up slowly.
NOTE: It maybe take 15 minutes before fan speed increases.
12. The fan speed should be in accordance to the table below.
13. If fan speed does not increase, make sure that the jumper wire has a good connection. If so replace the electronically
control viscous fan drive.