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The water pump pulley is held with a special holder and the wrench is used on the fan nut to remove the clutch.

These usually can be loaned from an auto parts store loan-a-tool program.

Here are GM instructions for fan clutch replacement.

Cooling Fan and Shroud Replacement

1. Remove the hood latch support.
   
   
2. Disconnect the transmission cooler lines at the engine and release the lines from the fan shroud.
3. Remove the 2 upper bolts on the fan shroud.
4. Drain the cooling system.
5. Reposition the upper inlet radiator hose clamp
6. Remove the upper inlet radiator hose from the radiator.
7. Remove the electrical connector from the shroud.
8. Position the water pump so the bolts are aligned in the vertical.
   
   
   
9. Remove the fan hub nut from the water pump shaft in a counterclockwise rotation. Using the J 46406 , in order to secure the water pump pulley, loosen the cooling fan hub nut from the water pump shaft.
   
   
   
10. Unclip the fan shroud from the radiator at the side panels (1).
11. Tilt the radiator and the condenser forward.
12. Lift the fan and the shroud up and out towards the engine to release the fan from the radiator to clear the radiator inlet.

Remove the cooling fan and shroud.

Below is a picture of the fan clutch tools