It is threaded onto the pump shaft. You need a special tool that will hold the pulley and a large wrench for the bolt behind the fan. you can rent the tool at napa. These tools can be rented at any Autozone as well. Ask for the fan clutch wrench set.
If you don't have access to NAPA or one of those tools, here's another suggestion. Leave the serpentine belt on, put a large wrench on the nut and tap it with a hammer to break it loose. The belt will hold enough tension on the water pump pulley to allow most of them to be broke loose.
Here's my method: remove the water pump with the fan attached. be especially careful not to damage the radiator fins. put the whole assembly on a work bench and place a strap wrench (they're cheap and very useful for other projects) on the pulley and a big crescent on the fan nut. that nut is a right hand thread and isn't on there very tight so it won't take much to break it loose.
I just did my water pump on my 98 Silverado. It should only take about a half hour to do the job from start to finish if you have the right tools. I left everything on the water pump, but pulled the shroud off for more room, then got the right tool from NAPA, (loan tools) to get the clutch fan off the pump.
I had the same problem . Fooled around with it for a while (several hours) then asked someone and it was simple. Thread 2 nuts down on the bolt. Tighten them together. Put a wrench on the bottom one. Turn counterclockwise.
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The clutch fan on a 2004 Chevy Silverado is removed by loosening the alternator bracket and pulling the drive belt. The fan's retaining bolts can then be removed and pulled from the engine.
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You will need to remove the transmission to replace the clutch slave.
disconnect the hose and remove 2 bolts.
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you have to remove transmission to take out and replace throwout bearing
Replace the cabin filter. remove panel under dash on pass side.
To change the return filter on a 2003 Chevy Silverado is easy. Simply open the hood of your car and gently remove the filter and replace with new filter. that it!
You have to remove the transmission to get to the clutch slave cylinder on that truck. You can pull the engine if you want and get to it. YOUR CHOICE.
remove the bed with 2 persones and u will remove the pump from the top not the bottom.