The pulley is a press-fit so if it slid off chances are the shaft of the pump is chewed up too. The belt should have been making a horrendous squeal. If the pulley is on too far or not far enough by 1/16" the belt is likely to squeal.
You need a special pressing tool that threads into the center of the pump's shaft. Many auto parts stores will rent or borrow that tool to you. Whether you need to remove the pump depends on the engine and how much clearance there is. Some of the new pumps and new pulleys come with a spacer that must be used with that tool to set where the pulley sits on the shaft. You can throw that spacer away after the pulley is installed. Normally we have the old pulley to look at the rust in the center hole to determine how far it was on before. My best suggestion is to look at a similar van in a salvage yard to see if the end of the pulley's hub is flush with the end of the shaft or if it sticks out a little. If it's flush, you don't use any spacer with the tool. Just press the pulley on as far as it will go, which is flush.
Monday, April 29th, 2013 AT 4:41 PM