Hi,
I have a Whirlpool SAU500W1. The lower spray arm is not spinning. If I try to turn it by hand, it bumps into a post sticking up from the upper pump housing assembly. There is some play (up and down) in the spray arm; if I lift up on it, I can rotate it freely.

This leads me to think that there is - or was - something like a washer that held the spray arm up higher that may have broken away.

I found the parts diagrams on this site and compared the relevant pieces, but I can't find anything missing.

Diagrams here, btw, and #4 is the pertinent one:
Model SAU500W1

I did not remove the pump housing assembly to check within it. I'm wondering if the spray arm itself should have had a washer that maybe was adhered to it and therefore not listed as a separate part? It's all metal, and I don't see any sign of leftover adhesive.

Any ideas? Thanks in advance.

Does it sound like the pump is moving any water, and does the tub fill with water? The spray arm will ride high and probably up and over that post when water is pumping through it so I don't think you have a washer of some sort that up and disappeared. I would guess your pump is not moving any water, or if it is, not under enough pressure to get the spray arm up and over the post. Verify you have water to the tub and that the pump is working, then get into the thing and see if you find anything that may prevent it from pumping water such as a broken impeller.

No problem. Give it a try and see what happens. If indeed it looks like a pump, a new unit may be the way to go simply for the energy efficiency of the new models, but those are personal decisions. Let us know if you need further assistance.