With the truck on, the fluid should have some "motion" to it. With the cap off, if I turn the wheel, you can see a little reaction in the fluid. No motion at ALL makes me wonder if
a) pulley is free-wheeling on the impeller shaft. I've never seen it happen, but I've had pulleys dead spin on alternators. Afaik, the pulley CAN seperate from the shaft....check out a pic from a thread a while back on ls1tech.
