I have a 1995 F150 5.0 A/C 4R70W with a shudder problem.
It happens at 1500 RPM while driving or parked.
Take the serpentine belt off and no shudder at 1500 RPM.
Replaced fan clutch and that did not change it at all.
Engine idles very smooth with no shudder until you raise the RPM and then the bad shudder at 1500 RPM, above this RPM the engine is smooth again.

I am thinking Air Pump, does anybody have any other thoughts.

 

water pump?

with belt off try moving it on its axis.

does air pump seem loose or making any noise?

 

could be, it does squeak when you turn it.
No, not that I know of but it does have a lot of fan noise.

 

Yeah I would follow quicklook's procedure for all of the pulleys. Move them up/down and side-to-side. Any that have a lot of play could be your culprit.

Does it do it with both the A/C on and off? ie, the A/C clutch both engaged and disengaged.

-Chris

 

my bearing went bad in my water pump and the belt started moving front and back a little on the w.p. pulley.

had a strange vibration if i remember right.

 

Could also be a weak spring on your tensioner (depending on how bad the shake is), although that would typically cause a vibration at lower RPMs if I remember correctly...

-Chris

 

Thanks, I will look at it in a day or two, I have to work on a BII and a Ranger today.