This is a bit different from some of the other threads on climate control. Car is a 94 6.0 and when I turn it to max cooling and AC on, I'd estimate about 70-80% of the air comes out the floor vents. Freezing feet are not nice! The centre vent does open and a trickle of air comes out, but nothing like the speed that it should move at. I have removed the right cheek panel and watched the centre vent vacuum motor and it does move from open to closed as I move the temperature control.

When I switch to defrost, the floor vent closes and the air comes out at the screen, so I know the lower blend door motor is working. Effectively, everything seems to be working as it should, except for very little air from the centre vent. Both blowers are working, I checked by pulling the fuse for the right side and left was running and there was a noticeable drop in air volume.

Any further tests I can run to diagnose this? I have read Warren's two PDF's and they don't seem to address the issue.

 

Try putting it in manual and full cold, this function is the same as the Mk3 the centre vent should open and blow cold air. If it doesn't then I would suspect a vacuum leak on the centre vent solenoid.

I know with the Mk3 in auto mode shuts the centre vent when not blowing cold and opens the feet, I really hate the system as there is not enough control where the air comes from

 

Jag64 - I feel your pain.
While i had NO air coming from the centre vent, I do know thanks to Warren and the vacuum tube not being connected.
The soliniod did look manky but I was happy with the final result.
Everything works as it should BUT I am still getting cold air from the foot vents, and freezing my feet but no where as bad as it was.
So the foot vents are not closing as they should..... what closes them?

 

The lower blend flap closes the foot vents, download the Delanair Mk3 introduction (I've posted it a few times so shoul d turn up in a search) which had diagnostic info in to for finding these type of faults.

 

The center vent does open, but only a trickle of air comes out. I looked under the dash and I can the vacuum motor moving through it's range. Most of the air still comes on the floor vents. I think I'll stick my inspection camera in there and see how much the center flap is moving.

 

The vacuum actuator for the centre vent is hidden up inside the dash and is not visible without the entire dash out. There are 2 motors on the RH side of the Climate Control unit these are the upper and lower blend flap motors. In your 96 there will be a rectangular box that has 5 vacuum hoses this is the solenoid valve pack. The documentation does not say which output controls the centre vent.

There are restrictors in vacuum lines these get clogged and could cause the issue you have, remove them and see if the vent opens. The restrictors are there so the flaps open and close slowly so they don't thump.

 

I removed the centre vent and the vac motor is right there. I put my hand vacuum pump on it, and the flap moves about half way open under 25" Hg. I can move it fully open with my finger, but there seems to be a fair bit of slop at the joint between the flap and vac motor. I can't see the joint without removing the dash though. The vac line in the photo was pulled off for testing.

 

Bugger........ that is probably going to be a dash out job to fix it.

 

From an ebay photo there appears to be a bolt adjustment, perhaps it has slipped? I'll have to dig deeper and see if I can get to it without a dash out. I have pulled the dash before and it's not too bad now that I have made the tool for disarming the airbag.

 

Maybe the screw has come loose,