When in heat mode, blower operates normally - vents operate normally but (sometimes!!!) when air is selected, dash vents fail to open and blower will not go to max speed whether manual speed selected or automatic speed.
Other times the vents operate normally and the blower goes to full speed (car is hot inside) but the blower speed slows down during normal acceleration (don't accelerate hard since my fuel is 2.60/gal these days) and speeds up again when not accelerating.
Thought maybe an vacuum leak only when AC is selected but normal operating blows that away.
Any ideas from anyone?

More than likely your "double acting" vacuum motor that controls the air mixing had developed a leak. Look in the FAQs for the replacement methods. (Not for the faint of heart!) I used the "factory" procedure to replace mine and it took the better part of a weekend. I can e-mail you a scaned copy of my manual in .PDF format if you want.

BTDT!
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'96 965, 16' wheels, rear 18mm bar & Koni + 204HP cams at 124K. Had '85 745 Turbo Diesel for 200K.

It is possible that you have some sort of leak in the vac system, but here is one thing else to look for. A while back my car was experiencing some intermittant problems with the ac where the blower would be running but no air was getting in because the vents were closed. Turns out there is a control module not too far up under the dash on the passenger side that has a connector on it with multiple electrical and vac connections if I remember right. To get to it, you have to remove some of the lower panels on the passenger side dash. It is small, gold colored box that is held in with one or two screws. I removed it and saw several cold solder joints (cracks). I resoldered them and put it back in and that fixed the problems. You might be having the same problem. Let us know.
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97 S90 with 182K miles still going strong

have the same problem, and just live with it sad to say. look in the 200/900 faq for a lot of tips but none worked for me

You probably have a leaky vacuum motor somewhere under the dash. Others in this forum can pinpoint the trouble spot.
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Roy Olson

Just a thought, but your AC may cut out on accelleration to give you more power for a short time. Most cars do this, but it's not really noticeable. The heater is just using the fan and hot water to provide heat, but the AC actually draws power from the engine via the belt system.

If you're doing a decent speed and start to go up a steep hill, does it also cut out? Check your owners' manual to see if they say anything about this.

Good Luck!

Steve

Thanks Steve, but this just started. Is not really an advanced feature of the system and previously always maintained speed required for cooling, automatically decreasing or increasing as cooling load demand varied.
Thanks