740 740 stuck on defrost

I have a 1989 740 with the manual heater, not auto climate control.

Thing is, the position is stuck on windscreen defrost outlet, sliding the position bar left or right does nothing. It seems as though nothing moves behind the dash when moving the knob.

What to do?

As far as I'm aware it's cable controlled. Therefore, I'd check the cable.

Different car but the principle should be the same as on my 97 940 with manual control.
I had the same issue and found the cable came loose where it's fixed on the heater pipe and was just bending everywhere when I rotated the knob and didn't have enough pressure to actually move the flap in the distributor box. It's a sturdy cable like the throttle cable.

Check the cable is still connected and operating the flap. You should be able to have a look at it by removing the panel below the steering wheel, or maybe on the passenger side in RHD case.

Everything, for the most part, is vacuum controlled. There's a series of hoses connected on a wheel behind the slider. So either you got a bad vacuum leak or a hose or 2 is disconnected or the connection to the wheel is broken. Pull the dash apart and physically look is your best option. Be careful about how hard you pull on the hoses when doing so though.

Sorry about the misinformation on my part. It looked like a cable on the diagrams.

Interesting that the 740 is vacuum controlled but my late 940 is cable only.

What about the floor? Usually the dash vents fail but you have floor and defrost. Dual vacuum actuator failure- Just disconnect yellow line from it and plug with the screw.