1978 450SEL Heater Controls

[WGB]

My 1978 450SEL has a working A/C but a wavering temperature control
and the up and down vent controls seem to work somewhat
ineffectually and the opposite to what is intended. When I set the
lever for the upper setting it blows better at the feet and Vice-
versa. The right hand temperature control lever also seems to Jam
unless the nut at the bottom is loosened off suggesting a sticking
vent flap deep in the bowels.

If the heat controls are set to off the water valve shuts as
intended and allows the A/C to work but the heat is either on or off.
The system has improved dramatically since I replaced the relay in
the drivers kick panel which energises the system and at least I get
either hot or cold and not both when it feels like it.

I would appreciate any single or collective experience before I
blindly pull the dash apart and re-invent the wheel.

The system has the two rotating dials one above the other in the
console - top works the fanspeed and the bottom works the A/C with a
heater slide either side with a vent control inside and in parallel.