Climate Control in 420SEL flips from cold to hot
 

Climate control in my 420SEL has a mind of its own. With the thermostat wheel set for 22C, with ambient temp 90F, the climate control starts out with high fan speed and cold air. Just when the cabin temp is getting comfortable, and fan speed is reducing........suddenly comes warm air from the side vents which quickly turns to hot air and the fan revs up again to high speed and the cabin heats to the point where windows have to be opened and the unit turned off. Lowering the thermostat almost to maximum cooling, and restarting the climate control, will once again bring cold air but it isn't long before it starts blowing hot air again.

I've searched the forum archives but can't seem to find another siuation exactly like what I've described. I've had the car to the best a/c shop in town, but they haven't been able to determine the cause.

Any input will be appreciated.

You have checked the mono valve, haven´t you?