Did you try pressing O to turn the system off?

If the mono valve is not working correctly then turning on the A/C would have no affect.

I have heard of people removing the hot water hose going into the mono valve and stuffing it with a rag to slow down and stop the flow of hot water getting to the heater core. This may make it tolerable till you can fix the mono valve.

Hope This Helps!

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William
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1987 420 SEL - Like a fine wine only better with age