560 SEC Heating Problem

I need help tracking down or diagnosing a heating problem If I have the Rotary dial set at Max heat, and the EC button depressed, I get a bit of heat from the bottom vents, If I select either full defrost or
the complete up and down setting it blows mostly cold air. Could this be the control unit? Is there a simple troubleshooting procedure that I could use to eliminate possible causes one at a time? This problem seems to be intermittent along with the high pithed squeal that I have been hearing
Any help or pointers are very much appreciated
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Defrost IS the diagnostic. If it doesn't blow lots of hot air then, it's busted.
It's full heat on, full blower un, upward windscreen vents open.
I'm tempted to say monovalve:
    http://articles.mbz.org/hvac/plumbing/monovalve /
But because you can get both hot air and cold air, maybe not, usually (USUALLY( it failed stuck open or closed but seldom varying. But it is possible. The mono valve controls the temperature of the water reaching the hater core by being pulsw width modulated by a controller near the blower which is told what to do by the pushbutton assembly.
At this age you probabky want to replace the monovalve anyway - the insert is cheap and that rubber bit is about worn out anyway.
If that doesn't fix it (it might) then find a good used pushbutton assembly (ebay?) as 99% of the problems with heat in these chassis is that wretched thing. I've still never seen any car that had any other problem with any other bit of this one of the worlds most arcane and overcomplicated heating systems. Keep in mind the manual for the HVAC is larger than the manaul for the entire engine/chassis.
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