RE: a 1985 300TDT - I don't have any repair manuals to work with so can someone tell me what to check next to repair the constant flow of hot air? Regardless of where the thumbwheel is, it's always hot. The mono-valve was recently replaced (about six-months ago). What's next??

You can test the monovalve -- unplug and apply battery voltage across it, should close and the air will cool off. If it won't close, the solenoid is probably corroded from leakage and you need to replace the whole thing, not just the valve and armature. Make sure it's not just stuck, too.

Test pushbutton unit by checking for voltage at the monovalve connection. If none, the PBCU is bad, get a new or used one and swap it out.

This happened to a friend of mine a couple years ago, he had to replace the PBCU. $75 at the junk yard.

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Thanks, but what's the PBCU??

The "push-button control unit"

The box in the dash with the buttons and temp wheel. If the monovalve is good, the PBCU isn't working.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!