No servo on the 84, you have a monovalve. Should be easy to locate, has a two wire connector.

Unplug the connector -- you should be roasted out of the car -- full heat. IF not, you have either a plugged heater core or stuck monovalve (four screws in plastic -- be carefull going back in) -- if the bellows is wrinkled and bulged, replace valve. If the armature is stuck in the solenoid part, you will have to replace that, too.

Also check the aux. heater pump. If it isn't running, it will cause heat problems. Sadly, when it burns out it usually takes the pushbutton unit with it, you may need to replace both.

Peter

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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!