No, the '85 300tdt bears very little similarity. Your car has no monovalve although it has a heater control valve that some folks will mistakenly call a monovalve. Forget about comments from those who are familiar with the earlier car and not familiar with yours.

From your symptoms, the first thing I would check would be to see that the system is drawing a cabin air sample over the temp sensor. There is a little grille near the inside rear view mirror. From that is a tube that snakes around to a little fan motor that draws air and blows it across the temp sensor.

To check it, put some smoke near the grille while the ignition switch is ON and ensure that the smoke is drawn through the grille. If not find the little fan motor and see if it is turning. If not, check for voltage. If voltage is present, replace the fan motor.

The location of the fan motor varies throughout the 124 series. Mine is in front of and to the right of the glove box. It is easy enough to remove the glove box and look for it there. If it isn't there, it may be somewhere near the center console.

Best of luck,