The HOT "click" position of the temperature wheel forces the ACC to go to maximum heat. Once out of the "click," the ACC is supposed to modulate the monovalve to reduce the heat to the temp set by the wheel. Air coming out of the center vents means the ACC is going into "cool" mode. With the temp dial set to "almost" the 'click' position, the ACC should still be in mostly heat mode. The fact that you are getting cool air (after the heater core has a few minutes to cool off) means that the ACC is not operating the monovalve correctly.

The monovalve defaults to "open," a fail-safe intended to make sure you can always defrost your windows. The 12 Volt control line from the ACC tells the monovalve to close part of the time (it pulses on/off with a variable duty cycle), thus reducing the amount of hot water allowed through the heater core. If you have a choice between "all heat" and "no heat," then the monovalve is working but the ACC is not controlling it correctly.

To be on the safe side, and to check the easy parts first, you should take the monovalve apart and make sure it's clean and moving up and down easily. There is a small chance that it is sticking and so working only "all or nothing." Since the monovalve is easier to fix than the ACC, look at it first. If it is troublesome, you can get a kit for about $40 to replace the guts.

If the monovalve seems to be OK, the problem is the ACC. Resoldering all of the contacts has worked for many forumites, including me. There are also rebuilt switch units available, unfortunately fairly expensive.

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970