You say up to operating temp - what is that? My SD stays around 82C fully warmed.

If it's not that warm, try a new t-stat.

If it is, check or replace monovalve again (unplug to get flow) and check aux pump again. Otherwise, you might have a clogged heater core.

The ACC turns on the compressor in all modes except econ. Sounds like your A/C works better than your heater!

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1984 300Sd 210k

Former cars:
1984 300D 445k (!!) (Strider) Original (and not rebuilt) engine and transmission. Currently running on V80 ( 80% vegetable oil, 20% petroleum products). Actually not, taking a WVO break.
1993 300d 2.5 275k. Current 120/day commuter
1981 300SD 188k (Hans) Killed by a deer