There are two reason for high idles...

1. Bad idle module... this is verified by doing a MB specified diagnostic
procedure and measurement... Simple test is to unplug the air valve wire
(not module unit)... if unplugging raises the RPM even higher then see #2.

2. Massive vacuum leak.... most likely is the black vacuum tube... rigid
one... on passenger side of the engine. Otherwise, spray carb cleaner to see
where engine idle would change (stumbles a bit)

Did they know in '84 what a computer is like?

The idling system in the Bosch injection systems of those times were
mainly vacuum regulated. Test the vacuum inside crankcase. With
decreasing idle and vacuum the idling air bypass increases the amount of
air taken from the crankcase just by its pressure with respect to the
intake after thottle. There is as system of membran cells to test the
different vacuum differences:  athmospheric, crankcase, intake manifold
before and after throttle.

Your symptom suggests that the vacuum system reports a dying idle and
tries to achive the maximum speed.

Check: Worn crankshaft gasket, clogged vacuum hose from crankcase to air
filter, a hole in the differential pressure cell, open idling air bypass.