the throttle microswitch (gray) shuts off fuel on the m103 after you take your foot off the pedal, if I remember correctly.

What problem are you chasing?

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'90 300SE 298k
-300K and it gets put into retirement.
'80 300D 255k Purchased new by family in 1980.

Had a:
1973 220 (gas)
1980 300SD
1992 400E