What year 300D? The W123 models had an oil pressure line, not a sender. The same test can be done, of course. If you try this, have someone hold the STOP lever down or use a remote start switch so the engine cranks but does not fire. If you do not see oil squirting out then the pump is not working.

Jeremy

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"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