This has long been an aux pump problem with the W123's ACC but I've never heard of one involving the monovalve. You did a good job diagnosing and repairing. A fuse would be a good idea. The later ACC versions (W124) were built to be short-circuit resistant so MB must have gotten a lot of complaints.

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