Auxiliary pump runs at low engine speeds, but many people have deleted it with no ill effects. It's supposed to run when you're sitting in stop and go traffic or idling for a extended time, but when it breaks it slows coolant flow through it.

That's just what I've read from others on this forum.

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Milan Brown 1979 240D, rebuilt OM617.952 turbo diesel, rebuilt 722.315 transmission - engine spun a rod bearing

1979 300SD, ~90k original miles, all stock