The aux water pump was added to the climate control system in response to whining from customers who said the heater didn't put out enough heat at low engine speeds (i.e., around town, low speeds, lots of sitting at traffic lights, cold weather, etc.). The main water pump doesn't send enough hot water through the heater core, apparently. I suggest removing the aux pump and replacing it with a short piece of copper pipe. If your heater still keeps you warm in your climate and style of driving, forget about replacing the pump.

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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