After reading horror stories about a seized water pump frying peoples ccu's, I thought I'd try unplugging the aux water pump and see how much heat I get at idle. The cabin was plenty warm. Granted, this is in 40*F California weather so it might not be suitable for everyone, but I thought I'd throw it out there.

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'81 300SD

Huh?

The aux water pump on the heater core line is a flow-through impeller design. Whether it is spinning or not, coolant flows in that line. Just flows faster when it's spinning.

I don't see the purpose of unplugging it, and how this would save your CCU?

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Cheers!
Scott McPhee

1987 300D

Last time I replaced my CCU, I replaced my pump at the same time just to be safe. Putting an online fuse in the wire to the pump is probably a good idea, but I've never bothered.

The older CCU design (before our 124s, Scott) had no short-circuit protection. A stalled aux coolant pump draws more current and can fry the printed circuit traces in the PBU. My '85 300D came with such damage and I repaired it, then replaced the pump and added a fuse to the pump power leads.

The aux pump was added by Mercedes due to customer complaints of insufficient heat at idle and city traffic in cold/snowy weather. Those who are blessed with a warmer climate can get along without the pump.

Disconnecting the electrical leads won't prevent the pump from corroding; as a permanent modification, the pump can be removed and replaced with a piece of pipe.

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

Ahhh... Problem on the electrical side. That explains everything, thanks.

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Cheers!
Scott McPhee

1987 300D

Put a 1 amp fuse inline with the pump.