Some vehicles may actually *NEED* the Aux pump, some don't. The 126 with the OM603 sure doesn't. My pump was seized and I ran the car last year with it unplugged but still in the circuit. It was capable of heating the car to a comfortable temperature and the vent temps were "pretty warm".

This year I decided to try unseizing the pump. Worked for 2 days before the shaft seal went belly up. With the Aux pump working, you could just about cook on the air coming out of the vents - VERY hot air with the temp wheel cranked to "MAX".

Before shelling out for a new pump, I tried replacing the pump with a simple brass elbow. In my car, it works nearly as well as with the Aux pump functional. The air is ultra-hot and still heating fine at idle/sitting - quite a noticeable difference from the Aux pump installed but non-functional. Makes sense that the impeller isn't blocking partial flow.

I won't be replacing the Aux pump since it works fine without it - no need to shell out money for a new one and far less to go wrong.

Some other engine/chassis designs may not be so lucky, but the 126/603 combination seems to be a winner when the Aux pump is replaced with an elbow.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]