Originally Posted by snookwhaler
I did a lot of testing on this a few months ago. But, this is only on my 85. Not sure if other years are affected.
I found that the aux. pump only runs when the defrost button is depressed or if the CCU wheel is all the way to max. heat (in the detent). In any other position, the aux. pump is off.
If you think about it.... It makes sense. These are the only 2 positions that demand maximum heat (coolant flow to the heater core).
How did you reach this conclusion? Did you verify with anyone (i.e. MBZ tech or service dept)? I've been told that the aux pump should engage whenver the CCU is engaged. Also, I thought the aux pump forced water into the heater core when driving at low speeds or at a stop to prevent the cabin air from going cold while the wheel is heat mode (not just max heat mode).