What causes the aux. water pump to kick in (1989 300E) - PeachParts Mercedes-Benz Forum

yambrovich · #1 03-04-2004, 12:02 AM

Hi everyone,
What mechanism / device controls the behavior of the auxiliary water pump?
I was told that mine needed to be replaced, so I bought one from Parts Shop and replaced it. Not too tough...
However, I don't know if that was the problem or not. Sometimes it still feels like there's no hot water coming through the system when at idle or crawling along in traffic on a cold morning.
Not as bad in the '93 300CE CV (A124) but then I just flip the seat heater switch and all is better!!!

Your help, wealth of knowledge and generous spirit are greatly appreciated.

Mike Murrell · #2 03-04-2004, 12:36 AM

Aux. water pump, What's it's purpose?

Location of aux water pump 126

#3 03-04-2004, 07:04 AM

The aux pump gets its signal from the pushbutton climate control unit. If the aux pump gets crudded up it will start drawing more current which will "blow" the CCU. If the pump was bad in that way, it very well may have knocked out the CCU.

That said, this is not the only thing that could cause lack of heat. With the engine at temp, feel of all the heater hoses with the heater turned full on. That way you can see if it is getting hot water past the pump, then past the heater valve. Also the engine must be reaching operating temperature on the gauge.

Hope this helps,

TROVERMAN · #4 03-04-2004, 05:05 PM

How can I tell if the CCU is blown or not. My problem is everything seems to work except being able to change the air flow direction. Currently will only blow out through front main vents and defroster. (not center vents) It is stuck in the default defroster position. A/C works, only through the defroster and front vents. Also, recirc feature no longer works. (lights up, though) Previously, everything had worked perfectly. Then the batt went dead on a cold morning and I jumped it. Never worked correctly after that.

yambrovich · #5 03-05-2004, 06:26 PM

OK OK, so how does the system know when the car is "at idle"?? I have to believe that there's a sensor (on the crankshaft maybe) that would tell when the engine is running below a certain RPM threshold... If the car is at idle, a signal goes to the CCU which then (in turn) sends a signal to the aux. water pump to 'go for it'.

Is that true? If so, what sends the signal to the CCU that the engine is below a certain RPM? Where do I find that? Is that possibly a "crudded up connection" or is there an inherent flaw (under-hood wiring harness burning out...) that I need to know about?

Your help is greatly appreciated. THANKS AGAIN