W124 (87) Auxiliary coolant pump
 

Hi, I have searched the forums but can’t get a clear answer without reading several hundred I guess. My 87 (new to me) had intermittent heat, I figured the pump as I had one of these before. It was dead as a nit so I had a spare and I swapped it out. I don’t seem to have power to the pump. Fuse is good and I was thinking maybe something else controls wether it come on or not. Is it on all the time when the key is on? Seems a bit simplistic for Mercedes to me but one never knows. Any help would be appreciated of course.
Thanks in advance

Does your heat get weak at idle but otherwise fine? That's the tell-tale sign of a bad aux pump.

I know the pump is bad but I changed it with a good pump. What I want to know is it always on or is it controlled by the CCU or duo valve or when does it come on?

I believe it is always running....perhaps someone else can confirm.

Mine doesnt seem to be.

Does the car have the ccu unit or dual controls?