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

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1987 300 TDT
1990 300 SEL