AHP has constant voltage but doesn't seem to be working, heater contol valve has constant vacuum which keeps valve closed making cabin without heat regardless of dash switch position. Any thought on how to get vacuum modulated on the contol valve or the aux pump to start working?

The vacuum to the heater valve is either on or off. Blending is done with the blend air doors that the "feedback" potentiometer sits on. The pushbutton allows variable vacuum to the element for feedback thru the switch-over valve assembly. NOT a simple system!!

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

Thanks, where can I get to the potentiometer and the valve assembly to see that my vacuum hoses are connected and not leaking?

Would the potentiometer be the climate control dial on the dash?

No, the feedback pot is inside the heater box. Sends a signal to control unit regarding the position of the doors.

__________________
Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.

Can I get to the heater box by going behind the glove box? I can hear doors opening and closing when I switch on the recirc and the vent buttons but no sound as I move the climate dial from hot to cold or vice versa. A/C is currently not working so I'm dealing strictly with the blend of fresh air and a heater?