1979 300sd (w116) ACC
 

Yes this topic once again.

The car: A 1979 300sd purchased from Original Owner, @ 3 years ago, mileage @ 260K great shape except for...
Never has had a functional heat/blower system, until recently I could get 'adequate" heat from the Defrost system while cruising Highway speeds (fan did not operate). I was able to turn off the 'heat by turning down the heat setting. Servo replaced by PO with aluminin body one, no leaks. Defrost /heat buttons work but not very smoothly not all return cleanly/quickly to 'rest' position when another button is selected.

Items replaced: New heater blower, New Heater recirc pump.

Now I get no hot water to interior, the (new) recic pump does not work when plugged into the wiring harness (it runs great when jumped with 12v). Fan blow crazy(cold) air on defrost, no other settings get air flow, and fuse blows with fan operation of more than 3-5 min.

Voltage at pins 1-7: (ignition ON) at the amplifier 5.4v (low temp) 8.2v (high), at servo pins 1,2: 4.23K ohm (low temp) to 5.59K ohm (High), with(ignition OFF) pins 1-7 4.8v. Servo motor operates when jumped with 9v battery at pins 1 & 5.

Vacuum system 'okay' door locks will work after sitting 2-3 days.

??Amplifier bad, servo bad, wiring to recirc pump shorted?? Hints for further trouble shooting?

Rick Witte

Some info for you here.