Yes it the dreaded ACC question

The car 1979 300sd 260K miles, great shape purchased from original owner @ 3 years ago. During my ownership the heating/cooling system has only had 2-3 brief moment of fan forced air movement. Until recently I did get hot air from the defrost when I was cruising highway speed (ala air cooled bug, no fan movement), and was able to turn off the heat by dialing down the temp control.

Items checked
Original owner replaced ACC servo, (Aluminum body type) servo appears to be leak free, servo motor operates when powered by 9v battery at servo terminal 1 & 5.

Items replaced: New recirc pump, new blower motor and fan.

Amplifier: Voltage at pins 1 & 2(Ignition on) heat set at 65 deg: 5.4v, Heat set at 85deg 8.2v. Ignition off pin 1 & 2: 4.8v.

Servo: Pins 1 & 2 (amplifier unplugged, grounded to pin3) heat set at 65 deg 4.23K ohm, heat set at 85 deg 5.54k ohm.

Recirc pump: works when jumped with 12v, does not operate when connected into wiring harness.

Vacuum system is 'okay' car generally shuts of quickly, door locks work after sitting 3 days usually. Pulls 20" Mercury at brake reservoir.
New fan blows huge amounts of air on defrost only, no air movement on other settings, new fan fries the fuse if on for more that 3-4 minutes.

Heater control buttons work but occasional do not return to 'rest' position.


??? Bad amplifier (I think yes) bad servo I hope not, how to verify recirc pump operation?
Further trouble shooting ideas.

Rick Witte

Make sure the foam sensor tube has been replaced....if the servo and the aux pump has been replaced last item on the list is the amplifier....just make sure to fuse the new aux pump....so it won't blow the amp again...

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