C3 1972 A/C equipped blower fan and fresh air plenum actuator valve operation

[Nick K.]

I need to confirm the proper operation of the blower fan of a C3 C60 a/c equipped car. On my '72 when function and fan controls are in the off position the fan still operates in low speed. Is this correct? If not what would cause the fan to operate when all controls are turned off? I've checked in archives and found an older thread from '09 that had conflicting opinions.

Also want to confirm the proper operation of the fresh air plenum vacuum actuator located in the right kick panel area. The actuator holds vacuum to open the plenum door but does not return the door to it's closed position. I am guessing that there is a return spring inside on the vacuum side of the actuator that has disintegrated and no loger returns the door to its closed position. Thanks in advance for aany input. I'm coming down the home stretch in finishing my '72 and sorting out these last few gremlins.

[Mark E.]

Re: C3 1972 A/C equipped blower fan and fresh air plenum actuator valve operation

Let me qualify my reply by saying it's been a few years since I've torn into one of these systems, and I'm not home with my service manual.

As I recall, the blower is off when the left thumb wheel is set to "off". But I could be wrong.

The fresh air plenum door is held in its default position by spring tension, but I don't remember if that's open or closed. Either way, because of its location (bottom of the cowl) it's vulnerable to corrosion, so it's not uncommon for it to be stuck. Check for freedom of movement, integrity of the diaphragm and vacuum lines, and that it's receiving a vacuum signal at the proper HVAC settings.

For definitive answers, the service manual has a nice diagram showing the on/off state of the electrical switches and vacuum ports for each HVAC setting. That was invaluable as I troubleshot my system.

A couple of lessons learned-
- Carefully check all grounds if circuits don't check out as expected.
- Use a hand vacuum pump to check vacuum integrity and operation rather than running the engine. It's easier to hear vacuum leaks and the doors operating with the engine off. Be sure the ignition is "on" (without starting the engine) while doing this to power the system.

[Nick K.]

Re: C3 1972 A/C equipped blower fan and fresh air plenum actuator valve operation

Thanks Mark. I rebuilt the plenum door with new rubber so that part I know well. It should close when there is no vac signal to actuator I beleive so that leads me to beleive that the actuators internal spring is no longer functioning due to corrosion I would guess. The fan motor operation when all is turned off is a puzzle so I am guessing that that is the way it was designed to operate contrary to the service manual.

[Bill C.]

Re: C3 1972 A/C equipped blower fan and fresh air plenum actuator valve operation

on all AC c3 cars (at least through 75), blower motor runs low even when the switch is put in the off position.
but if the thumbwheel is in the off spot, then the fan is all the way off (pretty sure).

as for the plenum, there is a spring behind it that loads it in the closed position by default.
spring is either missing, or stretched way out.

Bill

[Mark E.]

Re: C3 1972 A/C equipped blower fan and fresh air plenum actuator valve operation

To clarify, by "off" I mean the left thumb wheel is in the off position. The left most position of the fan switch is the lowest speed, not off.