Heater control on 911SC

[deemsriddle]

After owning scores of Porsches for over 50 years (no typo) I am never surprised with the lame design ideas coming out of Porsche.

I am finishing the restoration of a 1979 SC Targa. I have had several cars equipped with the heater control unit with no problems. This car seems to have a little heat at all times, but no control. Trouble shooting the heater control unit, it doesn't even open and shut the flapper valves much less change heat to defrosting.

My go to medium has been YouTube but there is not much there. Armed with the factory workshop and Bentley manuals I open up the unit. A previous owner seems to have proceeded me as there seems to be parts missing.

As I never use my vintage Porsches in inclement weather, I only need heat. I have a 1971 911T in the paint shop. It occurred to me that an early heater control lever would work for my needs. Can you retrofit the old lever to work the flapper valves. I can then wait for my IQ to double or triple and can then make the control unit work.

As always, thanks to all you, owning an older Porsche is a lot easier. Thanks, Deems

[kltarga72]

Deems,
Are you wanting to eliminate the two heater handles located between the seats? The handles do control the flapper valves and activates the heater blower motor.
I may be misunderstanding what you are trying to accomplish.

[PeteKz]

Take a close look at your heater control boxes, under the car ahead of the engine. The flaps have rubber/fabric gaskets around the edges. If that's worn or torn, then you will get some heat through the vents into the car because the car cabin is at a slight vacuum when moving due to air getting sucked out the rear vent above the rear window. So it tends to suck hot air from the control box through the ducts into the car. A workaround is to run the fresh air fan, which pushes air the other way

[JSV798]

On my 1978 SC I've never got the heater control unit as Porsche intended. At present after nearly 30 years' of ownership I have it working as a just a switch to the blower in the engine compartment. The original heater control unit had been "modified" by some PO. I got that to work a little bit but then bought another unit which has also failed. The one I'm using now is a mixture of the two but since the thermostat above the rear view mirror was missing the system on my car was never going to work properly. My car has the red lever so I have that detached from the control unit and when I want heat I pull up the lever and turn the control knob (which switches on the blower). Works quite well.
You don't mention a red lever nor a blower fan in the engine compartment. Does your car have these?
Roy

[deemsriddle]

Originally Posted by PeteKz

Take a close look at your heater control boxes, under the car ahead of the engine. The flaps have rubber/fabric gaskets around the edges. If that's worn or torn, then you will get some heat through the vents into the car because the car cabin is at a slight vacuum when moving due to air getting sucked out the rear vent above the rear window. So it tends to suck hot air from the control box through the ducts into the car. A workaround is to run the fresh air fan, which pushes air the other way

We have been checking this out and have multiple problems. Leaving the control unit aside for a later time, first and foremost the lever does not open and shut the flaps. The flaps appear to have the seals on them but they won't respond when the lever is pulled up. The flappers remain closed (no heat.) You can manually rotate the flappers from under the car and the springs closes them appropriately. So the first problem is between the the lever and flapper box. The wire controlling the flapper box and the white plastic covers are all new. When you pull up of the lever, it moves but nothing happens at the flapper boxes.

When we solve this problem, the lever is missing the ball that attaches to the controller rod .

The motor is from a 1987 3.2 Carrera and has the heater blower motor on it. I will cross that bridge when I get to it.

[joe912/62]

The wire's on the red levers must not be connected. You need to remove the levers and see what's going on in there. I have a 82 SC and did it last year.

[EC900]

Read somewhere that converting from the dial type heat control to the older “two red lever” style is common and doable when the newer style no longer functions.
If you have existing red levers and not the dial type, then could it be the cables just came loose from the tightening screw-down? Is the looped end of cable broken that connects to module?

You say the flaps manually operate fine by hand…can you pull on the cable(s) from the red levers to see if that can manipulate the flaps?

Note: when I work alone, I set up my iPhone on a small tripod (or mirror) to view work when I’m not in a position to see it functioning.

[deemsriddle]

Taking the coward's way out, I substituted the two lever control of of an older model.
Thanks to all. Deems

[theiceman]

i guess my first question would have been what type of heat do you have manual or autoheat control, but sounds like it is sorted..

BTW this was the first car to have dual zone heating so lame design ? i dont think so , i would say lame understanding ..

[EC900]

Nothing cowardly about analog. Heck, even that switch contact mechanism on the old 2 level system is high tech enough. However you do want the engine blower to help boost the airflow so make sure it’s connected.