How '80s 911 ventilation systems work

[Globulator]

In the '80s 911s there are a few heater controls that somehow always defy rational explanation.

In the BMW E30 for example you've got three sliders, top (windscreen), middle (for the middle) and bottom (for the feet). However Porsche always liked to go one better and in a quest for knowledge (and to tighten up the back of my cigarette lighter power (it actually unscrews from the inside I discovered later. Much later.)).

So, the big red slider at the bottom - heat? well - not quite. The middle slider, up or down? Well, nearly. The A/C fan and ducts - how do they fit it? And the little slider at the top left - what does that do? And the switch in the middle of course..

OK so the first thing to note is that the Porsche system is not an integrated heating and AC system that you'd find on other cars. It's actually 3 separate systems, two of which use the same exit vents.


Firstly the hot air runs from the rear heat exchangers and into two cylindrical air pumps, in each front footwell at the corner. Heat and the fans are controlled by the rotary control in the middle of the seats. This hot air goes into one valve each side operated by the red slider, at the bottom. When you slide that lever, you divert the air either into the footwells or onto the windscreen.

Secondly cold air rushes along the top of the bonnet and into that cute grille. Along with any rain etc. That goes into a box with a fan, and a big 25mm drain. Check your drain actually connects up to the drain pipe - you may be surprised. This fan has multiple speeds (controlled by the little top-rhs slider I think) the little slider to the left of that actually just closes the exit of that box. Assuming the exit is open (little lhs slider to the right) then the air goes into a valve controlled by the middle slider valve, which directs it down to the footwell or up to the windscreen.

So the First and Second air systems (hot and cold) use a siamesed but isolated valve on each side diverted up or down by the bottom and the middle sliders, into the footwell or onto the screen.

Thirdly we have the A/C, which is an entirely different system again. Behind the LHS (passenger) side feet is the A/C cooler (evaporator). This has a fan on the back attached to a big tube, so it sucks air from inside the car (just from under the dash) through the evaporator, and through that tube and then straight into the centre and side mid level vents. The operation is controlled by the two 'climate' controls in the center console, the LHS knob being the cooling enthusiasm, the RHS knob being the fan. There is no connection with the other two systems.


So this explains quite a lot about why the heating controls are a little unfathomable on the 911, and I can see why no one ever worked out what they do. Quite simply, you'll never get fresh air from the bonnet into those mid-level vents.

I'll do a little more research tomorrow when I stick it back together again but I'm pretty confident that's the basics covered. I'm wondering if there is a better way to connect it up or if anyone has, I suspect the 964 and the 993 are a little more integrated.