Improving the Cabin Air Ventilation System on the 3.2 Liter Porsche 911

[dkirk]

Improving the Cabin Air Ventilation System on the 3.2 Liter Porsche 911

Article by David Kirk

Concept by Brian Kearns

I’ve always enjoyed every aspect of my 1988 Porsche Carrera coupe except for the cabin air ventilation system. The marginal amount of air that moves through the ductwork combined with the inability to direct this air into one’s face, makes driving this fine car a windows-down experience on warm days. Windows-down is not a full remedy however—the air buffeting around in the cabin and wind noise above 60 mph does not add pleasure to driving this otherwise fine automobile. One can always run the A/C (air conditioner) but why be forced to do this when outside air, directed properly, would suffice? Certainly there must be a simple way to improve on this flaw and bring the fresh air flow and directional control up to an acceptable rate without too much rework or modification.

My friend Brian and I had discussed this problem at length. He owns an ’88 Carrera coupe also, and was well-aware of the lackluster ventilation system. One thing that we both observed is that adequate air is supplied to the cabin with the diverter valve set in the half-way position, the cabin ventilation selector
full-open, and ventilator fan on. The problem is that this air comes out of the defroster and floor vents and not into the drivers face. Other observations were that the air conditioner vents are placed such that air is directed properly, but this is recirculated cabin air only. To compound the problem, the A/C system energizes when the A/C fan is switched on, even if the temperature control is set to the warmest position! As in many European cars of this vintage, the heating/ventilation system and air conditioning system were two, totally separate devices with no interfacing of ductwork or controls.

Brian came up with the elegant solution of an electrical bypass switch, shutting off the A/C compressor and front mounted evaporator fan when the A/C fan is turned on. This allows running the 3-speed A/C fan along with the cabin air ventilation system without the A/C system operational. The A/C fan and ductwork now circulates the cabin air, blowing it where directed via the A/C vents, while the ventilation system keeps the cabin air replenished. It works extremely well! Furthermore, it is an easily implemented modification taking about 90 minutes to install and the total parts cost is under $20.00.

Note: I elected to install the A/C bypass switch in the High-Intensity Headlamp Washer Switch location on the instrument panel. If your 911 is equipped with this option, you will have to mount the bypass switch in another location.

Parts Required

Here are the parts required to install the A/C bypass switch:

• One 57-inch length of ¼-inch diameter flexible electrical conduit in black
• Two 60-inch lengths of 16-gauge automotive electrical wire with green insulation
• One single-pole/single-throw rocker switch capable of handling 4A @ 12V (see Notes section at end of article)
• One each ¼-inch male and ¼-inch female wire spade terminals
• Two each ⅛-inch female wire spade terminals
• Black wire ties

The parts layout is shown below. It is suggested that wires be cut to length with ends stripped for terminals, and conduit cut to length before proceeding.



The Installation Procedure

Begin by removing the clock and speedometer from the instrument panel. There is no need to disconnect any wiring. Your panel should appear as follows:



In the instrument panel, under and between the clock and speedometer location, is a rectangular hole where the High-Intensity Headlamp Washer Switch installs. The hole is already punched in the underlying sheet metal but will be covered by the dash material. It can be felt by moving one’s finger around in this location:



Once located, carefully cut away the dash material using an X-Acto knife to the same dimensions as the underlying rectangular hole.



Now with the trunk lid open and the covering moved out of the way, open the smugglers box lid. To the passenger’s side of the A/C blower motor is a wiring harness. Locate the harness connector that has a green wire running through both sides of the connector. On my car, the female connector side is white and the male side is black. Unplug this junction and using a small screwdriver, depress the tab lock on the green wire spade terminal such that the terminal and wire can be removed from the white connector.



Repeat the same procedure on the black, male connector’s green wire and spade terminal.



With both green wires removed, the connectors may be plugged back together.
Now crimp a ¼-inch male spade terminal to one end of a 60-inch wire lead. Crimp the ¼-inch female spade terminal to the other 60-inch wire lead. Plug in each wire lead to the appropriate terminals on the two green wires previously removed from the factory connector plug. At this time, you may insert the wires through the 57-inch plastic conduit and tape the exposed wire leads to the conduit end. Your newly assembled wiring harness end and terminals should appear as shown below:



Route the wiring harness through the forward portion of the smugglers box, exiting toward the driver’s side of the car. Lift up the foam seal and route the wiring harness underneath the seal. The picture that follows shows how this should appear:



The smugglers box lid may now be closed and latched. Route the wiring harness behind the brake booster, adjacent to the defroster tubes. The new harness is almost invisible!



The free end of the wiring harness should now be routed such that it exits the instrument panel opening where the clock installs. At this time, crimp the two, ⅛-inch female spade terminals on the ends of the protruding wires. Now push the wires from the clock mounting hole and route down to the switch installation hole, previously cut in the instrument panel material, located below the clock. With both terminal ends protruding through the switch hole, attach each wire terminal to a terminal on the single-pole/single-throw switch. Orient (rotate) the switch such that the “on” position faces to the right (passenger side). Now press the switch into the dash opening and it should click into place. (Note: some cutting of the excess dash covering material may be required such that the switch snaps into place correctly. See Notes section at end of article). Your switch installation should appear as follows:



The speedometer and clock may now be reinstalled. Now the job is complete!

Testing

Open the engine lid and start the engine. Turn on the air conditioner fan on high and temperature control set at the coldest position. Verify that the compressor clutch is engaged (compressor clutch hub rotating). Switch your newly installed compressor bypass switch to the “off” position. Verify that the compressor clutch is now disengaged (compressor not rotating).

Using

After implementing this modification, I’ve found the best settings for optimizing the ventilation effects are as follows:

• Cabin ventilation controls: diverter valve control in mid position, ventilation selector full-open, ventilation fan on low or medium
• Compressor bypass switch in “off” position
• A/C fan on low or medium
• A/C vents adjusted for airflow directed into occupants faces

The suggested settings result in a good balance of well-directed airflow with tolerable fan noise. Experiment to arrive at your preferential settings.

Notes

Regarding the snap-in-place rocker switch, there seems to be several different switch body dimensions available. The dimensions I found are a rectangular body of .750 X .510 inch, or .776 X .516 inch. Either size will work in this application. I happened to use the larger size which necessitated some filing of the rectangular switch opening in the instrument panel before the switch would snap into place properly.

On cars equipped with the High Intensity Headlamp Washer option, a different placement of the A/C Bypass Switch will be necessary. One suggestion is to mount the switch between the two A/C temperature and fan speed controls, on the console. This will require longer wire leads, different wire routing, and a more difficult installation procedure. This is beyond the scope of that contained herein.

[DRACO A5OG]

Outstanding!

There is an access whole for a switch on the other side the steering column in between the Fuel and Oil Gauges.

I like the idea about puttin gthe switch in between the A/C dials. My consol has the usless fader switch. I've been meaning to delete and put a cover in it's place.

[Canada Kev]

Great writeup and one that will result in better air flow. This will be a mod that I'll do for sure. Thanks for sharing.

One thing I noticed in these cars is that the vent air coming in has no place to go. It gets 'trapped' in the cabin. That's where opening rear quarter windows would be a boon.

[dkirk]

Thanks guys. I like the suggestion on opening the rear quarter windows. Is there a retrofit kit available to make this possible?

[javadog]

Not really. You just have to round up all the parts that are different, which is all of them. Frames, glass, rubber, latches, trim...

Then, to do it right, you need to do a little welding on the B pillar. There is a different piece of metal there, to accept the screws for the hinges.

JR

[RoninLB]

my rear 1/4 are open all year and are great. If you have ever owned one you'll want the old windows back

I'd call a 911 junk yard for the whole package or place a WTB add on Pelican

[tcar]

Just get an old door window frame set with operating VENT windows.

Solved.

[GH85Carrera]

Thank you. Great article. Well done.

[FinallyGotOne]

Will this mod be the same for a 79 SC?