'86 A/C compressor bypass switch

[260chrisb]

I want to install a separate switch so that the fan can run and force air thru the dash vents without the A/C compressor coming on. Has anyone done this? I love this car but the ventilation system may be the dumbest design in the history of the automobile. These cars are tight as drums and I'm in the car for 2 minutes and I want ventilation. The only way to get any air thru the dash vents is to run the A/C which sucks when you don't need cool air but simply want any air. It's impossible to get any fresh outside air into these cars unless the windows are open. The slides and fan for outside air distribute to the screen and floor which is useless. I can't believe that a car as well engineered as this one has no way of getting outside fresh air without running a fan or anything thru the dash vents without the A/C running. Any suggestions? Also, what's the easiest way to disable the A/C compressor?

[kuehl]

Fresh outside air is controlled by the top sliders in the dash (side by sides; one is for fresh air and the other is for fresh air motor; yes, funky design).

The AC system is a recirculating system so there will not be 'fresh air' coming from the AC vents, which are (on 77ish through 89 factory ac cars), the primary ac vent above the radio, the bow tie vent in the bulkhead in front of the center console, and the side vents. Air is drawn into the evaporator box from airways under the driver and passenger side floor boards.

When you turn on the ac evaporator blower motor control knob, the right hand know in the center console, two things happen. First, the AC system is turned on in either of the 3 factory fan speeds. Second, power is sent to the thermostat which is the left knob in the center console.

The AC thermostat turns on and off the front condenser blower motor and the compressor clutch. Depending upon your AC thermostat, if the thermostat knob is turned fully to the left or CCW, it may or may not turn off your front condenser blower motor and compressor clutch. If it does then you can have the AC evaporator motor blowing without the front condenser blower motor and compressor clutch engaging. If it does not turn off those 2 components then you'd need a thermostat that does shut them off in the fully CCW position, such as this Thermost

[GH85Carrera]

When I autocross, I want my system to not run the compressor, but I want air blowing on me.

I turn the fresh air and have it all directed down to the floor, turn on my AC fan to max, and then open the engine compartment, and flip a switch. It is a simple toggle switch that opens the power to the compressor. It is easy to find the compressor wire and rig in a switch, or even easier, unplug the compressor and tuck the wire away in a safe place.

I end up with the air actually blowing on me, and not me feet or the windshield. All for the cost of a simple toggle switch and some wire.

My thermostat works, but it never shuts off the compressor. Flip my switch, no AC compressor, but a lot of air blowing on me.

[jlex]

Originally Posted by GH85Carrera

When I autocross, I want my system to not run the compressor, but I want air blowing on me.

I turn the fresh air and have it all directed down to the floor, turn on my AC fan to max, and then open the engine compartment, and flip a switch. It is a simple toggle switch that opens the power to the compressor. It is easy to find the compressor wire and rig in a switch, or even easier, unplug the compressor and tuck the wire away in a safe place.

I end up with the air actually blowing on me, and not me feet or the windshield. All for the cost of a simple toggle switch and some wire.

My thermostat works, but it never shuts off the compressor. Flip my switch, no AC compressor, but a lot of air blowing on me.

But you must also deal with the front condenser blower... doesn't it continue to operate even if the compressor is switched off? Would be worth getting the Keuhl switch if it shuts off both units when the thermostat switch is CCW.
Maybe you need to install a switch on the front blower as well...

[Missed Approach]

Hi 260chrisb- Having installed Charlie's Kuehl Thermostat, I can say that having the ability to run the a/c fan without the compressor or condenser frunk fan kicking on is a wonderful thing. As with any Kuehl product, great instructions, great fit and great customer support.

[jlex]

Originally Posted by Missed Approach

Hi 260chrisb- Having installed Charlie's Kuehl Thermostat, I can say that having the ability to run the a/c fan without the compressor or condenser frunk fan kicking on is a wonderful thing. As with any Kuehl product, great instructions, great fit and great customer support.

How difficult was the install? Does it require you to pull out the center console?

[kuehl]

You do not have to pull out the entire console, sometimes simply remove the fasteners that hold it down to the floor to lift it a bit;
let's say the job is harder than a brake job, easier than replacing an alternator or doing an engine drop (ohhh, drama; no really it's not that hard).

For cars with center consoles, there are 2 designs of factory consoles. From the mid to late 70's up through 1985 (for discussion we will call these 'early"), and 1985-1989 (we will call 'late' or the rubics cube).

The early years are rather easy in terms of R&R of the thermostat and its aluminum sensing tube that leads to the evaporator. The late rubics cube is more challenging.
In either console model, you have to use caution in not breaking the aluminum sensing tube brazed to the thermostat switch which leads to the evaporator.

Simply, the thermostat switch is held in the console, there are 2 wires with spade connectors on the back, one from the fan speed switch and the other provides
power to the front condenser blower motor relay and the compressor clutch. A thin aluminum tube, filled with refrigerant gas, is brazed a bellow in the thermostat
and leads to the evaporator (through the back of the console, under the carpet, through a hole in the bulkhead (firewall).

You are kinda working in an awkward position, both doors open, spending time laying down on both seats (not at the same time naturally, the stick shift is a rib breaker).
So, I suggest at least 10 minutes of warm-up stretching exercises. Here is an excellent video to get you started.

I can do a thermostat swap out in about 1 hour, so for first-timers maybe 3 hours.

Common tools.

I'd say our instructions are pretty decent vs factory instructions.

[jlex]

Thanks for the link. Am wondering what model Porsche she drives

[blucille]

but, to answer the OP's question, if he wants a toggle in the center console that would absolutely shut off power to the compressor clutch but still allow the fan to blow ambient air, isn't it as simple as installing a toggle that interrupts the path either just before or just after the thermostat itself.

In other words, take either wire off the back of the thermostat switch, attach it to a toggle, and then connect the other end of the toggle to the place where you removed the wire from the thermostat switch.

[kuehl]

Originally Posted by blucille

isn't it as simple as installing a toggle that interrupts the path before or just after the thermostat itself the thermostat switch.

Yup!

There are a 100+ ways to skin the cat. GH85Carrera offered up a solution he likes and it works for him. You could also just unplug the compressor clutch wire and yank out the front condenser blower relay or unplug the front blower.

However. If you are adding another switch in the console you have gone through 70% of the work at that stage, added 2 more connections which can add resistance to the circuit and propensity to have connection issues down the road, completed your Kegel exercises, installed a switch that either requires you to bend down taking your eye off the road or puts ugly eye candy in front of you. So, why not just replace the thermostat?

The thermostat's electrical contacts, alike the stock factory fan speed switch's contacts, do oxidize and break down over time (a lot of amps passing through them).
You will have to replace the thermostat sooner or later.

[jlex]

Well said. Maybe I can talk the wife into getting one for me for my birthday

[dkirk]

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.

[dkirk]

Here is how the well-ventilated 911 appears from the outside.

[jlex]

Terrific writeup...thanks for taking the time. My Guard's Red '88 thanks you!
Will the modification shut off both the compressor and front condenser?
Am wondering if an in-line fuse should also be integrated to the line?

[dkirk]

jlex - thank you...hope you can make use of it. The front condenser fan as well as the compressor clutch is controlled by the switch. I don't see the need for a line fuse as the A/C fuse (I don't recall the location number) still protects the circuit.

A red '88 - nice coincidence. Kindred spirits!