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Variable-Speed A/C Blower Switch Installation (on Your 1984-89 911)
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Pelican Technical Article:

Variable-Speed A/C Blower Switch Installation (on Your 1984-89 911)

Nathan Auldridge

Applicable Models:

Porsche 911 (1984-89)

Variable-Speed A/C Blower Switch Installation (on Your 1984-89 911)

Figure Figure 1

I have seen many complaints from owners of 911's with factory A/C systems regarding the air conditioner 3-position blower switch. The High position creates such a fan noise that conversation is impossible, while positions 1 and 2 are virtually useless.

Part of the fan noise (and the anemic air flow through the system) is due to a huge heat-generating resistor pack, used by the blower switch to regulate voltage to the blower. This resistor is located right in the center of the air intake for the A/C evaporator, disturbing and restricting airflow, as well as adding heat to the cabin air.

I have seen a variable-speed switch offered by an after-market supplier for the 911, but at some $200, the price just seemed way out of line. I decided to see if there wasn't some other solution to this annoying issue.

After some research, I discovered a couple of companies who sell variable speed controllers for 12v motors using "Pulse Width Modulation" technology. In a nutshell, these devices do not rely on a huge resistor pack to reduce voltage to the motor, wasting energy and generating heat. Instead, the circuitry pulses the power to the motor at some 18,000 cycles per second, varying the amount of power-on time during each cycle, as required, for the desired motor speed!

I chose a unit offered by a company called Critical Velocity ( www.CriticalVelocity.com). I went for the higher amperage model, Part No. SPD-328 which is rated at 28 Amps (the fuses for the 911 A/C circuitry are 30 Amp so this unit will be capable of handling power needs). I sourced a couple of other lower priced controllers, but went with this model because the speed-controlling potentiometer can be mounted remotely, rather than being permanently soldered onto the circuit board. All the on-board connections to external wiring is accomplished via sturdy screw terminals, making final installation and testing easy.

I also liked the fact that the entire unit is small enough that I could install it right inside the center console, so it would be out of sight.

Figure Figure 2

This project is one anyone can do with simple hand tool. Once completed only one single original wire must be slightly altered by splicing a new wire into it. All remaining connectors are left intact without modification in case the next owner should prefer originality over comfort.

The end result of this project provides a separate, small, infinitely variable speed control knob for the blower, and eliminates airflow-restricting OEM resistor pack. The original blower switch knob will remain in place, but it's only function will be to switch on the A/C system, independently of the blower motor speed.

Note that although my personal installation experience was with an 88-89 Carrera, the wiring schematic is pretty much identical back to 1984. For earlier cars with A/C, it should be simple to apply this instruction to your specific model.

To complete this job you will need from www.CriticalVelocity.com (or another source of your choice):

  •  A PWM Variable Speed control circuit P/N SPD-328
  • Potentiometer P/N POT-5K
  • A soldering iron (I prefer the butane-powered ones for portability)
  • Approximately 8 feet of 10 or 12 gauge stranded wire (preferably one length each of RED and BROWN to match the OEM wiring scheme)
  • A crimp-on ring terminal for the new ground connection
  • A couple of crimp-on female spade connectors (get the yellow ones) for the blower motor connection
  • A self-tapping sheet metal screw for the new ground connection
  • Several #6 short machine screws and nuts for mounting the circuit board inside the console
  • Electrical tape, preferably the cloth non-gummy "friction" tape

Okay, let's get started:

Figure Figure 3


First, for safety sake, disconnect the negative terminal from the battery, then remove the carpeting from the passenger footwell, and remove the footboard which hides the intake opening for the A/C evaporator. Remove the 4 screws that attach the air intake mounting to the bulkhead, and pull the mounting free of the evaporator. Disconnect the connector to the resistor pack in the center of the vent, then remove the two screws that hold the resistor to the center mount, and remove it. In the picture, the resistor pack has already been removed, leaving just the metal mount in the center, for future reinstallation if ever desired: The connector that went to the resistor pack will not be used after this installation, so you can just tape over the entire connector for safety, then tuck it back underneath the other wiring bundles, out of the way.


Remove the back outer cover from the center console by removing two screws at each side, near the front edge of the cover (there may be an additional two screws along the top of the cover). With the back cover removed, you will then see two more screws on each side of the inner housing. Remove these, and you will be able to slide the entire switch assembly out of the console.

Pull the knob off the original blower speed switch (it is just a friction fit), and then with a narrow flat bladed screwdriver, carefully turn out the inner bezel nut that secures the switch to the housing. It will be much easier to work with the wiring if you remove this switch from the housing.

The OEM blower switch actually performs two separate functions (refer to the annotated factory schematic shown later in this instruction). When the switch is turned from position I (off) clockwise to any of the 3 motor speed selections (II III IIII), a separate integrated switch closes a connection to the A/C compressor, essentially activating the cooling system.

We won't be using the 3-speed blower speed function of this knob, but we DO still need that switching function for the compressor. We are going to install the small potentiometer in the console, and that knob will henceforth perform the blower speed function. (NOTE: an optional configuration could be to complete replace the original blower switch with the new variable speed potentiometer control, and then mount a toggle switch separately to actually turn on the A/C system.. adventurous souls should be able to follow the factory schematic to accomplish this option, but it won't be covered here).

Now it's decision time: You can choose to wire the new controller, as I did (and as shown on the schematic), so that the blower fan can be activated, via the new continuously variable speed control, EVEN IF the A/C system is not on (i.e., the original blower switch is on position I (off). I chose this option, because I frequently lament the absence of any air flow through the upper vents except when the A/C is on. This wiring option is shown by the RED wire which leads to terminal V+ on the controller board.

The second option would be to wire the new controller so that the blower function only gets power if the A/C is turned on (original blower switch in any position except "I". This wiring option I have depicted on the schematic as a GREEN wire, and should be used instead of the RED wire to terminal V+ on the controller board.

After you have made your choice, identify the correct power wire on the back of the original switch, either the solid RED wire, or the Green/white wire as shown on the schematic.


Cut approximately 18" of at least 12 ga. stranded wire (RED preferably), and strip the insulation for about one inch from one end. Now carefully remove about 1/2" of insulation from either the solid RED or one of the GREEN/WHITE wires coming from the blower switch (your choice, per the discussion above) without actually cutting the wire. Wrap the 18" piece around the bare wire, then solder it together, and finally wrap the solder joint securely with electrical friction tape.

Figure Figure 4


According to the manufacturer, the controller must have it's own separate ground wire, and the blower motor must receive its ground from the controller board as shown in the accompanying schematic.

Cut a length of approximately 24" of 10-12 ga. Wire (BROWN preferably). Strip about 1/4" off one end, and crimp, then solder a ring connector on the end. Use sandpaper to remove all paint from a nickel-size spot on the center "hump", just below the console, to insure a good solid ground connection. Use a self-tapping screw to secure the ground wire, via the ring connector, to the metal chassis, as shown in Figure 4.

Figure Figure 5


Open the trunk lid, remove carpeting, and open up the "smuggler's cove" door on the trunk floor near the bulkhead on the passenger side.

You will see the blower motor right on top, with the 2-wire connector feeding its power (A BROWN and a RED/WHITE wire). Disconnect the two connectors, and tape up the side that comes from outside the box (leaving the wiring lead that comes from the motor itself).

Make up 2 lengths of 10-12 ga. Wire approximately 48" long (One RED, One BROWN). Crimp, then solder a female spade connector to one end of each wire. From inside the cabin, find where the existing power wires feed to the evaporator/blower box. Look for the larger WHITE "wire" (actually the temperature probe for the evaporator) and follow it through a small hole.

Temporarily tape your two prepared wires together (at the crimp connector end) then feed them both up through the hole into the evaporator box. After a couple of attempts, you'll be able to fish the wire through to the blower motor. Simple push each of the crimp connectors onto the existing white plastic two-wire connector that leads to the motor, matching the RED/BROWN color scheme to the incoming wires.

This completes the work in the "smuggler's cove"â Close up the lid to the evaporator box, reinstall your trunk carpeting, and close the lid!

Figure Figure 6


Decide where you want to mount the potentiometer in the cockpit. This will be the variable speed control for your blower. I chose to mount my control between the hazard and defrost switches in the upper quadrant of the console (I also cut about 1/4" off the potentiometer shaft so the final knob would fit flat against the panel: Determine the length of wire you will need to run from the controller board (to be mounted inside the center console) to your desired location for the rotary control. You could even choose to mount it somewhere on the dash panel for convenience if desired.

Cut three 18 ga. Wires (preferably using 3 different colors.. 18" in length if a console-mounted potentiometer location is chosen). Solder one wire each to each of the terminals on the potentiometer. Strip about 1/4" of insulation from the other end of each of the wires, and then tin the bare wire with solder so there won't be a stray strand when it is connector to the controller terminal block.

After positioning your potentiometer in its mounting, and running the wires to the console, follow the accompanying schematic (as well as the one included with the board) and connect the three wires to the controller board, using the screw terminals provided. It is important that the wire from the CENTER lug on the potentiometer is connected to the controller terminal marked D/CN. The two remaining wires may be attached to the P+ and P- terminals. The pot. Will function irregardless of which outer wire goes to P+ or P-. However, the position of these wires will determine whether motor speed increases with clockwise, or counterclockwise knob movement.


Using the schematic at the end of this instruction, run the power and ground wires previously prepared, and the two new wires from the blower motor, up through the back of the console, and then attach them to the appropriate screw terminals on the controller board.

That completes the functional wiring part of the installation!

Figure Figure 7


Double check all wiring connections at the controller, making sure that the positive and negative polarity is correct. Reversing the power leads can permanently damage the controller board.

With the original blower switch in the off position, and the new variable speed potentiometer control in full counterclockwise position, reconnect the battery, then turn on the ignition key.

Turn the original blower switch to ANY position other than position "I". This will energize the A/C system. Slowly turn the potentiometer clockwise and check that the blower speed increases. If the blower speed DECREASES with clockwise knob movement, then just reverse the position of the two outer-lug wires at the controller terminal block.

Figure Figure 8


Position the controller board horizontally in the back panel of the switch enclosure (this is the removable piece that remains attached with the switch assemblies when removed from the console). Position it as close to the top as possible so that board doesn't protrude into the open cubbyhole at the bottom of the switch panel. Mark and drill mounting holes, and mount the board using #6 machine screws. I put a little thread lock on each screw to avoid having to put too much torque on the board during mounting.

Now guide the new wires out the back of the console, slide the switch assembly back into the console and secure with two screws on each side. Finally, reinstall the outer rear console cover with the 2 additional screws on each side. Finish off the installation with an appropriate knob for the blower control. Radio Shack has a good selection.

Figure Figure 9


As always, these instructions represented my own experience with my own car. I make absolutely no representation as the correctness or suitability of anything contained herein. Use these instructions at your own risk, as I will not be responsible for any damage caused by their use. Also, please note that the particular controller I used does have only a 30-day warranty. It is possible that there are other similar products available at lower price with longer warranty. To date, I have had no issues with this installation.

Please note that the switching configuration suggested here does allow the possibility of having the A/C system turned on, while the separate blower switch could be in the off position. Such a selection will cause evaporator icing and render the system temporarily unusable. Always make sure the blower is running at least at minimum volume anytime the A/C system is switched on.

Figure Figure 10


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Comments and Suggestions:
Kuehl Comments: Peter,

Figure #1, I believe, shows the bracket without the shunt resistor pack; the author notes in the adjoining paragraph that he removed it. There are two variations of the bracket and shunt resistor pack found in the 911/930 however both mount inside the trapezoid air inlet plenum under the RH side floor board in LH drive models.

The original shunt resistor pack does generate heat depending upon what fan speed you have set on the original fan speed switch; power traveling through the shunt resistors springs is dissipated by design and release into the air stream going to the evaporator. There is also a feature built into the pack that shuts off the circuit when temperatures get too hot by way of a simple bimetal contact.

In LH drive factory a/c cars there are 2 air intakes in the cockpit. The one discussed under the RH side foot board; Air path is from behind the center console and moves to the right under the RH floor mat; the air gap is provided by the wooden foot board. The second air intake is located under the LH side foot board; hence there are two air inlets into the Behr evaporator box located in the 'smuggler's box' well located in the front trunk. Does the factory shunt resistor fan speed feature located in the RH side air intake reduce air flow? Marginally, not greatly in our experience. Does the factory shunt resistor pack heat up the air entering the evaporator box, yes, to a degree however not severe; it is located by design in an air stream to keep it cool.

Rather than using 2 devices OEM fan speed switch and potentiometer to turn on and control the fan's speed as described in this article you could simply use a "pot-switch"; a potentiometer that has a switch feature; as found in the Kuehl Variable Speed Fan Controller.
April 7, 2011
  Followup from the Pelican Staff: Thanks for the additional Info. We appreciate it. - Nick at Pelican Parts  
funk#49 Comments: hey guys:

i'm starting an a/c update on my '88 carrera coupe and, after reading nathan's article, i figured i'd start by locating the evaporator intake resistor pack.

if figure #1 is the resistor he pulled out of his '88, it's certainly different than what i discovered. mine appears to be original, and was exactly where nathan said it would be, but all i found was a unit 2"x1.5"x1.5" with 2 small springlike devices which i suspect are the resistors. it's hardly "huge".

have i found the right unit? i want to update to the variable fan switch but i'm wondering if it's going to make much of a difference. surely the little unit i located couldn't be generating much heat or greatly obstructing air flow. do you feel the variable feature is worth the effort?

are you aware of a toggle, or rocker switch, conversion for the power-on portion of the oem blower switch done in conjunction with the variable switch update? nathan makes reference to the possibility, "for adventurous souls".

January 14, 2010
  Followup from the Pelican Staff: Without seeing a photo of what you rmeoved I can't besure if it the right part. The upgrade is for sure worth it. However, I'll leave it to one;s taste to decide if it is right for them.

The toggle switch would be a custom install, you would have to work the bugs out. - Nick at Pelican Parts

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