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Porsche Club of America
Just Slightly Off-Camber

I should probably first explain the heater operation. This experience comes from an ’87 911, but I’m sure other models are similar. There are two handles between the seats that determine the amount of heated air that is allowed in the cockpit. Two mechanical dampers accomplish this, one on each side of the cockpit. These levers (either one) also activate a switch that turns on a fan in the engine compartment. This fan is located on the driver's side, at the end of a long black plastic duct. The purpose of this fan is to deliver heated air from the engine heat exchangers to the cockpit. There is a rotary switch in the plastic console between the seats. This switch has four positions - 0 (off), 1 (low), 2(med), and 3 (high), controlling the speed of the “auxiliary blowers” located behind the kick panels on the drivers and passenger side. These blowers operate at the same speed and will (should, in my case) operate only if one of the levers has been lifted.

Operation of the heater is also affected by the controls on the dashboard. There are four horizontal levers, in three rows. In the top row, there are a pair of levers that control the volume of outside fresh air allowed into the cockpit. The lever to the left controls two mechanical dampers, one on each side of the cockpit. The lever on the right controls the speed of the fresh air blower, which is located in the center of the fresh air blower case between the control dampers. The center row lever allocates fresh air between the defroster (and the large center vent in the dash) and the under-dash vents. The lower lever allocates heated air between the defroster (and the side dash vents) and the under dash vents.

While it can be complicated to get the correct temperature air to the desired locations in the cockpit while keeping the car between the stripes on the highway, it seemed that “de-bugging” the system would be straight forward. Little did I recognize the complexity of the German mind.

Armed with the proper schematics for my car, I started the search for the problem. In the interest if brevity, I won’t go into all of the wrong trails that I followed. In the end, I discovered that the seller’s mechanic probably knew less than I did about the operation of the auxiliary blowers. Rather than trace for a broken wire (which I did find), the mechanic simply wired the fans to run all the time. Fooled me. I do want, however, to share some of the surprises I found. Hopefully this will help the next person that has to look into the electrical operation of their heater.

The first thing that I discovered is that the (heated air) blower in the engine compartment is not controlled directly by the heater control levers between the seats. The blower is controlled via a sealed plug-in relay in the engine compartment. (Driver’s side, black plastic enclosure with two brass wing nuts.) In the event of a major problem, I would look here first. This is not a great environment for electrical contacts. Also, you should be aware that this blower can be activated automatically by a thermostat to help cool the engine. There are also two fuses in the engine compartment that can affect the operation of (all) the blowers. There is a 25 amp fuse supplying current to the engine compartment blower, and a 16 amp fuse for the Aux. Blowers (But only for three of the four fan speeds. More on that later). A third fuse on the engine compartment fuse block  is for the rear defogger.  Another area of interest is the resister block used to control the voltage to (and, therefore, the speed of) the Aux. Blowers. The resistor block is located in the luggage compartment, passenger side, against the firewall. Along with the resistor block is an in-line (large) diode. If the first three speeds on the Aux. Blower are operational, this is not your problem. At this point I should tell you that Stuttgart was not entirely honest with you. The Aux. Blowers will operate at the “Low” speed in both the first detent of the rotary switch (Off) and the second detent (Low). Both the schematic and a visual inspection of the switch verify this. But remember, the Aux. blowers will only operate if one of the levers is lifted. The forth, “High” position is a totally different circuit. There is an additional fan relay to supply the current required for the high speed. This circuit even has its own 25 amp fuse on the luggage compartment fuse block. Yes, the Aux. Blowers are supplied by two (not quite) independent circuits. I say “not quite” independent because the power for the lower speeds is used to activate the “additional fan relay” for the High Speed operation. Given this circuit diagram, a blown Aux. Blower fuse in the engine compartment (or wiring fault) would cause both fans to fail at all speeds. A blown fuse in the luggage compartment (or wiring fault or relay failure) would only disable the highest speed. The last insight is that both Aux. Blowers share almost all of the same circuitry. If there is a failure in only one of the blowers, the fault will be with the blower itself (bad news) or the individual in-line fuses located near each blower (behind the left and right kick panels).

By the way, in my search I discovered that there are (at least) seven electrical fans/blowers in my 911. With all that potential air movement, why do I still have to worry about not getting enough air on my brake rotors at the track. Is there no justice?

In closing, I thought I would pass on a random bit of wisdom I ran across recently. I can think of several instances where my reviewing this would have been appropriate ---

“The real art of conversation is not only to say the right thing at the right time, but also to leave unsaid the wrong thing at the tempting moment”.