Up until the mid-70's, 911's took their primary heater air source directly
from the air used to cool the cylinders, and Turbo's continued this practice
into the 90's. From the mid-70's on, non-Turbo 911's have used an auxiliary
motor in the upper left corner of the engine compartment to provide heater air.
This provides a more consistent source of air, especially when stopped in
traffic, as it's volume is not engine-speed dependant. 964's continued with an
auxiliary motor for it's heater air source, however, Porsche eliminated the
auxiliary motor from it's racers, substituting a bypass pipe. By retrofitting
the bypass pipe, you remove the weight of the fan motor, maybe 5 lbs worth, and
you open up the left side of the engine compartment.
Here's how I replaced the 964 engine mounted fan with a cup bypass pipe, and the
HVAC system seems none the wiser. As you may know, the HVAC system shuts down if
it determines the engine mounted fan is not working. (Note, since I'm not doing
the work, I can't guarantee the fitness nor success of modifications to your
964).
First, here's what the installed bypass pipe looks like:
The Cup bypass pipe can be bought directly from Porsche Motorsports, they
sometimes come up for sale as used parts, and some vendors may be able to order
them through European contacts. (Thom Fitzpatrick recently emailed me to say he
has had a batch made in carbon fibre and has some for sale;
Info Here. - John M)
So, to fool the system, two resistors were used - one for the motor and one for
the temperature sensor. (Thanks to Adrian Streather and Chris Duffey who
provided guidance on determining the correct wattages). For the motor, a 51 ohm
15 watt wirewound resistor (#1 in picture below) works, and it does get warm to
the touch when the heat is on. The temperature sensor is the wire going into the
top of your existing air pipe. This senses the temperature, and sends the fan
into overdrive if the airflow gets too warm. An 18K ohm 2 watt resistor (#2 in
picture) is used, which tells the computer it's around 40 degrees F. The last
resistor (#3 in picture) is the long one with the curly metal wire that's stuck
in the side of the original air pipe. It's there to break the electrical circuit
should the motor develop a short, so, for simplicity, it was left alone.
All the resistors are in a box that is mounted near the left upper shock mount,
behind the original fan. Below is a picture showing the box with the cover on.
Note there are ventilation holes in the box to deal with heat generated by the
wirewound resistor. The tape on the base of the temperature sensor was to hold
it in place better. You can also see the enclosed box in the background of the
bypass pipe picture above.
Note: there is a "T" connection in the white tube connected to the bypass pipe,
for both the distributor and the rear lights.
