By Steve Grosekemper
As the temperature drops this winter
and we remember that 911's do actually have heaters, (Well warmers at least)
I thought I would outline the operation and basic repair procedure for air cooled
The first series of heaters we will discuss
is 65-68 911's. These are the simplest of all the systems. When you pull the
single red lever between the seats a cable is pulled and two heater valves under
the car are opened, allowing heated engine air into the cabin. To direct warm
air to your feet, open the slide valves at the inside of the door threshold.
To direct air to the windshield turn the black lever on the dash (looks like
a little window crank). The biggest problem with this system is that most people
don't know what the defroster lever is for and it never gets used. The lack
of use gums up the cable and makes operation difficult if not impossible.
If you have no heat at all check the
engine heater hoses. There are two on either side of the main engine fan which
blow air into the heat exchangers. After the air is heated it exits the heat
exchanger via another hose to the heater valve on the bottom of the car. If
any of these hoses are loose, broken or detached heater operation will be greatly
The second series of heaters is 69-74
911's. This second series of heating is the same as the first series, with the
exception of the air control in the passenger cabin. Once the heater valve levers
are pulled (between the seats) all air direction and temperature mixing is controlled
by the 3 levers on the dash. The bottom lever (red) directs the hot air up for
defrosting or down for foot warming. The top lever controls the volume of fresh
air entering the car, the farther to the right the more fresh air. The middle
lever controls the direction of the fresh air, up or down just as with the lower
When problems occur with this system
and all engine heat connections are correct, it is best to check that all the
hoses and cables in the trunk are operating properly. All the heater/fresh air
controls are located behind the dash inside the trunk. Remove the large cardboard
panel and have someone operate the heater/fresh air system. If there are any
hose or cable malfunctions it should be easy to see. The most common failure
will be a hose or cable that has popped off. A quick repair here should get
you warmed up in no time.
The third series of heaters is 75-83
911's . This version of heater is very similar to the version II heater except
that in this version, an electric blower motor was added to the upper left side
of the motor. This blower was added to increase the volume of warm air entering
the car at low engine speeds such as idling in traffic. This greatly improved
the operation of the defroster system from the earlier cars. High speed operation,
such as on the freeway, was not noticeably affected.
The first model to have an electric booster fan was the 1975 911. In this model the fan stayed on anytime the engine was on. While this made it easier to wire the new fan, it did not make for long fan motor life. These fan motors fail quite often due to their continuous run time. In 1976 Porsche figured out that a switching system would be needed to prolong fan life. To accomplish this, a relay was added to the rear engine relay board. (Relay II in figure #1).
To activate the relay a micro switch was installed at the heater levers between the seats. The micro switch provides a ground signal to the relay when the levers are raised. The activated relay provides the power to the blower motor. The motor gets its ground from the engine wiring harness. (See wiring schematic figure #2) This system was used until the 1983 model year.
If this system fails, first go to the
electrical plug at the blower motor and test for voltage with the red levers
raised. If voltage is present repair or replace the blower motor. If there is
no voltage present, check the 30a fuse at the relay board. (Fuse #2 in figure
#1) if the fuse is ok then remove the relay and verify that the voltage is reaching
the relay plug at terminal #30. If it is present, bridge terminals #30 and #87
and the fan should operate. If it works, then check the ground circuit from
the relay to the micro switch. Connect a test light to fuse #2 and relay plug
pin #85. The test light should light up with the levers in the up position.
If it does not the micro switch may need cleaning and/or replacement.
The fourth series of heaters spans from
84-89. Starting in 1984 the earlier relay system was replaced by a control unit
system. The control unit determines fan motor speed by inputs such as engine
and vehicle speed as well as inside cabin temperature. Diagnostics for this
system are more complex and other that checking for blown fuses, should be left
to a professional.
Another boost to cabin heated air was made in 1984 with the installation of booster foot well blowers. (See figure #3)
Below the dash on the far left and
right sides, reside two small blower motors. These motors, in essence, supercharge
the heated air in the cabin to provide an even more powerful blast of air to
the defroster and foot well vents. These blowers are activated when the switch
above the hand brake is turned on. When these blowers fail, it is usually due
to lack of use. If the motors are not used the bushings in the motor bind up
causing a excessive load which will blow the fuses. The fuses are located on
the outside of the foot well blower motors. If the motor does not work after
voltage is restored the motors will need repair or replacement.
The fifth and final series of heaters
we will discuss is for 90-97 911's . This final series of heating system is
completely different from the earlier systems. The heat from the engine is controlled
by a set of back pressure valves. Their operation is quite simple, if a heater
flap is closed in the car all engine heat dumps out to the ground. However if
a hose inside the car falls off, the cabin will be filled with a full blast
of heat. This is the usual problem with this late heating system, too much heat
as opposed to not enough like in the earlier systems. As with the earlier systems
is best to check all the hose connections, as well as all the fuses for the
climate control system. After that, the checks are best left to a Porsche technician
who is familiar with all the complex workings of the system.
As you can see from this article many
of the problems with Porsche heater systems are with the blowers themselves.
While replacement of these items is quite expensive there are electrical motor
rebuilders who may be able to repair them for less than half of the replacement
cost. Give them a try you just may get a warm cozy feeling from it.
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