Tech Articles


March 2001


                                                                                                  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 911's.

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 diminished.

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 red lever.

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.

Good luck


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