Heater Adjustment

12/3/00. Updated 1/13/02.

I adjusted the heater today. Now that the weather has cooled off I am starting to rely on the notorious Old Porsche Heater which did not work well on my first 912 and I do not expect to work well on this one.

But wait a minute! Does it work as well as it could? Is is correctly adjusted? Well, I first looked at the factory manual which points out that system is the same as the the 356B/T6 heater and to look at that documentation. It then concentrates on remove and replacement of the heater cables and adjustment of the handle friction. Not much help.

The cables from the heater control in front of the transmission shift lever attach to a pair of air gates, one on each side of the engine. These can be readily seen by removing the rear wheels and viewing the duct work coming from the heat exchangers on the engine that terminate at these valves. The system is simple. With the control all the way forward the gate valves should be open. When the control is back the valves should be closed.

This is a simple adjustment, although I made some non-standard choices on the way in order to make the controls work a bit more freely. See the discussion with the pictures to understand what I am saying here.

The manual discusses the control handle and its friction plates that insure the control stays where you leave it. I do not have a problem in this regard, so this note is purely about the gate valves.
120300-66 Dirty isn't it! The heater control is at full heat for this picture. Clearly the valve is out of adjustment. In this position the gate valve should be closed, but you can see it is still partially open. To readjust the lever needs to be rotated on its shaft. The first step is to remove the cable and lever arm. Then clean off the dirt. The gate valve has a spring that forces it toward the open position. This spring works fine on both sides for my car. See the full size pictures for more detail on the process.
120300-68 Same view as above after cleaning, a quick spray with primer and reassembly. Reassemble with the heater in the off position which provides the maximum length cable. Position the lever arm at the end of the cable in order to have the maximum rotation when pulling the cable as the heater is turned on. Note: I turned the lever upside down in order to match better with the cable opening at maximum heat. The factory pictures show this lever the other side up.
011302-660 The description below refers to my 1966 heater. Looking at pictures of later engines suggest heat exchangers were used in later years.

The heater control also controls the flapper valves on the heater boxes below the cylinders on the engine. You can adjust these such that they close with the gate valves shown above. There are two flappers, one at the rear and a second that makes a box when they are closed.

This flapper valve arraignment is one of two heater designs and was the one exported to the U.S. and most other parts of the world.

When these flappers are open, hot air gets dumped down and out of the engine. When closed the box gets pressurised and hot air is forced to the gate valves. Air is forced past the cylinders and forced to the passenger compartment. There is no heat exchanger. This system is known as the 'Export Model' heater.

The other design is more like the 911 and was used in Germany and Sweden. Hot engine air goes into a heat exchanger and exhausted. Cool air is bled from the fan side of the engine and forced through the other side of the heat exchanger and to the passenger compartment.

If your car leaks oil into the cylinder area, oil smell will get into the passenger compartment with the Export model. The alternate design prevents this. I suspect there may also be a problem with carbon monoxide if the exhaust manifold leaks with this design. As a result, good maintenance is important in this area.
011302-663 Adjusting the flappers seems to work best if the heater control is set to maximum heat - control all the way to the rear. There are wire controls just like the gate valves that are attached to a bell crank with a small bolt that has a hole in it for the wire to pass through.

The control wire is at its shortest position when the heater is set to ON. You can close the flappers manually by pushing the rear most toward the front. The inner one should close in synchronization. It is easiest to take the bolt out, thread the wire on it and replace the bolt. The wire hole in the bolt is next to the bolt head. There is a small washer between th wire and the bell crank.

This picture shows the bell crank in the closed position.
Last modified: Sun, 13 Jan 2002


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