Michael,
I would check the sytem before I gave up on it. I had problems with mine and most of it was remedied by new heat boxes and cables. The whole system works fine now.
The cable replacement is an easy and inexpensive fix. The box replacement is fairly expensive and a real pain because the bolts are hard to get at and sometimes break.
The first to do is to turn the ignition on and then turn the switch on between the seat; put you ear close to the black box between the seats. You should hear a hum from the servo which moves the arm connected to the heater box cables. If you can hear the motor jack up the car and see if the heater flaps are opening and closing.
Good luck. Let me know how you make out.

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1984 Targa

Hear ye, hear ye! As most of you know, I love cars but this is my first 911. In my year of ownership I have agonized over this heater blower motor and now I believe I have some credible advice for those who want to avoid the expense of a new motor. Brian, your motor's front bushing is dry and perhaps shot.

You're not supposed to be able to replace the bushing but there is a way, if you have the patience, and the brushes can be replaced too. e-mail me, anyone, for a more complete discussion.

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'83 SC

Another question about the heater!

What is the purpose of the exhaust-mounted sensor?

Thanks!

OK, I've now taken the control box apart and found a couple of things.

First, the servo is working fine (phew)

Second, the sweeping arm on the pot was broken - I'm sourcing a secondhand part.

Now can anyone help with the following?

The sweeping arm hits a micro switch at each end of its travel. At the 'Defrost' end the switch winds the servo all the way in, presumably to bypass the auto control and give full heat. But what is the switch at the cold end for? I'm guessing it's for the blower in the engine compartment.

Someone had fitted an external switch for this blower. Am I correct in assuming that the lead to the blower should be connected to the two brown wires at the rear of the servo - these have spade connectors and are not linked to anything on my car.

Finally, what is the purpose of the sensor on the heat exhanger and what happens if this circuit is left open/shorted?

Many thanks!

The sensor measures the temperature of the heated air just before it enters the flapper valve. It would see cold air until the heat exchanger begins to heat the air.

I have always assumed that this measurement was read by the circuit board and resulted in the auto heat control NOT opening the heat flapper valves until the air is hot enough to do some good.

I further assumed that when the heated air gets REALLY HOT, it causes the flapper valve to close somewhat to reduce the amount of REALLY HOT air that enters the cabin.

Sounds like there may be too many assumptions in there ....

But when it began to malfunction and the dealer wanter $230 for it(10 years ago), the conversion to a manual lever looked really attractive. I have never regretted it.

Brian

Thanks for responding.

What were the symptoms when the sensor malfunctioned?

Cheers

Phil

Phil

The symptoms were that the control would not raise the lever when heat was obviously required. Definitely failed to the NO HEAT range. Never did OVERheat.

Reached into the control and disconnected the lever from the control arm, then used a small block of wood to hold the lever up. A 3/4" x 1" x 1-1/4" block gave three different heat levels. Drove that way for a couple of years.

Was always embarassed when inquisitive passengers asked about the high-tech solution ...

Somewhere along there (11 years ago or so), I had occasion to remove the engine. Took the flapper box sensor off and found what looked like metal fuzz between the sensor tip and the metal tube it was mounted in. Cleaned it off, theorizing that the fuzz was affecting the resistance of the tip (grounding it?) and throwing off the comparative circuitry in the controller. the unit worked for a while after that, but I had to turn the heat way up (8-9) to get what I used to get with 2-3. And a little later, it failed again (failed to raise the lever. Gave up and bought the friction discs. Reinstalled everything so it looks exactly as it came from the factory, except the lever is operated manually.

Brian

My sensor is missing - the wire is just hanging there, so I assume the circuit is open, and I have the same symptoms as you - ie the lever will not move back.

When the rain stops here in England I'll try shorting the sensor wire to give a closed circuit and see what that does.

Phil