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Join Date: Jun 2017
Location: Frankfurt Germany
Posts: 106
Porsche interior temperature sensor 91165910501

Porsche interior temperature sensor 91165910501

I recently purchased a 1977 911 that features the automatic heating control mechanism and soon learned that the hole between the visors stems from the missing interior temperature sensor. And that it is not avaiable anymore while used ones are very hard to find.

I was gearing up to building one myself when I finally found one. I thought it makes sense to document the part given that I only found very few pictures and limited info on the thermistor on the web. I will not repeat the readily available and good info on the automatic heating control.

The automatic heating control seems to be a straight forward control loop that is taking three inputs:
1) Setting on of the heater control dial (0 to 9)
2) Interior temperature: Via the interior temperature sensor 91165910501 which in the earlier cars is mounted between the sun visors and on later cars behind the dash
3) Temperature sensor left flapper box: This sensor measures the temperature of the heated air. It is located in the left flapper box (below the motor at the 'end' of to the heat exchangers)

All inputs are translated into resistance which is read as input by the heater control. In the case of 1) via a poti (which is directly dialed up and down by the heater dial). I measured 0 to 2kOhms at max temp (setting 9). In the case of 2) and 3) via a thermistor, a devise which changes its resistance depending on the temperature.

To find a fitting thermistors two parameters are primarily important: The resistance at ambiant temperature (defined as 25C). Second the 'B'-value which is a material constant of the thermistor that determines the temperature - resistance curve over a temperature range.

I measures the following temperature - resistance curve in the Porsche original (from a 911 SC 3.0).

It appears that Porsche used for the interior sensor a 1,5kOhms thermistor (resistance at 25C) - this is also the value stated in the service manual. I also depicted the measurements from a NTC thermistor with 1,0kOhms and a B value of 3800k. This is a 50cent part I ordered on Ebay. It appears that the B-Value results in a curve that is fairly similar to the Porsche Original. Any thermistor with 1,5Ohms and a B value of 3800k should do fine given we are talking Porsche interior temperature control and not rocket controls.

For the thermistor at the flapper box the manual states 135 Ohms resistance @ 100C (warm engine).

Please note: in some forums the use of resistors in parallel are proposed. That would change the temperature - resistance curve, but according to my measurements this is not necessary.

The cable to the sensor seems to be a cable that is shielded.

Sensor housing:
Here some close-up pictures of the original part. I was surprised to realise that the part is actually made of rubber instead of a plastics compound. The values in the hand-drawing below are all in [mm].

1973 2.4T gemini blue
1977 3.0 Turbo slate grey
1973 2.4s motor rebuilding project

Last edited by Topocalm; 01-12-2018 at 03:26 AM..
Old 01-12-2018, 12:08 AM
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Join Date: Mar 2015
Location: Yorktown, Virginia
Posts: 87
I have the same issue and I am trying to find the housing that the Thermistor goes into inside the cage. I have the rubber cage and actually my son is going to 3D print me another one out of plastic, but I do not have the metal piece inside of it that holds the thermistor and also has the plug at the bottom to go to the sensor wire.

Do you know where I could get one of those?

Old 10-01-2019, 04:52 PM
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91165910501 , cabin , interior , sensor , temperature

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