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Old 07-10-2010, 11:49 PM
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Mrmerlin
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Default Thermistor value

Does anyone know what the outside temperature thermistor vales are??
I bought a few thermistors a few years ago and dont remember where or what the temp/OHM ratings were.
I have been searching but i guess i didnt post these specs or where i got the parts or part number,
Thanks
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Old 07-11-2010, 12:01 AM
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3.5K ohm, according to the Nichol's web site....

http://www.nichols.nu/tip432.htm

But, don't you need to know how resistance varies with temperature? Why not buy the part used or new from Porsche?
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Old 07-11-2010, 12:13 AM
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I got a set of 5 thermistors for 3.00 a piece,
with some potting compound i had a fresh functioning part using the old thermistor housing
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Old 07-11-2010, 12:26 AM
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Will the new part match the temperature/resistance curve? I don't see how that's possible without getting the same thermistor's manufacturer's part. Compensation will be off by some amount other than what was designed by Porsche.

If you don't care about the temperature compensation function, then a fixed resistor or wrong value/slope thermistor will work just fine.
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Old 07-11-2010, 12:35 AM
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I found the specs for the thermistor in the Porsche Climate Control Spec Book, page 5: measured at the sensor wires directly...

At 32 degrees F.... 933 ohms
At 68 degrees F.... 588 ohms
At 86 degrees F.... 457 ohms

There are other measurements at the controller in this spec.
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Old 07-11-2010, 12:40 AM
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Ok I found them .
I got the thermistors from www.newark.com
PN 121-102EAJ-Q01
Resistance 1Kohm
Thermistor tolerance 120%
Power rating 1mW/0 degree Centigrade
Resistance ratio 7.04
These are glass enclosed parts similar to whats in the system now for the outside sensor.
They have 2 inch leads.
So with some potting compound ,(JB weld will work) drill out the sensor it has glass in it so be careful,
solder the wires onto the new part then insert into the sensor housing then fill with the potting compound try to put the harness wires into the tube so the new part wont get damaged ,
and pot the harness wires as well
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Old 07-11-2010, 01:00 PM
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Old 07-11-2010, 01:50 PM
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Apparently someone is The Man.

Let us know how it works.

A while back I faked out the HVAC system by just splicing in a 500 Ohm resistor. The sensor in the left-front wheel-well was completely missing. At least the brain was happy and it didn't go defrost on my constantly.
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Old 07-11-2010, 02:17 PM
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it would be cool for some one thats an elex wizard to check the specs of the thermistor i used and see if it is close to the correct one from the part i got it seemed like the closest thermistor they have .
I have installed one of these and the HVAC system seems to work as it should but there may be other thermistors that are closer to the factory spec, i dont know for sure though
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Old 07-11-2010, 02:22 PM
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The one for outside temp doesn't need to be as accurate as you might think. From that value and the value of the inner temp and slider settings, the setting motor brain makes a deceion on when to open the outside air flap. If it's off a little you won't notice a significant difference in CC operation.
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Old 07-11-2010, 06:57 PM
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I looked at the Honeywell data sheet for your substitute thermistor. It has a tolerance of 20%.

That's pretty wide manufacturing tolerance allowance for each part, so you could try the others to see if one has a better match to the factory specs for temperature/resistance.

But like dr bob says, you might not notice any difference if outside mixing is off by 10 degrees F outside air.
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Old 07-11-2010, 08:20 PM
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thanks so much for your time, Stan
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Old 07-11-2010, 10:09 PM
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Originally Posted by dr bob View Post
The one for outside temp doesn't need to be as accurate as you might think. From that value and the value of the inner temp and slider settings, the setting motor brain makes a deceion on when to open the outside air flap. If it's off a little you won't notice a significant difference in CC operation.
I don't believe it has anything to do with the outside air flap (e.g. the recirculate flap).

It does of course affect the temp mixing flaps - but as you say its a somewhat secondary effect - the cabin internal temp sensor is more important.

I did look into this I while back - but could not find easy matches to the characteristics. You need to look for baseline resistance at ref temp and then the correct polarity (PTC/NTC) and change rate (ohms/degree).

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Old 06-14-2011, 09:02 PM
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Sorry to dig up an old thread, but does the Porsche Climate Control book have values for the early 944? The evaporator thermistor died on my 84 944 and need to find a suitable replacement.
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Old 06-14-2011, 09:38 PM
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It would be in a different book - does anyone on the 944 boards have a factory workshop manual...?

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