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Outside temperature sensor resistance values...
You can check the resistance of your sensors to see if they are out of
The resistance of the sensors follows a gently sloping, declining curve from:
~3300 ohms @32F
~2200 ohms @ 46F
~1500 ohms @ 59F
~1250 ohms @ 68F
~1000 ohms @ 72F
~ 800 ohms @ 86F
~ 650 ohms @ 95F
(This data is for the 1986-88 digital climate control, as per the Bentley)
Based on this, if you were sufficiently motivated, you could change the
outside temperature that the control head sees and then verify it using the
Also, the A/C compressor clutch is regulated by the thermostat on the A/C
evaporator housing. It has a capillary tube that's inserted into the
evaporator and controls the power to the compressor clutch if the evaporator
temperature falls below 32F, preventing ice formation. The resistance values
for this sensor are:
~ 40 ohms @ 59F
~38 ohms @ 68F
~36 ohms @72F
~35 ohms @86F
~34 ohms @95F
You'll need a good meter to measure this one properly.
There is also an "Ambient temperature switch" that is _independent_ of these
sensors located on the A/C evaporator housing. It opens when the temperature
falls below 37F, interrupting a ground signal. The A/C control head will
then prevent the compressor from being switched on.
Other devices that can kill the compressor are:
A/C high pressure sensor (in plenum chamber, near left side of heat exchanger
A/C kick-down switch on auto trans. cars - sends a ground signal to terminal
9 of A/C control head, turning the compressor clutch off for 12 seconds.
Engine coolant overheat switch - sends a ground signal to terminal 20 of AC
control head, which then sends a signal to A/C programmer to shut off the
compressor clutch when coolant temperature rises above 247F.
If anyone needs the full procedure for diagnosing the outside temperature
sensors, you can email me.
Unfortunately, the Bentley doesn't just come right out and hand us the code
for the control head's integration of data from these sensors. Anybody want
to take one apart and disassemble the code? You have to reconstruct what the
engineers were thinking from the testing procedures. :-(. Now if _I_ was
publishing the "official" manual...
It ticks me off that the publishers will lay out all of the bits of the
"hard" mechanical aspects of the car but somehow the "soft" mechanics that
are ensconsed in the firmware of the many computerized systems have to remain
hidden. Why all this modesty? If they _had_ to make the system work without
the benefit of digital processing, it could be done, and then you can bet
your boots that the schematics would be explicit. I'm only ranting about
this because today I found the old owner's manual to an integrated B&W
television/radio/cassette "boom box" that I was given when I was a child.
The last section of the manual contains the _complete_ electrical
schematics of the entire unit. Of course, it didn't have any VLSICs or
microprocessors. No "black boxes".
You pay the money to own the thing, but you really only own its functions.