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Re: A4 climate control gripes (long)
>From the archives: (perform this at your own risk!)
> If you want to force the AC to operate below its lowest temp, you can
> find the wires going into the programmer and parallel a couple of
> resistors across the thermistor leads to fool it into thinking it is
> warmer than it really is outside. A single pole double throw switch
> could be easily wired so you can switch it on and off. You could even
> use microswitches that open and close with the activation of the
> defroster flap if you wanted. As a starting point, 2.2 k should make
> sure the indicated temperature is > 40 deg F.
> Hope this is helpful.
> Fred L. DeRoos
> > You can check the resistance of your sensors to see if they are out of
> > spec...
> > 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
> > fault-code channels.
> > 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:
> > 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...
Robert Houk - SMCC WorkGroup Server Firmware wrote:
> Recirculation *is not* disabled by the heater; it *is* disabled when the
> compressor is not running. So, if it's warm enough outside ( > ~35 deg.
> for the A/C compressor to run, but for the heat to be on inside, you
> engage recirculation.
> So which thermistor should be replaced by a resistor that always
> "reads" as 40F ???
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