Citroen BX AirCon Temp Control Sensor Substitute (Ken W)
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  1. #1
    Local Tyrant gibgib's Avatar
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    Default Citroen BX AirCon Temp Control Sensor Substitute (Ken W)

    A few months ago I put a post up on a few Cit forums asking if anyone knew the characteristics of these sensors (when they were working) and I got no replies. So I decided to reverse-engineer a solution.

    Symptoms: When it is working properly, the BX aircon slide is actually a variable temp control with off on far left then cool on the left graduating to cold on the right (yes I had one that worked once upon as time). Most controls now just seem to engage the compressor as you start to slide from the left and the compressor never disengages and the aircon gets very cold. Water condensate freezes onto and clogs the evaporator until no more air can flow through and constant compressor engagement chews up lots of petrol. The resistance of the temp sensor seems to only measure at about 500 to 800 ohms.

    Reverse-Engineering: The control module is electronic and the sensor wire come from the module which is behind the dorr open display panel and goes up the right side of the air box disappearing into the air box beside the coolant pipes going to the heater element. I cut the sensor wires, connected a variable resistor and found that the sensor resistance needs to be about 5000 ohms at the just on position and 12000 ohms with the aircon hard on ie the sensor has a negative temperature coefficient with resistance rising as tenperature drops. This is the same characteristic as a thermistor and on examining spec sheets, I found that a device that was 4700 ohm at 25 degress C that rises to about 12000 ohms at 0 degress C - perfect.

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    Fix: Unfortunately, Dick Smith only stocks a 100000 ohm one and Jaycar don't stock any. So I contacted Prime Electronics in Bowen Hills and they had a couple of hundred of the 4K7 thermistors at a cost of less than $2 each. I bought one on the way home from work, soldered it onto a piece of thin speaker wire, shrink sleeved the joins and lightly coated the unit in silastic to water proof it. Then I poked it about 10cm into the air box through the same hole that the other sensor wire use for entry and hooked it onto the sensor wires going to the control module.

    The system now controls the air temp at about 16 degress C when control is just to the right of off down to 0 degrees when far left and compressor cycles on and off quite frequently. It is now working better than original as far as I can remember.

    Good Luck with your unit. Email or post of you have any queries.

    Ken W
    Last edited by gibgib; 26th April 2004 at 09:18 AM. Reason: Change title only

  2. #2
    Moderator Alan S's Avatar
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    For the further information of members, we are presently doing this job and have had to order the parts in.
    They are available at any "Jaycar" reseller and are shown in the "Jaycar" catalogue as cat number:- RN3438.
    We had a problem tracking them down as we don't live in a metropolitan area, so have bought a handful in case we need them in the future, so if you get caught, give us a call and we can post one out to you.
    Word of advice; check with a meter prior to use as they can come faulty straight from the factory which is why we bought a few.
    Thanks again to KenW for this info; in current heatwave conditions, you're a lifesaver!!


    Alan S
    If it ain't broke, use a 12" shifter.....that usually does the trick!!

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