Heater Temperature Control

Why fit a temperature dial in a car that is about 8 degrees away from reality? I mean it seems to stabilise well, but does anybody know if it can be tweaked to make more sense?

Why not instead make an accurate, reliable petrol gauge that is calibrated in known units?


Michael

Michael, are you referring to the engine water temperature gauge or the outside air temperature gauge?

Derek

I am referring to the thumbwheel in the cabin to preselect the cabin temperature. The outside thermometer is quite good actually!

How do these things actually work? I thought the system measures the cabin temperature with the sensor in the roof and not the actual air temperature that comes out of the ducts!? These heater systems have some kind of mystic aura around them. Now, the modern cars they have even mores sensors and I have noticed they are radiation sensors and others.
Still the climate controls don't work. Is it really so difficult?

You are quite right, the internal temperature is sensed by the thermistor in the roof, but if you change the setting it will be in reference to what the thermistor temperature is at that precise moment, so, if you turn the setting down, the heater will blow coldish air into the cab until the temperature at the thermistor drops to the temperature selected, then the air will warm up until the required temperature is reached.

Peter.N.

Hi Peter/Michael,

I believe the sensor and the thumbwheel occupy the opposite nodes of a simple wheatstone bridge. In series with the thumbwheel there will be a calibration resistor to preven the every thing you are complaining about. If you can identify the calibration resistor on the control board you could replace it with one of the correct value to correct your 8 degree drift.

Cheers

noz

Just to add a little more to the control circuits for the climate control, there are 2 more temp sensors associated with it's operation. One is fitted in the inlet tract above the fan and the other in the plenum to the evaporator for the a/c.These are bayonet fit with the usual 2 pin cit connector and are easily accessable from inside the passenger footwell.

I was recently playing with the roof temperature sensor and, believe me, it is one of XM's details which really made me fall on my head! It is rather complex, bayonet-type assembly with the temperature sensor at the hole mouth (a simple PTC or NTC beacon). There is a silent propeller and a motor around it drawing the air from the cabin so that the sensor reacts more promptly! Of course, there are related electronics inside, even one integrated circuit.

When taking the assembly apart, I found it rather clogged with dust. Cleaning it helped restoring more accurate and faster temperature reaction and control. Lubricating the motor will make it more silent.

Wot he said up there! Works every time

Robert, could you please describe in more detail where and how to reach the other two temperature sensors? I am eager to getting hold of them as my automatic control still shows some temperature drift.

Hi John, to access these 2 sensors, drop the lower trim panel which accesses the fuse/relay power distribution board/fan on the passenger side footwell.The a/c evaporator sensor (prevents frost forming on the evaporator-blocking air flow) is inserted directly through the side casing of the evaporator and the connector protrudes towards the footwell-you may need to pull the carpet back to see it just above the footwell outlet(unscrew the 2 screws and remove the footwell outlet louvre)The inlet tract sensor protrudes inwards and is mounted above the fan.This requires reaching high up under the dash(you see it from the engine bay looking into the fan grill).They twist 90 deg to lock and unlock,approx 2k resistance value,