Planning the next
AO Silver Member
Join Date: May 2007
Location: United Kingdom
Hadn't spotted this before.
Is the behaviour the same when the air is fully directed at the windscreen and fully directed into the footwell? i.e. it doesn't matter which direction the air goes
There are two areas I'd initially investigate.
The mixed air temperature sensors. There are two of them, one for air directed to the footwell and one for air directed to the windscreen/face vents. If one of them is faulty, then that could explain the a 'yes' answer to my question above. They tell the ECU the temperature of the air that's about to exit into the passenger area after it has been mixed ( some having passed through the heater matrix to be warmed up and some either at outside temperature ( if the aircon is off ) or chilled ( if the aircon is on ).
Probably more likely ( to my mind ) is the cabin air temperature sensor. That lives behind the grille between the temperature control knob and fog light switch. It has a brushless fan built into it to draw air from the cabin over the sensor. If that sensor tells the ECU that the cabin is cold, the ECU will mix the air for maximum heat.
If you have access to a full ( paid for ) version of FiatECUScan ( it needs to be an up to date version as the functionality has only been added recently ) and a suitable lead ( to communicate with the climate ECU ), you should be able to diagnose fairly easily. The following is based off a 156 but the GTV/Spider should be similar. Plug in a laptop and go for a drive. On the screen your passenger will be able to monitor cabin temperature, both mixed air temperature sensors and the mix motor position ( expressed as a percentage ).
If for example the cabin air temperature sensor is faulty, I'd expect to see the mix actuator ( motor ) at 0% or 100% ( whichever way round it is for maximum heat ) and the cabin temperature at a very low level e.g. -30°C.
If you don't have access to FiatECUScan, you can remove the centre console and test the sensors with a multimeter. Let me know and I'll detail a procedure to use with a multimeter.