I am having an intermittent problem with the interior of my 2008 4.2 suddenly fogging up when it is around the freezing point outside. The climate control is on auto everything fine and then suddenly you can smell and feel the humidity, and the windows will fog so bad I have to open a window to clear so I can see. I am not loosing any coolant so it is not a leak in the heatercore. Any ideas please jump in.

 

If you're positive the fog is not from coolant, there are a few things to consider.

Next time it happens, drive the temperature selection down until LO is displayed. This bypasses some of the automatic control functions and will help determine what is happening.

You mentioned this only happens when ambient air is near the freezing point. Do you normally use the exterior temperature display on your control panel? If not, it's possible the sensor is not accurate and this will affect system operation. A single ambient temperature sensor (behind the front bumper) feeds the panel display and the controller. Below 27F, the compressor is commanded off, because in theory air below freezing is dry enough that dehumidification (AC operation) is not needed. If the sensor is reading a few degrees cooler than actual temperature, the AC will be commanded off above freezing, when there is plenty of moisture in the air.

Another possibility is an inaccurate evaporator discharge temperature sensor. A long-winded explanation is here, especially in post #3 and #4:

https://www.jaguarforums.com/forum/s...ol-how-185002/

If you remove the control panel, you can compare resistance readings for the evaporator discharge temperature sensor and the two duct discharge temperature sensors. All three are the same part number, so if all have stabilized at the same temperature, the resistance readings should be very similar. Details at the link above.

Another possibility is a low refrigerant charge. This lets ice build up on the evaporator. The thermal expansion valve tries to maintain the evaporator just above freezing by releasing extra refrigerant flow. However, if you've already got ice built up, this extra flow melts the ice and releases moisture into the discharge ducts. Details of how to check the refrigerant level are at the same link.

 

Thank you so much for the detailed answer, the sudden melting of the evPorator sounds like it fits the problem exactly. I will get into it and report back. Gotta love this forum.