Hello all. This is not my usual section but i figured I would ask. I have a buddy which bought a gem of gem 4G which is a 6-6 TL. We are going through and making it road worthy again. Right now he is having an issue with the HVAC where the compressor is not coming on. He ran the HVAC diag sequence and we see that the sunlight sensor is having issues. Since the sunlight sensor is a 2 lead I am expecting it is resistance on the line based on the amount of sunlight it receives. The trick here is that I know it is also used for the auto headlights. Right now his auto headlights work, but his HVAC says sunlight sensor issue. Is there a relay between the light sensor and the HVAC system we should look into? We are also having issues finding a wiring diagram on this if anyone has input.

 

So I did some digging into the service manual, and 1, the Sunlight Sensor for the HVAC system and the Automatic Lighting Sensor for the headlights and stuff are 2 different sensors (I didn't know that either). I guess the Sunlight Sensor for the HVAC system is looking for sunlight intensity in terms of how it'd heat up the cabin, and the Automatic Lighting Sensor is checking for overall sunlight and whether it's getting dark enough to turn on the headlights and dim the navigation screen.

Checking the sunlight sensor is simple if you have a multimeter - just check for these voltages with the sensor in direct sunlight and in the shade. If they're off, the sensor is bad. Considering the built in diag is throwing a code to say the sensor is bad, it probably is, but this would be easy to double check. (in that pic, the square looking thing behind the sunlight sensor is the automatic lighting sensor)



If it's not the sensor and you still need to figure out why the AC compressor isn't turning on, you could check fuse #20 (7.5A) in the under-hood fuse box, or the AC clutch relay (also in the under-hood fuse box) to see if those are working or blown.



And here's the wiring diagram for the relevant part of the HVAC system. Shows where the fuse and relay for the AC clutch are and where the sunlight sensor goes.

 

Hmmm. I'm no mechanic, but if the sunlight sensor is out of spec, does that actually prevent the compressor from turning on, no matter what? Even if you turn the temp to max cold?

 

That's right, the sunlight sensor helps mainly in auto mode and if you turn on maximum cold, then the sunlight sensor completely drops out of the equation.