So I figured I'd do a quick write up of this since this seems like a problem that I've read a lot about, but never really found a solution to my problem until I did a little more digging on my own.

The symptoms were intermittent - the AC condenser fan would sometimes go on at full speed for no rhyme or reason. If economy mode was off, it wouldn't go on while the car was running, but at times it would go on at full speed after the car was off in an attempt to cool off the car, even if I only had driven it about 5 miles. If the economy mode was on, it would almost always go on at full speed while the car was being driven, and it wound sound like the car was turbine powered. It would then also stay on for about 5 minutes after the car had been shut off.

Also, thanks to the switch's inability to operate correctly, I believe this is also the cause of a "phantom coolant leak" in which I was loosing coolant without a trace. I had pressure tested the coolant system and could never track down a leak. However, if the coolant wasn't being circulated correctly when the car was off and the pump should have been on and it wasn't, then coolant would have a tendency to boil, expand and leave the system through the coolant overflow port.

Please note: The C5 2.7t has three fans. One in front of the radiator closest to the front of the car known as the AC condenser fan, and then two behind the radiator; the one on the passenger side is the auxiliary fan, and the one on the driver's side if the belt driven main fan that is always on when the car is running.

The fix was pretty straight forward since I also noticed that the after run secondary coolant pump was running when it shouldn't have been running so it clued me in that the thermal switch was either bad or going defective.

Most people talk about the green temp sensor at the top of the engine on the passenger side, or the blue one on the bottom closest to the radiator, but what many people don't know is that there is another sensor on the back of the driver's side cylinder head that's known as the thermal switch.

It sits in between the driver's side EGR/Kombi valve and the secondary air pump, and if I recall correctly, its job on the 2.7t is to turn on the secondary coolant pump, which will also activate the fan to assist in cooling off the engine. But if it's malfunctioning, then all kinds of weird stuff will happen.

Here are the steps involved in gaining access to the thermal switch:

- move the coolant expansion tank out of the way
- disconnect the main air supply hose going to the kombi valve (Be careful not to break it as it's brittle!)
- Remove the y-pipe
- Remove the secondary air pump

This should give you plenty of room to gain access to the green switch. Mine was actually seized in the housing. What I did to break it free was disconnect the wiring and then stick a large flat head screw driver into the sensor and twisted it to break it free from all the crud that was locking it in there. Once you do this you're committed because you will destroy the contacts on the switch in the process.

Also make sure to use a pick to remove the o-ring that will most likely be left in the housing. You might not be able to see it in there, but trust me, there's one in there.

Some tips on installation: Use some silicone spray lube on the o-ring to help it in the housing's slot upon installation, as well as spray a little bit on the sensor itself to help slide it in. You should feel the sensor "fall" into place when you have it seated properly.

I figured I would do a quick write up on this problem as I've read a lot of threads out there on different sites of people describing similar symptoms, yet it seemed like they gave up and never reached a solution.

 

Thank you for taking the time out to post this. It's exactly what I needed to know in order to help solve my coolant leak & failing A/C condenser fan.