Overheating...testing thermo-switch

I have a 2003 Golf that is overheating at highway speeds after it gets up to temp. I can pull the gauge back down if we run the heat to dump heat away from the engine.

I pulled the thermo-switch off today and jumped between pin 1 & 2 and the small fan (passenger side) kicked on but the large fan (drivers' side) did not. I turned the key to "on" and tested the voltage out of the pin tied to the red wire, and it's bobbing around in the -0.5 to -3V range. Thoughts?

I'm following this document (http://pics.tdiclub.com/data/516/A4_Coolant_Fan_testing_Rev_7.pdf)

I should also mention I have checked all the corresponding fuses to the fans/cooling system, and everything looks perfectly fine.

Two thoughts:

- both fans should come on at idle when you turn the A/C on. If not, start there, and it sounds like the large rad fan is your first task.

- that said, at highway speeds it's very rare for the fans to come on at all, since the motion of the car is already providing all the airflow the rad should need.

So, I'd be surprised if your overheating on the highway is actually related to your fans. Other suspects would be the water pump, the t-stat, a clogged radiator, and, worst-case, a leaking headgasket.