You never really think about your ability to set the fan in the car at different speeds until all of a sudden you can't...
Angie has been driving our 2005 VW Golf and thus it wasn't for some time that I noticed the fan only worked on the high position. She doesn't complain much and since she had a work around she took it. Winter is an especially annoying time to have the fan not work at low speed since a warm car will tend to fog the windshield so I knew this had to be fixed.
The problem turns out to be a common one caused by a failed thermal switch:
That's the little blue headed pointy thing in the picture. The big green part is the resistor pack that actually runs the fan at lower speed. The thermal switch is there to keep the resistor pack from burning up if too much power is requested.
So I could just replace the thermal switch but that doesn't address the root cause of the problem which is a sticky fan. The 4th generation Golf and Jetta have a tendency to get some water in the fan which gums up the motor, the gummy motor draws more power and pops the thermal switch. So I popped out the fan, cleaned its bearings (well, bushings actually) good with carb cleaner and dripped a variety of lubricating substances in. I saw a YouTube video where the guy used WD40 but WD is "water displacing" its not really a lubricant. I started with Marvel Mystery oil lubricating formula and moved on to PB Blaster in an effort to find something that would make its way into the bushing. Then I reinstalled the fan and replaced the thermal switch.
I was a little nervous about soldering the new switch in, it is a thermal switch after all but I limited myself to just quick touches with the soldering iron and I'd bought a 5 pack of the switches from Amazon so I knew if my soldering didn't work out I could always resort to crimping on another one.
Fortunately I seem to have done a good enough job, the fan has been working correctly for a couple weeks now.
Of course I could have replaced the whole resistor pack, IDParts has them for $35 but for $1 and a couple minutes with a soldering iron I'm well pleased with this fix. If this happens again I'll probably replace the fan, its difficult to get oil in to the bushings on the old fan, although maybe if I used a needle oiler...