cycling of cooling fan

[Calvin Krug]

While working on a coolant leak problem on my 200, I noticed some odd 
behavior from my radiator fan.  It doesn't come on with the turbo 
pump after the engine is turned off, and when sitting in traffic, the 
fan cycles on an off at a very regular rate.  Approximately 20 
seconds on, and then off for about 30.  Is this normal?  I recently 
replaced the multifunction temp switch, but still get a fault code 
for a coolant sensor.  So, are these related problems, or do I have 
both a defect in a coolant sensor circuit AND a problem with the fan 
controller?
On a positive note, at least it doesn't leak coolant into the 
cylinders anymore.  A little bit of radiator stop leak "goo" did the 
trick.  I still don't recommend that stuff, though.

Calvin K