Coolant warning wrong??

[Mark Novak]

My 1991 200 has recently begun showing the coolant warning symbol on the
dashboard after only 15 seconds of starting the engine.  It persists the
entire time the car is running.  The coolant level is at maximum and the
temperature gauge on the dash is showing 1/4 to less than 1/2.  I am
assuming the problem is the multi-temperature function switch (MTFS) going
bad, but any other ideas on why the warning?????????