3rd/High Speed Fan Problem

[JAYTRES at aol.com]

Hello List,
Yesterday I was idleing the car for roughly 10-12mins in 90 degree weather. 
Steam began to rise out from under the hood. It so happens that i was 
watching my cooling gauge and saw it creep up to the 2/3 hot range on the 
gauge. (I heard the 2nd stage fan run at 1/3 hot and saw the voltage drop on 
the climate control voltage gauge) 

But the 3rd stage never came on, then things started to steam.

Now today i did all the cooling tests via SJMautotechnik and the after run 
1st stage works, 2nd stage works but no 3rd.

Any suggestions on where to look for problems about the 3rd stage not working?

TIA,
Jay Desmarais 1990 200TQ