Why Is My 1991 350 SDL Running Hot?

[ezrider]

Have had the car since new, 142K miles now. Car always ran a little over 80 degrees C in the past.

Jumped all of a sudden to just under 100C. Low speed electrical radiator speed electrical fan on all the time now, high speed electrical fan never comes on - it used to cycle frequently in the past even at the little over 80C temp. When I rev up the motor good with the hood open in the driveway the main engine cooling fan does not seem to change it's speed/pitch and I can't hear it
make a "whoosh" sound as I think it should.

My mechanic says he's stumped after changing the engine heat sensor,thermostat and coolant. He says the engine cooling fan is "operating normal at high speed" (am I wrong in my observation above?) He says the high speed electrical fans kick in as they should at 105 degrees C. He wants to change the engine water pump next.

My main questions now are:

1. At about what RPM should the main engine cooling fan clutch kick in and should it be obvious to see and/or hear it when it does?

2. What could be the reason(s) the high speed electrical cooling fans are not kicking in and cycling below 105 degrees C.

In the past I had a problem with the high speed electrical fans being stuck on high speed and replacing the main air conditioning relay fixed that problem. Sorry ror the long post. Thank you.