At what engine temp should the aux fan in front of the radiator turn on?

it's been cold/cool so far, but I want to make sure this is operational on my 89 300e.

I might also seek some general comment on this... when I got the car, no heat. So I switched out the monovalve and now have heat. However before the switch, the engine temp never went above 80 c. Now it goes to about 85-86 c... maybe up to 88 c when stopped in traffic.

any thoughts are welcome
thanks