Hi Bob,
my '87 420 also had the electric fan running this morning when I stopped after about a minute to pick up the paper. I assume various temp. and pressure switches do their job. I had the car at an AC specialist after I bought it about 2 years ago and they tested when (at what pressure) the electric fan came on and found everything ok. As the AC works and temp is max at 100 in stop and go, otherwise around 85, I leave it alone.
Hope this helped.
Reinhard Kreutzer