Thanks. I did follow the 50 step testing procedures here,

A/c Relay - Mercedes-Benz Forum

before buying a new speed sensor, then a Klima relay from Peachparts. It all works right after that, except when it cuts off.
Thinking more about belt tension, no glazing is evident, but will oRder a new damper and spring payday, it does have 220,000 miles on it.