Thanks Arthur for pointing that out ... I was referring to the '86 model 300E shop manual, so I suppose this circuit has changed since then. The sensor appears to be a 3 pole in the diagram, as opposed to the 2 pole that's on my '91.
The manual notes:
"As a protection against engine overheating an emergency cut-out is provided. After reaching approximately 115c engine coolant temperature, the switch is closed (grounded)(diagram pointing to head temp sensor) and the A/C compressor via control unit is cut-out. The switch opens at approx. 108c and the A/C compressor is then re-engaged."
'91 300E, 208k miles