I didn't read alan911's last post and tested it myself. 0VDC at pins 9 and 11 (compressor rpm) and 1 and 2 (engine rpm). I switched to duty cycle and got a varying 3-25Hz at the compressor at idle and 1500Hz at the engine at idle. It was fun testing the engine rpm signal, which seems to be twice crank rpm, while changing the setting on the ELR resistor. At least I know the ELR system works

I'll check for AC voltage and see what I get. The compressor frequency fluctuates at idle but is steady at revs. I expect the same will be the case with AC voltages, whatever they are.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD