Working on a 1982 380SL with a stubborn high idle (1200 in Park). The air valve and idle control module are known good. The problem is that the control module is only sending 1 volt to the valve.

Yes, the throttle position switch is working.

So I am going to check all of the inputs to the control module at the control module's plug. Simple enough except for one: how do I measure the "tach" (rpm) signal that comes from the ignition module?

TIA

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe