Doesn't the CFI Module have to be in the 'Diagnostic Mode' from '88 up to read duty cycle? On my '93, I use the homemade LED Tester to check for DTC codes and also to put it into diagnostic mode to check my duty cycle. Otherwise, all you'll read is battery voltage across pins 2 & 3.

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1993 190E 2.3
2001 SLK230
1971 LS5 (454) Corvette Convertible