Looks like it has not been used. I have the cables but the manual was not in the box.
Vintage Sears Inductive Engine Analyzer 2821423 W/ Box And Cables UNTESTED
SEARS CRAFTSMAN ENGINE ANALYZER MODIFIED WITH ORIGINAL MANUAL, CASE, AND CABLES
Vintage Sears Ampere Damper 28.7104 - With Original Box
Vintage Sears Craftsman Engine Analyzer Inductive Tach 161.210400 No Cables
Vintage Sears Engine Analyzer Model In Original Box Complete 9-2163
Vintage Sears Craftsman Engine Analyzer 28-21040 in original Box -
