Originally Posted by Arthur Dalton

"9-Pole Diagnostic Connector (1980-94)
The 9-pole Diagnostic Connector is used on earlier model vehicles. It can display
on-off ratio fault codes (1986 and later), RPM and Lambda sensor values. Various
on-off ratio Meters are available that provide access to this type of diagnostic
connector. Call Baum Tools at 800-848-6657 or 941-927-1414 for more information
on these meters."

Any DMM with Duty Cycle will work and they are easy to find.