Pulling codes on EEC-IV

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I am trying to pull codes on this truck with EEC-IV. Per instructions, I am jumping SIG RTN with STI (Self Test Input). Then turning the key to ON (Engine off). Check Engine Light should flash out the trouble codes but nothing happens at all.

What am I missing? Should I also use a voltmeter and jump STO to the battery? I think Check Engine Light should be sufficient.