My OBD2 to OBD1 Jumper harness has caused my RPM gauge not to work. I have searched and have not found a exact answer how to fix the issue. I have heard that you have to change certain pins or at least one pin. Thanks in advance.

 

look what i found for ya look for the A19 wire jumpered with the OBD2b B13.....all going to OBD1 A21 .

look at the 26 pin connector , pin number a21 and a25 tap in a 1k ohm resistor between the wires

or last run a wire from the engine bay connector should be a blue wire from the distributer to the connector , now from that connector run a new wire to the ecu pin a19 tach input

hope this helps there seem to be a few remedies