Even though I no longer have an Isuzu, I still have my FSM since I am still a moderator here.

There is a diagnostic connector under the removable tray in the center console. Two pins on the connector need to be jump to put it in diagnostic mode, which allows one to set the timing correctly. I am not sure but I believe that it is pins A and B which need to be jumpered. The FSM shows that.

To be honest, years ago I found a small GM diagnostic device which plugs into the diagnostic port. I even kept mine next to the connector in the console. Once I found that device, I never manually jumpered the pins again.