Like many other General Motors cars and light trucks, the 1990 GMC Pickup is equipped with an onboard computer system that has diagnostics capabilities to allow for diagnostics of component failure. The system in the truck stores codes relating to components and systems that can be used to narrow down the possible causes of faults. Also available on this system is a data stream that allows the observation of sensors and devices, while they are being operated, using a scan tool.
Turn the ignition key to the "OFF" position. Locate the diagnostic connector. On the 1990 GMC truck, it is a black, rectangular connector located below the driver side of the dash to the right of the steering column.
Unfold a paper clip and bend it into a "U" shape. Plug the paper clip into the two terminals on the right end of the top row of terminals.
Turn the ignition key to the "ON" position. The check engine light will begin to flash. Long flashes indicate "Tens" and short flashes indicate "Ones." So, for example, one long flash followed by two short flashes indicates a code 12. Count the flashes and write down the codes.
Compare the codes to the descriptions in the service manual.
