1994 GMC C1500 diagnostic

On the drivers side
bottom of the dash there's the obd1 connector where you would put a code reader on. Look at the top row of it far right side there's two pins side by side. Turn the key to the run postion don't start it and jump those two pins together and leave it that way. Watch the service engine soon light flash. You will see it flash once then two faster flash's. That's a code 12 that means the computer is good it will do this 3 times if it repeats the code 12 after the 3 times you don't have any codes. But watch the light for example if u see 3 quick flash's then 2 quick one's that would be a code 32 it would repeat 3 times then go to the next code when it gets back to code 12 then the code's are done. Let me know what you find and we will go from there.

Hello,

Here is the proceedure.

To extract a trouble from the computer for diagnostic purposes, the Assembly Line Communication Link (ALDL) connector is used.

Ground terminals A and B with the ignition "ON" and engine not running. The system will enter the diagnostic mode. In this mode, the computer will display a 12, three consecutive times, by flashing the "CHECK ENGINE", "SERVICE ENGINE SOON" or "MALFUNCTION INDICATOR LAMP" or light. A 12 consists of one flash, a short pause, then two flashes in rapid succession.

After 12 is displayed, any stored trouble s will be displayed by flashing the "CHECK ENGINE", "SERVICE ENGINE SOON" or "MALFUNCTION INDICATOR LAMP" light. All s are displayed 3 times each then a 12 will repeat. In the diagnostic mode, the computer will also energize all computer controlled relays and solenoids.