Prior to the introduction of OBD-II (on-board diagnostics, series 2) in 1992, several manufacturers including GM used a type of interface called ALDL (assembly line diagnostics link). While ALDL performed all of the engine diagnostic procedures of it's OBD successors, GM didn't design it with end-user interaction in mind. But even the less mechanically inclined can pull codes from a pre-OBD car with little more than a paper clip and two minutes of free time.
Look under the bottom left side of your dashboard, below the steering wheel and just ahead of your left knee. Look for a plastic wiring harness connector about 0.5 inch tall by 1.5 inch wide with twelve holes in it; this is the ALDL port. You'll notice that the holes in the port are in two rows of six, and that the two holes on the top right have very visible brass inserts.
Bend a paperclip into a narrow U-shape so that the tips will fit inside the two brass-inserted ports on the top row --- designated terminals A and B. Turn your ignition key to the "On" position but do not start the car.
Connect terminals A and B with the paperclip "jumper" wire. You'll see the check engine light begin to flash. Pay close attention to the flashes --- they refer to the two-digit error code. One long flash indicates a "one" in the tens place, two long flashes indicate a "two" in the tens place, and so on. There will be a pause after the long flashes, followed by a series of short flashes indicating the ones place. Example: three long flashes followed by five short flashes indicate a code 35.
Upon inserting the paperclip jumper into the terminals, you'll note that the check engine light flashes a code 12 --- one long flash, two short flashes --- three times. This is to notify you that the diagnostics system is functioning and about to display codes. Wait for the long pause after the third code 12.
Note the codes that follow, if any. If your computer has any current or active codes, write them down and refer to your reference source to decode them. You can use a print reference source, or you can click the link in the Resources section of this article to find the codes for your specific application.
