Diagnostic Link Connector on a 1993 Chevy S10 Pickup

GM's on-board diagnostic system on all pre-1996 S10/S15 vehicles have the capability of displaying trouble codes by jumpering two terminals of the diagnostic link connector (DLC).

In this tutorial, I'll show you where to find this connector and what 2 terminals you need to jumper together to retrieve the diagnostic trouble codes (DTCs) that are lighting up the check engine light (CEL).

Contents of this tutorial at a quick glance:

  1. Reading the On/Off Flashes of the Check Engine Light.
  2. GM OBD I Diagnostic Trouble Code Definitions.

En Español Puedes encontrar este tutorial en Español aquí: Cómo Leer Los Códigos De Falla (2.8L V6 GM) (en: autotecnico-online.com).

Reading The On/Off Flashes Of The Check Engine Light

Reading the On/Off Flashes of the Check Engine Light (How to Retrieve GM OBD I Diagnostic Trouble Codes)

The diagnostic link connector is located under the dash. More specifically, it's usually on the right side of the steering wheel column.

In GM tech speak, the diagnostic link connector (DLC) is known as the Assembly Line Data Link (ALDL) connector. This bad boy is the same across all GM vehicles that are pre-OBD II. The photo at the beginning of this tutorial is what the ALDL connector looks like.

You can use a paper-clip or a small piece of wire (with both ends stripped) to ‘jumper’ these two terminals together.

These are the steps to retrieve the trouble codes:

  1. Make sure your battery is fully charged before starting (its customary to place a charger on the battery while retrieving trouble codes to keep the battery from discharging).
  2. Turn the key to the On position but don't start the vehicle.
  3. Jumper terminals A and B, of the diagnostic link connector, together.
  4. If no trouble codes are present:
    1. The MIL will flash a code 12 (see illustration below) which consists of one flash, a short pause, then two flashes in rapid succession. Code 12 would look like this:
      1. ‘Flash’, pause, ‘flash’, ‘flash’, long pause.
    2. Code 12 will is repeated another 2 times (all codes are flashed 3 times before the next one).
  5. If trouble codes are present:
    1. The MIL will flash code 12 three times and then any other codes stored.
    2. Each trouble code, in addition to code 12, is flashed 3 times.
    3. A code 22 would look like this:
      1. ‘Flash’, ‘flash’, pause, ‘flash’, ‘flash’, long pause.

Once you've confirmed and written down the trouble codes, remove the jumper wire from the diagnostic link connector (DLC) and turn the key to the Off position.

Reading the On/Off Flashes of the Check Engine Light (How to Retrieve GM OBD I Diagnostic Trouble Codes)