Understanding your vehicle’s diagnostic system is crucial for both routine maintenance and troubleshooting issues. For owners of a 99 Suburban, the OBD2 ALDL plug serves as the gateway to your vehicle’s computer, allowing you to access valuable diagnostic trouble codes (DTCs). While modern scan tools offer sophisticated diagnostics, your 1999 Chevy Suburban also supports a method of flashing codes using the Malfunction Indicator Lamp (MIL), often known as the “Service Engine Soon” light. This guide will walk you through understanding and utilizing your 99 Suburban’s OBD2 ALDL plug to retrieve basic DTCs.
Understanding the ’99 Suburban OBD2 ALDL Plug
The term “OBD2 ALDL plug” might seem a bit confusing, but it refers to the diagnostic connector in your 1999 Chevrolet Suburban. “OBD2” (On-Board Diagnostics II) is the standard, and “ALD”L (Assembly Line Diagnostic Link) is a General Motors term used for earlier diagnostic systems, which in this context is somewhat of a misnomer as OBD2 is the relevant standard for a 1999 vehicle. This connector, standardized across most vehicles in this era, is typically located under the dashboard on the driver’s side.
This 16-pin OBD2 ALDL plug is not just a connector; it’s your interface to the Powertrain Control Module (PCM), the brain of your Suburban’s engine management system. Pin 6 of this connector is designated as the diagnostic terminal on some vehicles, and we’ll be using it to access trouble codes. Pin 5 is the ground terminal, which is crucial for initiating the diagnostic process.
Accessing Diagnostic Trouble Codes via Flashing MIL on Your 99 Suburban
While a diagnostic scan tool is the recommended method for reading DTCs, you can use a simpler technique on your 99 Suburban to retrieve basic codes. This involves grounding terminal 6 of your OBD2 ALDL plug to terminal 5. Here’s how to do it:
- Locate the OBD2 ALDL Plug: Find the 16-pin diagnostic connector under the dashboard on the driver’s side of your 99 Suburban.
- Identify Terminals 5 and 6: Referring to the diagram above, or the original article’s description, locate terminal 6 (diagnostic terminal, if equipped), which is the third terminal from the left on the top row. Terminal 5 (internal PCM ground) is directly to the left of terminal 6 on the top row.
- Ground Terminal 6 to Terminal 5: Using a jumper wire or a paper clip, carefully connect terminal 6 to terminal 5. This essentially grounds the diagnostic terminal.
- Turn Ignition ON (Engine OFF): Insert your key into the ignition and turn it to the “ON” position, but do not start the engine.
- Observe the MIL (Service Engine Soon Light): Watch the “Service Engine Soon” light on your dashboard. It should begin to flash a sequence of codes.
Decoding the Flashing Codes: DTC 12 and Beyond
The first code you should see is DTC 12. This code, represented by a flash, pause, flash-flash, long pause, repeated three times, indicates that the diagnostic system is working correctly. If you see DTC 12 flashing, it confirms you’ve successfully initiated the diagnostic mode.
Following DTC 12, if any diagnostic trouble codes are stored in your 99 Suburban’s PCM, the MIL will flash those codes. Each DTC will be flashed three times, and if multiple codes are present, they will be displayed from the lowest to the highest numerically. If no other codes are stored, the system will continue to flash DTC 12.
Important Note: This flashing method provides basic DTCs. For comprehensive diagnostics, including deeper system analysis and live data, using an OBD2 scan tool is always recommended. This method is a helpful alternative for quick checks on older vehicles like the 99 Suburban when a scan tool isn’t immediately available.
By understanding your 99 Suburban’s OBD2 ALDL plug and this simple flashing code method, you can gain valuable insights into your vehicle’s health and potentially address minor issues yourself. Remember to consult a repair manual or professional mechanic for proper diagnosis and repair based on the retrieved DTCs.
