Decoding Your 1990 Buick Century Check Engine Light: A Guide to OBD1 Diagnostics

Navigating car troubles can be stressful, especially when that check engine light illuminates in your 1990 Buick Century. While modern cars use OBD2 systems, vehicles from the early 90s, like your Buick Century, operate on an earlier system known as OBD1. Understanding how to diagnose your OBD1 system is key to addressing issues effectively. This guide will walk you through reading the trouble codes on your 1990 Buick Century, empowering you to understand what your car is trying to tell you.

Understanding OBD1 on Your 1990 Buick Century

The 1990 Buick Century, being a pre-OBD2 model, utilizes the first generation of on-board diagnostics, OBD1. Unlike the standardized OBD2 ports you find in newer vehicles, OBD1 systems in cars like the 1990 Buick Century use a different connector and method for retrieving diagnostic codes. It’s important to clarify that while you might be searching for “1990 Buick Century Obd2” information, your vehicle is indeed equipped with an OBD1 system. Don’t worry, accessing these codes is straightforward and doesn’t require expensive scanning tools. In fact, you can do it with a simple paper clip!

Locating the ALDL Connector

To begin reading your 1990 Buick Century’s diagnostic codes, you first need to locate the ALDL connector. ALDL stands for Assembly Line Diagnostic Link, and it’s the interface for accessing the OBD1 system.

For your 1990 Buick Century, the ALDL connector is typically found under the driver’s side dashboard. You’ll be looking for a connector that is often black or grey and has multiple terminals. It’s usually in a convenient, accessible spot, making it easy to perform a diagnostic check.

Reading OBD1 Codes with a Paper Clip

Once you’ve located the ALDL connector, you’re ready to retrieve the diagnostic trouble codes. Here’s how to do it using a simple paper clip:

1. Turn off the ignition: Ensure your Buick Century is turned off.

2. Locate terminals A and B on the ALDL connector. These terminals are usually labeled. (Refer to the image below for a visual guide).

   Alt text: Diagram showing a paper clip inserted into terminals A and B of an OBD1 ALDL connector for diagnostic code retrieval.

3. Use a paper clip to jump terminals A and B. Carefully insert a paper clip to create a connection between terminal A and terminal B. This essentially grounds the diagnostic terminal and initiates the code reading process.

4. Turn the ignition key to the “ON” position, but do not start the engine.

5. Observe the “Check Engine” or “Service Engine Soon” light. The light will begin to flash in a pattern. This flashing pattern is how OBD1 communicates diagnostic codes.

Interpreting the Flash Codes

The check engine light will flash a series of codes. It begins with “Code 12,” which indicates the diagnostic system is working correctly. Code 12 is represented by one flash, a pause, then two flashes. This “1-2” pattern will repeat three times.

After displaying Code 12 three times, the system will flash any stored trouble codes. Each trouble code is also flashed three times and follows the same pattern: flashes, pause, flashes. After all stored codes are displayed, Code 12 will flash again, indicating the end of the diagnostic code sequence.

For example, a “Code 32” would be displayed as three flashes, a pause, then two flashes, repeated three times.

1990 Buick Century OBD1 Trouble Codes

Here is a list of common OBD1 trouble codes for your 1990 Buick Century to help you understand what each code signifies:

• Code 12: Diagnostic System OK (No trouble detected)
• Code 13: Oxygen Sensor Circuit – Check wiring and sensor; replace if necessary.
• Code 14: Coolant Sensor Circuit (High Temperature) – Check for overheating, wiring, and sensor; replace sensor.
• Code 15: Coolant Sensor Circuit (Low Temperature) – Check wiring, ECM connections, and sensor.
• Code 16: System High Voltage – Check charging system; voltage above 17.1 volts.
• Code 21: Throttle Position Sensor (TPS) – Check for sticking or misadjusted TPS.
• Code 22: Throttle Position Sensor (TPS) – Check TPS adjustment; replace sensor.
• Code 23: Manifold Air Temperature (MAT) Sensor – Check sensor, wiring, and connectors for open circuit; replace sensor.
• Code 24: Vehicle Speed Sensor (VSS) – Ignore if car is stationary; check ECM connections.
• Code 25: Manifold Air Temperature (MAT) Sensor – Check voltage between sensor and ECM (should be above 4 volts).
• Code 26: Quad Driver Circuit – Requires professional diagnosis.
• Code 27: Gear Switch Diagnosis – Requires professional diagnosis.
• Code 28: Gear Switch Diagnosis – Requires professional diagnosis.
• Code 29: Gear Switch Diagnosis – Requires professional diagnosis.
• Code 31: Park/Neutral Switch – Check connectors and switch.
• Code 32: EGR System – Check vacuum hoses and connections for leaks/restrictions; replace EGR solenoid or valve.

Alt text: Image showing an EGR (Exhaust Gas Recirculation) valve installed on a car engine, a component related to OBD1 code 32.

• Code 33: MAF Sensor – Replace Mass Air Flow (MAF) sensor.
• Code 34: MAF Sensor – Check air duct, idle speed, vacuum leaks; inspect MAF sensor and connections.
• Code 38: Brake Input Circuit – Check brake light switch connections; adjust or replace switch.
• Code 39: Torque Converter Clutch – Check ECM connections; check for faulty solenoid.
• Code 41: Cam Sensor – Check connections and sensor; may indicate timing chain issue.
• Code 42: Electronic Spark Timing (EST) – Check connections and ignition module.
• Code 43: Electronic Spark Control (ESC) – Check knock sensor or Mem-Cal.
• Code 44: Lean Exhaust – Check ECM wiring, vacuum leaks at hoses and intake manifold gasket.

Alt text: Close-up image of an intake manifold gasket, a component relevant to diagnosing OBD1 code 44 related to lean exhaust conditions.

• Code 45: Rich Exhaust – Check evaporative charcoal canister and components for fuel presence.
• Code 46: Power Steering Pressure Switch – Check connections and switch.
• Code 48: Misfire Diagnosis – Check ignition and fuel systems.
• Code 51: PROM or Mem-Cal – Ensure proper installation; replace Mem-Cal or PROM.
• Code 52: Cal-Pak – Check Cal-Pak operation; replace Cal-Pak.
• Code 55: ECM – Check connections; replace ECM if necessary.
• Code 63: EGR Flow – Requires professional service.
• Code 64: EGR Flow – Requires professional service.
• Code 65: EGR Flow – Requires professional service.

Conclusion

Reading OBD1 codes on your 1990 Buick Century is a simple process that can save you time and money by providing valuable insights into potential issues. By using a paper clip and understanding the flash code patterns, you can effectively communicate with your car’s computer and begin the troubleshooting process. Remember to consult the trouble code list to understand what each code means and take appropriate action, whether it’s a simple fix or requires professional attention. This DIY approach empowers you to take control of your car’s maintenance and ensures your classic Buick Century stays running smoothly.