Is your “Service Engine Soon” light illuminated in your 90 Chevy Silverado? Diagnosing your truck’s engine issues might seem daunting, but with a simple paper clip and this guide, you can easily retrieve the diagnostic trouble codes from your 1990 Chevy Silverado’s OBD1 system. While the term “OBD2” is commonly used for modern vehicles, your 1990 Silverado, part of the fourth generation (1988-1998), utilizes the earlier OBD1 (On-Board Diagnostics I) system. This system, while simpler than OBD2, is still incredibly helpful in pinpointing potential problems.
How to Check OBD1 Codes on Your 90 Chevy Silverado
Retrieving codes from your 1990 Chevy Silverado’s OBD1 system is a straightforward process that doesn’t require any specialized tools. All you need is a paper clip or a short piece of wire to bridge two terminals in the Assembly Line Diagnostic Link (ALDL) connector, often referred to as the DLC connector. This connector is typically located under the driver’s side dashboard.
Here’s a step-by-step guide:
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Locate the ALDL/DLC Connector: Look under the driver’s side dashboard for the ALDL connector. It’s usually a trapezoidal connector.
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Identify Terminals A and B: The connector has lettered terminals. You need to locate terminals “A” and “B”. In most cases, terminal “A” is the top right terminal, and terminal “B” is directly to its right, as shown in the image below.
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Jumper Terminals A and B: With the ignition switch turned off, use your paper clip or wire to create a jumper between terminal “A” and terminal “B” of the DLC connector. Ensure a secure connection.
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Turn Ignition On (Engine Off): Turn the ignition switch to the “ON” position, but do not start the engine.
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Observe the “Service Engine Soon” Light: Watch the “Service Engine Soon” light on your dashboard. It will begin to flash a series of codes.
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Decipher the Codes: OBD1 codes are communicated through flashes. Code 12 is the starting point and indicates the system is working correctly. It’s represented by a flash, pause, flash flash (1-2). Each digit of a code is represented by flashes, with a pause between digits and a longer pause between codes. Each code will be displayed three times before moving to the next code, and the entire sequence will repeat.
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Record the Codes: Carefully count the flashes and record each code. Codes are displayed in numerical order.
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Code 12 is Normal: Remember that code 12 is a normal diagnostic code indicating the system is functioning and is not indicative of a problem.
90 Chevy Silverado OBD1 Trouble Codes List
Once you have retrieved the codes from your 90 Chevy Silverado, you can use the following list to understand what each code signifies. Keep in mind that a code points to a potential area of concern, not necessarily a failed component. Further troubleshooting is often required to pinpoint the exact issue.
- 13 – O2 (Oxygen Sensor) circuit open
- 14 – ECT/CTS (Engine Coolant Temperature) Sensor circuit low (high temperature)
- 15 – ECT/CTS (Engine Coolant Temperature) Sensor circuit high (low temperature)
- 16 – VSS (Vehicle Speed Sensor) buffer fault
- 21 – TPS (Throttle Position Sensor) circuit high (signal voltage high)
- 22 – TPS (Throttle Position Sensor) circuit low (signal voltage low)
- 24 – VSS (Vehicle Speed Sensor) circuit low
- 28 – TR (Transmission Range) pressure switch assembly
- 32 – EGR (Exhaust Gas Recirculation) valve error
- 33 – MAP (Manifold Absolute Pressure) Sensor circuit high (low vacuum)
- 34 – MAP (Manifold Absolute Pressure) Sensor circuit low (high vacuum)
- 35 – IAC (Idle Air Control) error
- 36 – Idle speed control actuator error (when equipped)
- 42 – IC (Ignition Control) error
- 43 – KS (Knock Sensor) error
- 44 – Lean exhaust
- 45 – Rich exhaust
- 51 – PROM (Memcal, Chip) error
- 53 – System voltage high (supply to ECM/PCM)
- 54 – Fuel pump relay circuit low voltage
- 55 – ECM/PCM error
Understanding and Clearing OBD1 Codes
It’s crucial to remember that an OBD1 code is a starting point for diagnosis. It indicates a circuit or sensor is reporting data outside of the expected range. The actual problem might be with the sensor itself, the wiring, or related components. Thorough troubleshooting is essential to accurately identify the root cause of the issue.
After addressing the problem, you can clear the OBD1 codes in your 90 Chevy Silverado by disconnecting the negative battery terminal for approximately 20 seconds. Alternatively, codes will automatically clear after 50 engine starts if the fault condition is no longer present.
By using this guide, you can effectively diagnose your 90 Chevy Silverado’s engine issues using its OBD1 system. This simple paper clip method empowers you to take control of your vehicle’s maintenance and understand potential problems, saving you time and money. Remember to consult a qualified mechanic for complex issues or if you are unsure about any diagnostic or repair procedures.
