For owners of the classic 90 Chevy Silverado, understanding your truck’s onboard diagnostic system is crucial for maintenance and repair. Unlike modern vehicles that utilize OBD2, your Silverado is equipped with an earlier OBD1 system. This article will delve into the specifics of the 90 Chevy Silverado OBD1 port, its pinouts, and explore the options available for reading diagnostic codes, especially considering the desire to use modern OBD2 readers.
Understanding the 90 Chevy Silverado OBD1 System
The 1990 Chevy Silverado, like many vehicles of its era, uses an On-Board Diagnostics generation 1 (OBD1) system. This system is designed to monitor various vehicle parameters and alert the driver to potential issues through a “Service Engine Soon” (SES) light. Accessing the diagnostic information on an OBD1 system requires understanding its specific connector and communication protocols, which are different from the standardized OBD2 systems we see in vehicles from the mid-1990s onwards.
The diagnostic port on your 90 Chevy Silverado is a 12-pin connector, often referred to as the Assembly Line Diagnostic Link (ALDL) connector. This connector is the gateway to retrieving diagnostic trouble codes (DTCs) and accessing some live data from the engine control module (ECM).
90 Chevy Silverado OBD1 Port Pinout: Decoding the 12-Pin Connector
Navigating the 12-pin ALDL connector can seem daunting, but understanding the function of each pin is key to diagnosing your Silverado. Here’s a breakdown of the pin assignments based on common information and schematics for GM OBD1 systems of that period:
- Pin A: Ground. This pin provides the necessary ground for diagnostic tools and self-test procedures.
- Pin B: ALDL Data (ECM/Codes Only). This is the primary data output pin for retrieving diagnostic trouble codes from the ECM. It’s crucial for initiating the self-diagnostic mode.
- Pin C: Canister Purge Solenoid (For Testing). This pin is specifically for testing the canister purge solenoid, a component of the evaporative emissions control system.
- Pin D: Not Used. This pin is typically not connected or utilized in standard diagnostic procedures for this model year.
- Pin E: Serial Data (For Scan Tool Data). This pin is used for serial data communication, primarily for connecting with scan tools to read live engine data.
- Pin F: TCC (Automatic Trans) or O/D Relay (Standard Trans). This pin’s function depends on your Silverado’s transmission type. For automatic transmissions, it relates to the Torque Converter Clutch (TCC), and for standard transmissions, it’s connected to the Overdrive (O/D) relay.
- Pin G: Fuel Pump. This pin is used for directly activating the fuel pump for testing purposes.
It’s important to note that while there are 12 pins in the connector, only 7 are typically populated and relevant for most diagnostic and testing procedures on a 1990 Chevy Silverado.
This image illustrates the typical pinout for a 12-pin OBD1 ALDL connector, highlighting the positions and labels of each pin.
Baud Rate Considerations: 160 Baud vs. 8192 Baud
When working with OBD1 systems, you might encounter discussions about different baud rates, particularly 160 baud and 8192 baud. Baud rate refers to the speed of data transmission. Early OBD1 systems, like those in the 90 Chevy Silverado, often used a slower 160 baud rate for data communication. Some later OBD1 systems, and potentially certain modules within the Silverado (though less common for the ECM itself in this year), could use a faster 8192 baud rate.
The original article mentions circuits designed for 160 baud ALDL communication. These circuits are built to convert the ECM’s data signal into a format readable by a standard computer serial port (RS232).
This diagram shows a basic hardware interface for 160 baud ALDL systems, utilizing a transistor level converter and resistors to adapt the signal for RS232 communication.
The resistor R3 in the 160 baud circuit is interesting. As noted in the original text, it’s recommended to initially try the interface without R3. Some vehicles might require a 10k resistor to initiate data transmission, but using it can alter the ECM’s operating mode, potentially leading to a high idle and disabled spark knock retard. Resistor R4 is specifically cautioned against for fuel-injected vehicles and is only intended for certain carbureted vehicles that don’t work with the standard interface.
The mention of pin M and 8192 baud in the original article is also relevant. While pin E is commonly used for slower speed serial data, pin M was sometimes used for higher speed scanners in certain applications. However, as pointed out, “The terminal ‘E’ and ‘M’ share the same data line back to ecm,” suggesting that for many diagnostic purposes, especially with the ECM data stream, the specific pin might be less critical than ensuring the correct baud rate and signal conversion.
Detailed Functions of ALDL Terminals
Expanding on the terminal descriptions provided in the original text gives a clearer picture of their diagnostic utility:
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Terminals “A” and “B” (Ground & ALDL Data): These are the key terminals for initiating self-diagnostics. Grounding terminal A to the chassis and connecting it to terminal B with the ignition in the “on” position (engine off) will activate the diagnostic mode. This allows you to retrieve stored trouble codes, often displayed as flashes of the SES light. Leaving these terminals unconnected is the normal driving mode. The article also mentions using resistors (3.9K ohm and 10K ohm) across these terminals to force backup modes or closed-loop operation for specific diagnostic scenarios, but these are less commonly used for basic code retrieval.
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Terminal “C” (Canister Purge Solenoid): On Chevrolet vehicles with an air pump, this terminal is connected to the ECM and can be used to monitor the air switching system via the oxygen sensor. Voltage readings on this terminal can indicate the status of the air pump system during warm-up and normal operation.
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Terminal “D” (SES Light Check): This is a handy terminal for quickly checking if the “Service Engine Soon” light bulb is functional. Applying 12V to this terminal should illuminate the SES light, helping to rule out a burnt-out bulb if you’re not seeing any warning lights.
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Terminal “E” (Scan Tool Data): This is the primary terminal for connecting a scan tool. Scan tools designed for OBD1 systems can interpret the data stream from the ECM through this pin, providing real-time data and more in-depth diagnostic capabilities than just reading codes.
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Terminal “F” (Torque Converter Clutch Solenoid): This terminal is specifically for automatic transmissions and allows you to check the status of the torque converter clutch (TCC) solenoid. Voltage readings will indicate whether the TCC is engaged or disengaged based on brake pedal position and ECM commands.
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Terminal “G” (Fuel Pump Test): Applying 12V to this terminal directly powers the fuel pump. This is a useful test for diagnosing fuel pump issues. If the pump runs when powered directly, it indicates that the pump itself is functional, and the problem likely lies upstream in the fuel pump circuit or control system.
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Terminals “H”, “J”, “K”, “L”: These pins are typically not used in the 1990 Chevy Silverado OBD1 system.
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Terminal “M” (High-Speed Scanner Data): As mentioned before, this terminal was sometimes used for higher-speed data transmission with advanced scanners, often in conjunction with terminal “E”. While potentially relevant for some specialized diagnostic tools, it’s less critical for basic OBD1 code reading on a 90 Chevy Silverado.
This image provides a visual guide to the ALDL connector terminals and their typical wiring connections within the vehicle’s diagnostic system.
Can You Use an OBD2 Reader on a 90 Chevy Silverado? The OBD1 to OBD2 Conversion Question
The core question for many 90 Chevy Silverado owners is whether they can use modern, readily available OBD2 readers on their OBD1 system. Directly connecting an OBD2 reader to the OBD1 port of your 90 Chevy Silverado will not work.
OBD1 and OBD2 are fundamentally different systems. They use different connectors, communication protocols, and diagnostic codes. OBD2 is a standardized system, while OBD1 was manufacturer-specific and varied significantly between makes and models.
While there are OBD1 to OBD2 adapters available, their functionality for older vehicles like the 90 Chevy Silverado is often limited. These adapters might physically adapt the connector, but they cannot magically translate the OBD1 communication protocol into OBD2.
Therefore, “converting” your 90 Chevy Silverado’s OBD1 system to be fully readable by a standard OBD2 reader is not a straightforward or practical solution.
Your best options for diagnosing your 90 Chevy Silverado are:
- Using an OBD1 Scan Tool: Invest in a scan tool specifically designed for OBD1 systems, particularly GM OBD1. These tools can communicate with your truck’s ECM and retrieve codes and live data.
- Manual Code Reading (Paper Clip Method): For basic diagnostic trouble code retrieval, you can use the “paper clip method.” This involves using a jumper wire or paper clip to connect terminals A and B of the ALDL connector with the ignition on. The “Service Engine Soon” light will then flash in a sequence to indicate trouble codes. You’ll need an OBD1 code chart specific to your 1990 Chevy Silverado to interpret these flashes.
- Specialized OBD1 to PC Interfaces: As hinted at by the 160 baud circuit diagrams, you can build or purchase interfaces that connect your OBD1 port to a computer serial port. Combined with appropriate software (like WinALDL mentioned in the original text), this allows you to read data and potentially perform more advanced diagnostics using a PC.
RS232 Pin Assignment for Interface Considerations
For those considering building or using a PC-based OBD1 interface, understanding the RS232 serial port pinout is helpful for making the correct connections.
This image shows the pin assignments for a standard DB9 RS232 serial port connector, commonly used in older PCs and for serial communication interfaces.
This RS232 pinout is relevant because the 160 baud interface circuit is designed to output a pseudo-RS232 signal, which can then be connected to the receive data (RxD) pin of a computer’s serial port for data processing.
Conclusion: Embracing OBD1 for Your 90 Chevy Silverado
While the desire to use modern OBD2 readers on a 90 Chevy Silverado is understandable, it’s essential to recognize the limitations and focus on appropriate OBD1 diagnostic methods. Understanding your Silverado’s OBD1 system, its 12-pin ALDL connector, and the functions of each pin is the first step in effective diagnostics and maintenance. Whether you choose to use a dedicated OBD1 scan tool, the manual code reading method, or a PC-based interface, embracing the OBD1 system is the most direct and reliable way to troubleshoot issues on your classic Chevy Silverado.
