Navigating the world of car diagnostics can be confusing, especially when dealing with older vehicles during transitional periods in automotive technology. If you’re facing issues with your 95 Riviera OBD2 port malfunction, you’re not alone. The 1995 model year for General Motors (GM) vehicles is a notorious gray area when it comes to On-Board Diagnostics (OBD) standards. This article dives deep into the complexities surrounding 1995 GM OBD systems, particularly focusing on why your 95 Riviera or similar models might be giving you trouble when you try to use a standard OBD2 scanner.
The 1995 OBD-II Dilemma: Not Always Standard
The automotive industry’s shift towards OBD-II standardization began in 1994, with GM phasing it in during 1995. However, this transition wasn’t uniform. Depending on the specific vehicle model and its production date within 1995, GM produced vehicles with varying diagnostic systems. Notably, some 1995 GM vehicles feature a 16-pin connector that looks like a standard OBD-II port but doesn’t function as one. These vehicles can be broadly categorized into at least two groups with non-standard 16-pin connectors.
Group 1: ECM and Modified ALDL (Think S-10)
The first group of 1995 GM vehicles retained an Electronic Control Module (ECM) rather than adopting the more advanced Powertrain Control Module (PCM). These vehicles still relied on the older system of flashing the Malfunction Indicator Lamp (MIL), often called the “Service Engine Soon” (SES) light, to communicate error codes.
Crucially, their 16-pin connector is not a standard OBD-II Diagnostic Trouble Code (DTC) connector. Instead, it’s a modified Assembly Line Diagnostic Link (ALDL) connector. The 1995 Chevrolet S-10 pickup is a prime example of this system. Vehicles in this group are NOT OBD-II compliant and will not communicate with standard OBD-II scanners.
The pin configuration for these modified ALDL connectors typically looks like this:
_________________________
X | X | X | 4 | 5 | 6 | X | X
`\9 | X | X |12 | X | X | X |16/
``------------====-------------Where “X” denotes unused pins.
To retrieve error codes from these vehicles, you need to bypass the non-standard connector and revert to the older method. By shorting pin 5 (ground) and pin 6 (test), you can initiate the ECM’s diagnostic mode. The SES indicator light will then blink in a specific pattern to display the error codes.
Key takeaway for this group: You can still get diagnostic information without an OBD-II scanner, but you need to use the traditional method of shorting pins and reading the blinking SES light. Trying to use an OBD-II scanner on these vehicles will be futile.
Group 2: PCM and the Confusing 16-Pin (LeSabre/Riviera Mystery)
The second group presents a more perplexing situation, potentially affecting models like the 1995 Buick Riviera. These vehicles are equipped with a PCM, indicated by wiring, plugs, and terminal counts, a more advanced system than the ECM found in the first group. Component location diagrams for engines like the VIN L 3800 SFI V6 in models like the LeSabre also identify the module as a PCM and refer to a “DTC connector,” not an ALDL.
However, despite having a 16-pin connector and a PCM, these vehicles do not respond to the pin-shorting method used for ECM-based systems. Extensive multimeter testing across various pin combinations to ground will likely yield no results in initiating the code-by-blink mode, a characteristic feature of PCMs.
The 16-pin connector pinout in these vehicles, as observed in a 1995 LeSabre, differs from both the modified ALDL and the standard OBD-II configurations:
_________________________
X | X | X | 4 | 5 | X | X | 8
`\9 | X | X | X | X |14 | X |16/
``------------====-------------Where:
- X = Unused
- 4 = Chassis Ground (black)
- 5 = Signal Ground (black/white)
- 8 = Door Lock Programming (short to ground, 4 or 5) – (black/white)
- 9 = Unknown, Hot with Ignition (tan or pink or orange)
- 14 = Unknown, Constant Hot (blue or navy or purple)
- 16 = Battery+ Constant (red)
The suspicion is that GM intended these vehicles to be OBD-II compliant and installed 16-pin DTC connectors accordingly. However, changes in the OBD-II standard, wiring misconfigurations, or other factors might have led to a non-standard implementation. The Vehicle Emission Control Information label under the hood might even state “OBD-I Certified,” adding to the confusion.
For owners of a 1995 Buick Riviera experiencing OBD2 port issues, it’s highly probable your vehicle falls into this second group. Simply plugging in an OBD-II scanner may not work. While some might speculate about rewiring pins 9 and 14 to the standard OBD-II positions for GM vehicles (pins 2 and 10), this is risky and not recommended without expert knowledge and proper wiring diagrams.
Understanding Standard OBD-II for Comparison
To further clarify the issue, let’s look at the standardized OBD-II 16-pin DTC plug configuration:
_________________________
X | 2 | X | 4 | 5 | 6 | 7 | X
`\X | 10| X | X | X | 14| 15| 16/
``------------====--------------Where:
- X = Unused
- 2 = SAE J1850 Bus+
- 4 = Chassis Ground
- 5 = Signal Ground
- 6 = CAN High (SAE J2284)
- 7 = ISO 9141-2 K-line
- 10 = SAE J1850 Bus-
- 14 = CAN Low (SAE J2284)
- 15 = ISO 9141-2 L-line
- 16 = Battery+
Pins 4, 5, and 16 are mandatory in the OBD-II standard. Pins 2, 6, 7, 10, 14, and 15 are used for data communication, and their specific usage depends on the vehicle’s communication protocol.
OBD-II Protocols and Pin Assignments
Different manufacturers and vehicle types utilize different OBD-II communication protocols. Here’s a breakdown of common protocols and their pin assignments:
- SAE J1850 VPW (Variable Pulse Width Modulation): Used by GM cars and light trucks. Employs pins 2, 4, 5, and 16, but not pin 10.
- ISO 9141-2: Used by Chrysler, European imports, and most Asian imports. Utilizes pins 4, 5, 7, 15, and 16.
- SAE J1850 PWM (Pulse Width Modulation): Used by Ford vehicles. Uses pins 2, 4, 5, 10, and 16.
Conclusion: Navigating the 1995 GM OBD Maze
Diagnosing a 95 Riviera OBD2 port malfunction or similar issues in other 1995 GM models requires careful consideration. The 1995 model year represents a transitional phase where OBD-II implementation was inconsistent.
Recommendations for 1995 GM Vehicle Owners:
- Check the Vehicle Emission Control Information label: Located under the hood, this label might indicate “OBD-I Certified” or “OBD-II Certified.” However, even an “OBD-II Certified” label doesn’t guarantee a fully standard implementation in 1995 GM vehicles.
- Identify your vehicle’s system: Determine if your vehicle uses an ECM (older system, likely modified ALDL) or a PCM (potentially miswired 16-pin connector). The presence of a PCM suggests a possible intended OBD-II system, albeit potentially flawed.
- Consult a professional: Due to the complexity and potential wiring issues, seeking help from a qualified mechanic with experience in older GM vehicles is highly recommended. They can properly diagnose the system and avoid potential electrical damage.
- Avoid DIY rewiring: Unless you possess expert-level automotive electrical knowledge and access to accurate wiring diagrams for your specific 1995 GM model, do not attempt to rewire the OBD port. Incorrect wiring can damage your vehicle’s computer system.
Understanding the nuances of 1995 GM OBD systems is crucial for effective diagnosis and repair. While frustrating, recognizing the non-standard nature of some of these systems is the first step towards resolving your 95 Riviera OBD2 port malfunction or similar issues.
