Upgrade Your 95 SC400 to OBD2: Solve Lean Issues and Enhance Diagnostics

If you’re the proud owner of a 1995 Lexus SC400, you might be facing a common issue that plagues many early OBD1 systems, especially when pushing the performance limits. This article dives into the problem of lean air-fuel ratios in modified 95 SC400s and provides a comprehensive guide to upgrading your vehicle to OBD2 for improved engine management and diagnostic capabilities.

Understanding the OBD1 Lean Issue in 95 SC400s

Owners of 1995 Lexus SC400s (and Toyota Supras of the same era) with OBD1 ECUs often encounter a frustrating problem: a lean condition between approximately 4,000 and 6,000 RPM under high boost. This issue is particularly noticeable in turbocharged models or those with performance modifications aimed at increasing boost pressure. The engine starts running dangerously lean in this critical RPM range, as illustrated in the datalog below from a ’95 SC400 under high boost conditions. Attempting to compensate for this lean condition by simply adding fuel through piggyback systems like SAFC-II often proves ineffective and doesn’t fully resolve the underlying problem.

This lean running condition is not only detrimental to performance, robbing you of power, but it also poses a significant risk of engine damage due to increased combustion temperatures and potential detonation.

Why Upgrade to OBD2 for Your 95 SC400?

The solution to this OBD1 lean issue is surprisingly straightforward and offers a host of additional benefits: upgrading to an OBD2 ECU. By swapping your original OBD1 ECU with an OBD2 unit, you can completely eliminate the problematic lean condition. As demonstrated in the before-and-after datalogs, simply installing an OBD2 ECU resolves the lean issue without any additional fuel adjustments. OBD2 ECUs are known to run richer in stock configuration, providing a safer and more stable air-fuel ratio, especially under boost.

Beyond fixing the lean issue, upgrading to OBD2 unlocks a significant advantage in terms of diagnostics. OBD2 is a standardized system that allows you to connect a wide range of readily available OBD2 scan tools to your vehicle. This capability enables you to:

• Read and Clear Diagnostic Trouble Codes (DTCs): Easily identify and clear Check Engine Light (MIL) codes, saving you trips to the mechanic for basic diagnostics.
• Monitor Engine Parameters in Real-Time: Access live data streams for crucial engine parameters such as:
  • Timing advance
  • Short and long-term fuel trims
  • Coolant temperature
  • Throttle Position Sensor (TPS) voltage
  • Intake Air Temperature (IAT)
  • Engine load
  • Vehicle speed
  • Mass Air Flow (MAF) rate
  • Oxygen (O2) sensor voltage

This real-time data is invaluable for performance tuning, troubleshooting engine issues, and generally understanding your engine’s health. Further fine-tuning the OBD2 ECU with a piggyback controller like SAFC-II after the swap can then optimize the air-fuel ratio for even better performance and efficiency, as shown in the tuned OBD2 datalog below.

OBD2 Upgrade: Step-by-Step Guide for Your 95 SC400

Upgrading your 95 SC400 to OBD2 involves three main steps: swapping the ECU, addressing the primary O2 sensor heater wiring, and installing an OBD2 diagnostic port. While the ECU swap itself is plug-and-play, the additional wiring and diagnostic port installation require some careful modifications.

1. ECU Swap: Plug and Play

The physical installation of the OBD2 ECU is remarkably simple.

1. Disconnect the Battery: Always start by disconnecting the negative terminal of your car battery for safety.
2. Locate and Disconnect the OBD1 ECU: The ECU is typically located in the passenger side footwell. Disconnect the wiring harness from your OBD1 ECU.
3. Connect the OBD2 ECU: Plug the wiring harness directly into the OBD2 ECU.

At this point, your car should start and run with the OBD2 ECU. However, to ensure all systems function correctly, particularly the primary O2 sensor heater and diagnostic port, you need to address the following steps. Note that the TRAC system might not function perfectly after just the ECU swap and may require further investigation beyond the scope of this guide.

2. Addressing the O2 Sensor Heater Ground

OBD2 systems utilize an additional ground wire for the primary O2 sensor heater circuit, which is not present in OBD1 wiring. Without this ground, the O2 sensor heater may not function correctly, especially in colder conditions or during prolonged idling. This can prevent the system from entering closed-loop operation, leading to a rich running condition at idle. Adding this ground wire is crucial for proper OBD2 functionality.

Procedure:

1. Identify a Donor Pin: You’ll need an extra ECU pin. Locate the yellow protectors on the ECU wiring harness connectors and carefully pop them off using a small screwdriver. Identify pin TE2 (E10-19), which is a pink wire with a green stripe on OBD1 ECUs and is not used on OBD2.
2. Remove the Pin: Using a small safety pin, gently push up the plastic tab that secures the TE2 pin in the harness connector. Lightly pull the wire from the back of the connector; it should slide out easily. Cut the wire a few inches from the connector.
3. Insert Pin into New Ground Location: Locate the new ground wire location E03 (E9-78) on the ECU harness. Insert the pin you removed from TE2 into this location.
4. Extend and Ground the Wire: Lengthen the wire attached to the pin you just inserted so it can reach a suitable ground point. A convenient ground point is the mounting location for the TRAC ECU. Attach the wire securely to the ground point.

This modification ensures the primary O2 sensor heater functions correctly with the OBD2 ECU.

3. Installing the OBD2 Diagnostic Port

The final step is to install a functional OBD2 diagnostic port, as the original OBD1 port is not compatible with the OBD2 communication protocol. This involves moving a pin in the ECU harness and wiring in a standard OBD2 connector.

Procedure:

1. Disconnect Battery and ECU Harness: Ensure the battery is disconnected and remove the wiring harness from the ECU again. Remove the yellow protectors from the ECU harness connectors.

2. Move Communication Pin: On OBD1 cars, pin E9-29 (VF1) is used for serial communication. This pin is not used on OBD2. Conversely, OBD2 uses pin E10-8 (SDL) for serial communication, which is unused on OBD1. You need to move the wire from pin E9-29 to E10-8. Pin E9-29 is a light green wire.

3. Remove and Relocate Pin: Using a safety pin, carefully release pin E9-29 from the harness connector and gently pull it out. Locate pin E10-8, remove the yellow plug if present, and insert the pin E9-29 into the E10-8 location until it clicks into place.

4. Wire the OBD2 Connector: Obtain an OBD2 J1962 female connector. These can be sourced from online retailers specializing in automotive connectors.

   Make the following connections:

   • Pin 16 (OBD2 Connector) to +12V Ignition Power: Connect this pin to a 12V source that is powered when the ignition is on, such as the cigarette lighter circuit. Crucially, install an inline fuse for safety between the power source and the OBD2 connector.
   • Pin 4 (OBD2 Connector) to Chassis Ground: Connect this pin to a suitable chassis ground location. A ground point above the driver’s side kick panel is a convenient option.
   • Pin 2 (OBD2 Connector) to ECU Pin E10-8 (SDL): This is the light green wire you moved in step 3. The easiest way to connect is to tap into pin 7 (light green wire) on the OBD1 diagnostic connector (D5).
   • Pin 5 (OBD2 Connector) to ECU Ground E9-69 (E1): This is a brown wire. Tap into pin 3 (brown wire) on the OBD1 diagnostic connector (D5).

5. Test the OBD2 Port: Once wiring is complete, reconnect the battery and ECU. You should now be able to plug in an OBD2 scan tool and read diagnostic information from your 95 SC400.

Conclusion: Modernize Your 95 SC400 with OBD2

Upgrading your 1995 Lexus SC400 to OBD2 is a highly beneficial modification. It not only resolves the common lean running issue experienced with the OBD1 ECU under boost but also provides modern diagnostic capabilities that are essential for maintaining and tuning your vehicle. While requiring some wiring modifications, the process is well-documented and achievable for the moderately skilled DIY enthusiast. By following this guide, you can bring your classic 95 SC400 into the modern era of engine management and diagnostics, ensuring optimal performance and engine health for years to come.