So, you’re looking to squeeze more performance out of your OBD2 BMW? Many enthusiasts, especially those with E36 models, consider converting from OBD2 (On-Board Diagnostics II) to OBD1 (On-Board Diagnostics I) engine management systems. It’s more than just swapping parts; it’s a comprehensive process that, when done correctly, can unlock hidden potential. As a content creator at carparteu.com and a seasoned auto repair specialist, I’ve broken down the complexities of this conversion to provide you with a clear, step-by-step guide. This article will delve into exactly How To Convert Obd2 To Obd1, outlining the necessary components, procedures, and considerations for a successful upgrade.
Understanding the OBD2 to OBD1 Conversion: Is it Right for You?
Before diving into the nitty-gritty, let’s understand why someone would undertake an OBD2 to OBD1 conversion. The primary motivation often revolves around performance tuning. OBD1 systems, particularly the Bosch Motronic 3.3 found in earlier BMWs (like the ‘red label’ 413 ECU), are favored for their simpler, more easily tunable engine management. They offer a direct and arguably more accessible pathway for aftermarket performance chips and custom tuning compared to the more complex OBD2 systems.
While OBD2 brought advancements in emissions monitoring and diagnostics, some argue that for pure performance, especially in older BMW models, OBD1 offers a more direct route. This conversion is popular among enthusiasts modifying their engines for track use or seeking a more raw, responsive engine feel.
However, it’s crucial to understand that this is not a simple plug-and-play modification. It requires mechanical aptitude, a solid understanding of automotive electrical systems, and meticulous attention to detail. It’s also important to note that modifying your emissions system can have legal implications depending on your location.
Essential Components for Your OBD2 to OBD1 Conversion
To successfully convert your OBD2 system to OBD1, you’ll need a specific set of parts. Sourcing these components is the first crucial step. Here’s a comprehensive list of what you’ll need:
- OBD1 Engine Harness: This is the backbone of the conversion, providing the necessary connections for the OBD1 ECU and sensors. Sourced from a 1994 325i in the original example, ensure compatibility with your specific engine type.
- OBD1 ECU (Engine Control Unit): The “red label” 413 ECU is highly sought after because it typically comes from non-EWS (Electronic Immobilizer System) vehicles, simplifying the process.
- OBD1 Performance Chip: An aftermarket chip, like the Active Autowerkes chip mentioned, is essential to optimize engine performance with the OBD1 system.
- OBD1 Intake Manifold: The M50 intake manifold is known for its superior airflow compared to OBD2 manifolds (M52/S52), contributing to performance gains.
- OBD1 Fuel Rail: Necessary to accommodate the OBD1 fuel pressure regulator and fuel line configuration.
- OBD1 Crank Position Sensor: Different location and type compared to OBD2, requiring a swap.
- OBD1 Cam Position Sensor: Again, system-specific and must be replaced.
- OBD1 Knock Sensors (x2): OBD1 systems use different knock sensors.
- OBD1 Oxygen Sensor: OBD1 utilizes a single pre-catalyst oxygen sensor, unlike the multiple sensors in OBD2.
- OBD1 HFM (Hot-Film Air Mass Meter): Matches the OBD1 system’s requirements for air flow measurement.
- OBD1 Main Engine Coolant Hose: Due to differences in coolant pipe fittings, this hose is essential.
- OBD1 Throttle Boot: Connects the HFM to the throttle body and is specific to the OBD1 setup.
- OBD1 Throttle Body (Optional): While the OBD2 throttle body can be adapted, an OBD1 throttle body simplifies installation.
- OBD1 Valve Cover and Coil Packs (Optional): While OBD2 valve covers and coil packs can be retained with modifications, OBD1 components offer a direct fit.
Step-by-Step Guide: Performing the OBD2 to OBD1 Conversion
Now, let’s break down the conversion process into manageable steps. Remember, safety first! Disconnect your battery before beginning any electrical work.
1. Valve Cover and Coil Packs: Choosing Your Path
The OBD2 valve cover differs from the OBD1 version primarily in how the coil pack wiring is routed. OBD2 wiring enters from the passenger side, while OBD1 enters from the driver’s side.
You have two main choices here:
- Option 1: Retain OBD2 Valve Cover and Coil Packs: This is often preferred to keep newer OBD2 coil packs. However, it requires modifying the OBD2 valve cover using a tool like a Dremel to accommodate the OBD1 harness wiring routing.
- Option 2: Switch to OBD1 Valve Cover and Coil Packs: This provides a direct fit for the OBD1 harness but necessitates sourcing OBD1 valve cover and coil packs.
The original article author chose to keep their OBD2 valve cover and modify it, a viable option for those comfortable with minor fabrication.
2. Vanos Solenoid: Ensuring Proper Connection
The Vanos (Variable Nockenwellen Steuerung – Variable Valve Timing) solenoid connector on OBD2 systems is shorter than its OBD1 counterpart. To address this, you have two approaches:
- Option 1: OBD1 Vanos Solenoid: Replace your OBD2 solenoid with an OBD1 version for direct compatibility.
- Option 2: Extend OBD2 Wiring: Retain your OBD2 solenoid and use a BMW extension harness (part # 12-52-2-274-971, originally a transmission harness) to lengthen the wiring and reach the OBD1 harness connector.
Using the BMW extension harness offers a clean, plug-and-play solution without cutting and splicing wires.
3. Coolant Pipe: Adapting to the OBD1 System
The main coolant pipe from the timing cover differs significantly between OBD1 and OBD2. OBD2 uses a fixed metal pipe, while OBD1 uses a rubber hose connected to an aluminum neck. You have a couple of solutions:
- Option 1: OBD1 Timing Cover: Replace your OBD2 timing cover with an OBD1 version. This is a more involved mechanical task but provides a factory-like OBD1 setup.
- Option 2: Coolant Pipe Adapter: A coolant adapter, available from BMW performance parts vendors like Bimmerworld or Turner Motorsport, fits into the OBD2 timing cover and allows you to connect the OBD1 rubber coolant hose. This is a simpler and more cost-effective solution. The adapter is typically secured with JB Weld for a permanent seal.
4. Intake Manifold: Unleashing Performance
This is a key component for performance gains. The OBD1 M50 intake manifold boasts better airflow than OBD2 manifolds. It’s a direct bolt-on to M52/S52 cylinder heads without modification. The OBD1 manifold will have provisions for an air temperature sensor and a vacuum port for the fuel pressure regulator on its underside.
5. Throttle Body: Adapting or Swapping
You can utilize your OBD2 throttle body, but it requires adaptation due to different gasket sealing surfaces. OBD1 throttle bodies have a flat mating surface, while OBD2 throttle bodies have a recessed gasket. Solutions include:
- Option 1: Throttle Body Adapter Plate: An adapter plate, placed between the OBD2 throttle body and OBD1 manifold, provides compatible mating surfaces for both gasket types.
- Option 2: Extended Gasket: A thicker, extended gasket can seal the OBD2 throttle body directly to the OBD1 intake manifold.
- Option 3: OBD1 Throttle Body: Using an OBD1 throttle body eliminates the need for adapters or special gaskets, offering the simplest installation.
6. Coolant Temperature Sender: Wiring Considerations
OBD2 uses a single coolant temperature sender, while OBD1 uses two. For the conversion, you can:
- Option 1: Splice and Adapt OBD2 Wiring: Splice the OBD1 harness wiring and use the OBD2 connector from your old harness to connect to the single OBD2 temp sender.
- Option 2: Coolant Temp Sender Wiring Adapter: A plug-and-play wiring adapter, like those from Turner Motorsport, simplifies the wiring connection for the OBD2 sender to the OBD1 harness.
7. Crank Position Sensor: Location Matters
The OBD2 crank position sensor is located on the engine block, while the OBD1 sensor is on the timing cover. You must use an OBD1 crank position sensor and install it on the timing cover. Leave the OBD2 sensor in place to plug the hole in the block.
8. Fuel Lines: Major System Change
The fuel delivery system is significantly different. OBD2 fuel rails have both lines at the rear, and the regulator is under the car. OBD1 fuel rails have the supply line at the front and return at the rear, with the regulator on the rail itself.
You must use the OBD1 fuel rail. This requires modifying your fuel lines. Remove the OBD2 fuel pressure regulator from under the car. Run new 8mm fuel lines to the OBD1 fuel rail, bridging the gap left by the regulator’s removal. Connect the fuel filter feed to the front of the OBD1 rail and the return line from the rail to the car’s return line. Don’t forget to connect the OBD1 fuel pressure regulator vacuum line to the intake manifold.
9. PCV (Positive Crankcase Ventilation): Adapting the System
OBD2 and OBD1 PCV systems differ. Options for adaptation depend on your valve cover choice:
- OBD2 Valve Cover: Retain the OBD2 PCV setup and find a way to mount the OBD2 breather valve under the intake manifold.
- OBD1 Valve Cover: Use the OBD1 breather valve that clips onto the valve cover and connect its vacuum and oil drain lines.
- Breather Catch Can: An alternative is to use a hose from the crankcase vent to a breather catch can.
- Hybrid Approach: The original author used a hybrid method, connecting a hose from the OBD2 valve cover to the OBD1 breather valve and then connecting the oil drain line to the dipstick.
10. Idle Control Valve (ICV) and Fuel Tank Breather Valve
Good news! The OBD1 and OBD2 ICVs are the same and interchangeable. You can reuse your OBD2 ICV. You will need the correct hoses for the OBD1 intake manifold and throttle boot connections.
Similarly, you can reuse your OBD2 fuel tank breather valve. You’ll need fittings to connect its vacuum hose to the vacuum port on the OBD1 throttle boot.
11. Oxygen Sensors and Secondary Air Pump: Emission System Changes
OBD1 systems eliminate the secondary air pump and post-catalyst oxygen sensors present in OBD2. Remove the secondary air pump entirely. You’ll only use one pre-catalyst OBD1 oxygen sensor. Don’t forget to plug the ports for the removed OBD2 oxygen sensors in your exhaust headers and catalytic converter (M18 bolts are suitable plugs).
12. Oil Pan and Dipstick: No Change Needed
Contrary to some misinformation, you can reuse your OBD2 oil pan and dipstick without any issues or modifications.
13. EWS (Electronic Immobilizer System): Bypassing if Necessary
Some E36 models have EWS, others don’t. If using a non-EWS ECU and encountering ignition issues, you might need to bypass EWS. A common method involves cutting wire #66 on the main engine harness connector at the ECU. Consult wiring diagrams (like Bentley manuals) to confirm the correct wire (typically green or black/violet). Tape off both cut ends for safety.
14. Power Distribution and Grounding: Careful Reconnection
Before disconnecting anything, label all power and ground connections. Crucially, ensure the negative battery post is disconnected throughout this process.
The OBD2 power distribution box will need to be slightly relocated to reach the OBD1 harness power connections. Removing the secondary air pump creates space to remount the distribution box closer to the engine.
Pay close attention to power (RED wires) and ground (BROWN or BLACK wires) connections. Double-check wiring if unsure. There will be power feeds to the starter and fuse box, and grounds including one under the OBD diagnostics port and a small ground wire from the spark plug rail (ground to the engine hoist loop on the Vanos unit bolt).
15. General Wiring: Label Everything!
Label every connector on the OBD1 harness before installation! Use painter’s tape and a marker to clearly identify each connector based on Bentley wiring schematics. This is critical as many connectors can look similar, and the harness becomes a maze once in the engine bay.
Take pictures of OBD2 power, ground, and starter wiring connections during disassembly for reference.
Final Configuration and Performance Results
After completing the conversion and meticulous reconnection of all components, you should have a fully functional OBD1 system. The original author’s final configuration included:
- 1999 M3 OBD1 S52
- AA Cam Chip
- Sunbelt Cams
- Euro 3.5″ HFM
- Conforti 3.5″ Intake
- 24lb Injectors
- Sunbelt Valve Springs
- ARP Headstuds
- AA Race Headers
- AA Race Exhaust
- Zionsville Competition Radiator
- Euro Oil Cooler
Dyno results after the conversion showed an increase in horsepower and torque, further improved with software revisions. The initial dyno run yielded 253 RWHP and 228 TQ, increasing to 262 RWHP and 230 TQ after tuning adjustments.
Conclusion: OBD2 to OBD1 Conversion – A Rewarding but Complex Upgrade
Converting your BMW from OBD2 to OBD1 is a significant undertaking, but as this guide shows, it’s definitely achievable. It’s a path chosen by enthusiasts seeking enhanced tunability and performance, particularly in older BMW models like the E36 M3. While not a “simple electronics swap,” with careful planning, the right parts, and meticulous execution, you can successfully perform this conversion and potentially unlock more performance from your engine.
Remember to consult wiring diagrams, label everything, and double-check your connections throughout the process. If you have any questions or insights from your own OBD1 conversion experiences, feel free to share them in the comments below!
