So, you’re thinking about converting your OBD2 E36 to OBD1? It’s a popular modification for BMW E36 owners looking to enhance performance and simplify engine management. While some might downplay it as a simple “electronics swap,” the reality is that an E36 Obd2 To Obd1 Conversion is a detailed process. Having navigated this conversion myself, relying heavily on resources like bimmerforums.com, expert consultations, and countless hours studying Bentley manual wiring diagrams, I want to share a comprehensive guide to help you through it.
Before we dive in, let’s take a look at what an OBD2 S52 engine bay typically looks like before the conversion:
My project car was a 1999 M3, originally equipped with OBD2 management and the EWS (Electronic Drive-away Warning System). To undertake this e36 obd2 to obd1 conversion, I sourced an OBD1 engine harness from a 1994 325i, along with the sought-after red-label 413 ECU. For engine tuning, I opted for a performance chip from Active Autowerkes.
Let’s break down the essential components you’ll need for your own e36 obd2 to obd1 conversion:
Essential Parts for OBD2 to OBD1 Conversion:
- OBD1 engine wiring harness
- OBD1 ECU (Bosch 413 “red label” preferred for non-EWS applications)
- OBD1 performance chip (tuned for your modifications)
- OBD1 intake manifold (M50 manifold)
- OBD1 fuel rail
- OBD1 crankshaft position sensor (CKPS)
- OBD1 camshaft position sensor (CMPS)
- OBD1 knock sensors (x2)
- OBD1 oxygen sensor (pre-catalytic converter)
- OBD1 mass airflow sensor (HFM – Hot-Film Air Mass Meter)
- OBD1 main engine coolant hose (connecting timing cover, heater core, expansion tank)
- OBD1 throttle intake boot
- OBD1 throttle body (optional, but recommended for a complete conversion)
- OBD1 valve cover and coil packs (optional, depending on valve cover choice)
While the concept of an e36 obd2 to obd1 conversion might seem straightforward, numerous nuances require careful attention. Let’s delve into each critical step:
1. Valve Cover and Coil Packs: OBD2 or OBD1?
The OBD2 valve cover lacks the necessary routing provisions for OBD1 coil pack connectors. This is because OBD2 wiring enters from the passenger side, while OBD1 wiring enters from the driver’s side. You have a choice to make here, impacting your e36 obd2 to obd1 conversion:
- Option 1: Retain OBD2 Valve Cover: This allows you to keep your OBD2 coil packs. However, it requires minor modification to the OBD2 valve cover, typically using a Dremel tool, to accommodate the OBD1 wiring harness routing.
- Option 2: Switch to OBD1 Valve Cover: This necessitates using OBD1 coil packs. Both OBD1 and OBD2 coil versions are compatible with the OBD1 harness electrically.
I opted to keep my OBD2 valve cover and modify it slightly for wiring clearance.
Here’s a comparison visually:
2. VANOS Solenoid Wiring Adaptation
The OBD2 VANOS (Variable Nockenwellen Steuerung – Variable Valve Timing) solenoid wire connector is shorter than its OBD1 counterpart. To address this in your e36 obd2 to obd1 conversion, you have two primary solutions:
- Option 1: OBD1 VANOS Solenoid: Replace your OBD2 solenoid with an OBD1 unit.
- Option 2: Wiring Extension: Keep your OBD2 solenoid and extend the wiring. BMW part # 12-52-2-274-971, a transmission harness originally designed for connecting the engine harness to the backup light switch, is perfectly suited for this. It features identical connectors.
I chose to extend the wiring using the BMW harness for a clean and reliable connection.
3. Coolant Pipe and Timing Cover Differences
The main coolant pipe from the timing cover (located behind the thermostat housing) differs between OBD1 and OBD2 E36 models. OBD2 utilizes a metal pipe fixed within the timing case cover with sealant. OBD1 employs a rubber hose connecting to an aluminum neck protruding from the timing case cover. For your e36 obd2 to obd1 conversion, you have these options:
- Option 1: OBD1 Timing Cover: Replace your OBD2 timing cover with an OBD1 version. This is a more involved solution, costing around $100 for the part.
- Option 2: Coolant Pipe Adapter: Utilize a coolant pipe adapter (approximately $20) available from BMW performance parts suppliers like Bimmerworld, Active Autowerkes (AA), or Turner Motorsport. This adapter fits into the OBD2 timing case cover and is secured with JB Weld. It allows you to then clamp the OBD1 coolant hose onto it.
I opted for the coolant adapter for its simplicity and cost-effectiveness.
4. Intake Manifold Upgrade: The Performance Advantage
The OBD1 (M50) intake manifold is a key component for performance gains in an e36 obd2 to obd1 conversion. It offers superior airflow compared to the OBD2 (M52/S52) intake manifold. This swap is essential for unlocking the full potential of the OBD1 conversion. The OBD1 manifold includes an air temperature sensor and a vacuum port for the fuel pressure regulator on its underside, near the firewall. Crucially, the M50 intake manifold bolts directly onto M52/S52 cylinder heads without any modifications.
5. Throttle Body Considerations and Adapters
You can technically reuse your OBD2 throttle body in an e36 obd2 to obd1 conversion, but it requires adaptation for proper sealing. OBD1 throttle bodies have a flat mating surface that seals against a gasket on the OBD1 intake manifold. OBD2 throttle bodies are inverted, with a gasket within the throttle body mating to a flat surface on the OBD2 intake manifold. Solutions include:
- Option 1: Throttle Body Adapter Plate: Install an adapter plate (around $20) between the OBD2 throttle body and OBD1 manifold. This plate provides correct mating surfaces for both gasket types.
- Option 2: Extended Gasket: Use an extended gasket (approximately $15) that allows the OBD2 throttle body to seal directly against the OBD1 intake manifold.
- Option 3: OBD1 Throttle Body: The simplest solution is to use an OBD1 throttle body, eliminating the need for any adapters.
6. Coolant Temperature Sender Configuration
OBD2 engine management uses a single coolant temperature sender located on the cylinder head under intake runner #1. OBD1 systems utilize two senders, positioned under intake runners #1 and #2. For your e36 obd2 to obd1 conversion, you can:
- Option 1: Splice and Adapt: Splice the OBD1 harness wiring and reuse the OBD2 plug connector from your old harness to connect to the single OBD2 temp sender.
- Option 2: Wiring Adapter: Employ a plug-and-play coolant temperature sender wiring adapter (approximately $50) from Turner Motorsport for a cleaner installation.
7. Crankshaft Position Sensor (CKPS) Relocation
The OBD2 crankshaft position sensor is situated on the engine block, in front of the starter motor. In contrast, the OBD1 CKPS is located on the timing cover, mounted on a circular tab with a 6mm Allen bolt. For the e36 obd2 to obd1 conversion, you must use an OBD1 crank position sensor. You can leave the OBD2 sensor in place to plug the hole in the block.
8. Fuel Line and Fuel Pressure Regulator Modifications
Significant differences exist in fuel delivery setups between OBD2 and OBD1. OBD2 fuel rails have both fuel lines connecting at the rear, near the firewall. The fuel pressure regulator is located under the car, forward of the fuel filter. OBD1 fuel rails have the supply line at the front and the return line at the rear, with the fuel pressure regulator integrated onto the fuel rail itself. For a successful e36 obd2 to obd1 conversion, you must use the OBD1 fuel rail, necessitating fuel line modifications.
This involves removing the OBD2 fuel pressure regulator from under the car and running new 8mm fuel lines to the OBD1 fuel rail. Bridge the gap left by the removed OBD2 regulator using new fuel line, connecting the feed from the fuel filter to the front of the OBD1 fuel rail and the return line from the back of the fuel rail to the existing return line under the car. Finally, connect the OBD1 fuel pressure regulator vacuum line to the one-way valve on the underside of the OBD1 intake manifold, located at the rear corner closest to the firewall.
9. Positive Crankcase Ventilation (PCV) System Adaptation
The OBD2 crankcase ventilation system differs from OBD1. Several approaches exist for your e36 obd2 to obd1 conversion:
- Option 1 (OBD2 Valve Cover): If retaining the OBD2 valve cover, keep the OBD2 PCV setup and find a mounting location for the breather valve (cone-shaped plastic valve) under the intake manifold.
- Option 2 (OBD1 Valve Cover): If using an OBD1 valve cover, use the OBD1 breather valve that clips onto the crankcase vent port. The OBD1 valve has a vacuum line connecting to the ICV/intake manifold plug and a large oil drain line to the dipstick.
- Option 3: Breather Catch Can: Connect a hose to the crankcase vent and install a breather catch can system.
I opted for a hybrid approach with my OBD2 valve cover:
10. Idle Control Valve (ICV) Compatibility
The good news is that OBD1 and OBD2 ICVs are identical! You can reuse your OBD2 ICV for your e36 obd2 to obd1 conversion. You will need the correct connector and hoses for the ICV-to-intake manifold connection and the ICV-to-throttle boot vacuum port connection.
11. Fuel Tank Breather Valve
Similarly, you can reuse your OBD2 fuel tank breather valve. You’ll need fittings to connect the vacuum hose to the vacuum port on the throttle boot. Standard hardware store barb fittings (3/8” and 5/8”) can be adapted for this purpose.
12. Oxygen Sensor and Secondary Air Pump Elimination
OBD2 systems include a secondary air pump, an emissions control system absent in OBD1. This system is completely removed during an e36 obd2 to obd1 conversion. Furthermore, you’ll eliminate the two pre-catalytic converter OBD2 O2 sensors in the OBD2 exhaust headers and the two post-catalytic converter OBD2 O2 sensors in the catalytic converter. Remember to plug the O2 sensor ports. M18 bolts are suitable plugs and are commonly available at auto parts stores (FYI, Toyota Land Cruiser oil pan drain bolts are M18). OBD1 management uses only one pre-catalytic converter oxygen sensor.
13. Oil Pan and Dipstick: No Changes Needed
Contrary to some rumors, you do not need to swap the oil pan or dipstick for an e36 obd2 to obd1 conversion. Your OBD2 oil pan and dipstick are perfectly compatible and require no modification.
14. EWS (Electronic Drive-away Warning System) Deactivation
EWS presence varies across E36 models. While using a non-EWS ECU (like the red-label 413) helps, ignition issues can still arise post-conversion. An easy wiring modification in the main engine harness resolves this. Locate the connector to the ECU, remove the protective rubber boot, and cut wire #66. This wire is typically solid green but might be black/violet according to Bentley diagrams. Cut the wire and insulate both ends with electrical tape.
15. Power Distribution and Grounding Considerations
Crucially, disconnect the negative battery terminal before any electrical work. Label power and ground connections during disassembly. The OBD2 main battery positive post is near the ECU compartment on the passenger side. The OBD2 distribution box is parallel to the fender, secured by two M10 bolts. For the e36 obd2 to obd1 conversion, you’ll need to slightly relocate this distribution box to access the OBD1 harness power connections. Removing the secondary air pump frees up screw holes to relocate the distribution box closer to the engine and parallel to the firewall. It’s a tight fit, but it works. This relocation is necessary to reach the main power feed for the OBD1 harness.
There’s a ground connection under the OBD diagnostic port. Carefully check power and ground wires: RED wires are power feeds, BROWN or BLACK are grounds. Double-check by peeling back wiring sheathing if unsure. A large power feed goes to the starter, and a smaller one to the fuse box. Another ground wire (small, with a round terminal connector) from the spark plug rail needs to be grounded to the bolt securing the engine hoist loop on the VANOS unit.
16. General Wiring Harness Management: Label Everything!
Label every connector before attempting to install the OBD1 harness! Use Bentley wiring schematics and methodically identify each plug on the harness. Label each with painter’s tape and a marker for clear identification. The harness becomes a confusing tangle once in the engine bay. Fortunately, wire lengths are generally practical, and connectors are located near their corresponding components. Take pictures of OBD2 power and ground connections during disassembly, and especially photograph the starter wiring connections to avoid confusion later.
Conclusion: OBD2 to OBD1 Conversion Complete
That summarizes my experience with the e36 obd2 to obd1 conversion. This guide should provide valuable insights and assistance for your own project. Remember, meticulous planning, careful execution, and thorough labeling are key to a successful conversion.
Here’s my final configuration after the e36 obd2 to obd1 conversion:
1999 M3 OBD1 S52, Active Autowerkes (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.
And here’s the final result under the hood:
Dyno Results and Performance Gains
Initial dyno results (9/23/06) yielded 253 RWHP and 228 TQ. After revisions to the chip by Active Autowerkes to lean out the mixture, subsequent dyno runs (10-19-06) showed an increase to 262 RWHP and 230 TQ. Air-Fuel Ratios (AFR) were optimized, and I was very pleased with the final performance gains achieved through this e36 obd2 to obd1 conversion.
