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MS45 to MS43 ECU Conversion

BMW used an Siemens engine computer, or engine control unit (ECU) to control the fuel injection system in the E46 model range (and various other Series cars). There were three versions for non-M cars: MS42 for 1999-2000 323i/328i, MS43 for 2001-2/2003 325i/330i, and MS45 for 2/2003-2006 325i/330i. Another version was used on the M3 model (MSS54). The ECU is one component in the engine management for the car and is tied into other modules and systems.

2003 model year: there was a period of six months from 9/2002-3/2003 where BMW mixed MS43 and MS45 components. See the parts list below for more detail.

Why Convert from One to Another?
For our purposes, we need to convert certain models to MS43 to meet the requirements in the Spec E46 racing class. As a spec class where all of the cars use the same equipment, it was decided that one ECU and one accepted tuned file (from Epic Motorsports) would be allowed. This eliminates complex differences between systems and any claimed advantages of one system over the other. For other tuning work MS43 may have some advantages as well (and vice versa) but we will not be addressing those here. Our focus is only on the Spec E46 racing class.

Parts Required
Backdating to MS43 requires more than just the ECU. The entire engine harness was changed with different connectors for various sensors and components. For example, the valve cover must be changed because the ignition coils use a different mounting method. Many of the parts required for a conversion can be purchased used, while some should only be bought new. For a complete and hassle-free conversion, the entire list below should be completed.

- Main engine wiring harness
- MS43 Siemens ECU
- Ignition coils (see 2003 note below)
- Ignition coil wiring harness (see 2003 note below)
- Valve cover (see 2003 note below)
- Intake air temperature sensor (located in middle of intake manifold)
- Alternator or voltage regulator (depending on alternator brand)
- HFM/mass air sensor
- Pre-Cat O2 sensors (post-cat sensors are deleted for Spec E46 racing)

2003 was an odd mix of early MS43 wiring and late MS45 wiring. Ignition coils, valve cover, and coil harness changed early (in 9/2002) but the ECU and engine harness remained MS43 until 2/2003. For MS45 you might be able to find a 2003 325i/330i and only need the ECU and engine harness and keep your valve cover, coils, and coil harness.

A small change may be required on the coil harness power/ground plug to change your MS45 to MS43 (see image below). Otherwise, the ECU plug and other coil wiring appears to be the same. Unfortunately, RealOEM and ETK are missing data specific to E46 330i/330ci so we do not recommend relying solely on that data to give a clear picture.

Spec E46 Related Information
For Spec E46 racing the ECU must be shipped to Epic Motorsports for the flash software and series-required seal. We strongly advise adding the EWS-delete option with all Spec E46 software to eliminate the need for additional wiring or coding work. This lets the ECU run the engine without a key and the ECU can be used in multiple cars. If using a stock instrument cluster, additional wires will need to be added to your chassis harness to retain oil pressure warning lights. However, this will be unnecessary if an aftermarket data acquisition system that accesses the CAN data, such as AiM, is used. Our experience is most racers are using such a system so additional wiring won't be required.

BimmerWorld can supply new versions of the complete list of conversion parts, but we encourage buying used when it makes sense. The ECU, wiring harness, and valve cover would be the limit of our recommendation typically. As with any used part, ensure they come from a running donor vehicle. And for any electrical part new or used, make sure you install it correctly and completely, as most electrical parts are non-returnable, and improper installation can damage or destroy them.

We do not provide installation assistance or guidance on this conversion. This tech article is intended as a guideline and gathering point for information.

NEW MS43 Conversion Parts Available from BimmerWorld:

Main Engine Wiring Harness - 2001-2/2003 E46 325i/330i (MS43)

MS43 engine wiring harness. Genuine BMW part #12517513554.

Price: $427.77

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Spec E46 Epic Tune ECU Small.JPG

Epic Motorsports Spec E46 Race Software (E46 330i)

Fully sealed and tamper-proof spec racing software for the Spec E46 class

Price: $599.00

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Ignition Coil Pack, OEM - E36, E46, E39, E38, E31, X5, Z3

Replace your old and failing coils to optimize the health and performance of your engine

Price: $57.99

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Ignition Coil Harness - 2001-2/2003 E46 325i/330i (MS43)

MS43 wiring harness for ignition coils. Genuine BMW part #12517551908.

Price: $189.99

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Genuine BMW Valve Cover - E46, E39, X5, Z3

New valve cover with gasket to replace BMW part #11121432928

Price: $438.99

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13 62 1 739 510 Air Temperature Sensor Small.JPG

Intake Air Temperature Sensor - E36, E46, E39, E38, X5, Z3

Sensor that provides the ECU with intake temp data. Replaces BMW part #13621739510.

Price: $15.99

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HFM/Mass Air Sensor - Early M54 3.0-liter Engine (E46, E39, Z4)

High Quality Replacement to BMW # 13627567451

Price: $209.99

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Pre-Cat Oxygen Sensor, Bosch - E46, E39, E38, X3, X5, Z3, Z4 (11781742050)

E46 325/330: -2/2003. OEM Bosch bank 1 or 2 pre-cat O2 sensor. 24 month warranty.

Price: $63.99

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