Adapting the Round Motor Plug
The wiring harness of the M30 is connected by a round engine plug.  This plug will require some modifications.  Furthermore the e23/e24 had the battery in the left of the engine bay, this mandates that the thick cable that runs to the starter be extended.  The wiring harness cover of the e30 M3 that runs along the firewall can be altered and used for the conversion.  This option provides very pleasant engine compartment aesthetics.
 

Mounting Plate: BMW Part Number: 51 71 1 380 390 Cover: BMW Part Number: 51 71 1 380 391 Positive Terminal Link (suitable for use with the mounting plate): BMW Part Number:  1 710 719 291

It is best to modify the plug at the engine, rather than at the ECU.  The engine plug parts needed for this can all be purchased separately.  The existing plug is cut off and new connections are to be made.  A simple pair of pliers should do the job. It is best to test the system before fully assembling the plug (far right picture above).  In this situation, should anything be found to be incorrect it can be repaired without disassembling the entire plug.

The following links provide information about engine plug pin placement and wiring information.  Note that BMW changed the wiring arrangements and wire colors from year to year.  The links below should serve to identify many cable colors, all those not represented below can be determined logically.  This information can also be found in a number of repair manuals. 

Engine Wiring Diagrams Useful to the Project  (318i, 325e, M3, 535i, 735i, 745i...)

Pin Placement in the Engine Plug of the e23 735i 1980 **

Pin Placement in the Engine Plug of the e24 635CSi 1987 **

Pin Placement in the Engine Plug of the e28 535i 1985 **

Pin Placement in the new Engine Plug e34 1990 **

Pin Placement in the Engine Plug of the e30 325iC 1986 **

Wiring for Automatic Transmissions
Wiring the electronic controls for the automatic transmission would prove to be a very difficult task, indeed. In the case of the M30, the engine plug also carries the controls for the automatic transmission. If an originally automatic engine is to be used it is highly recommended that the entire wiring harness be swapped for that of a manual engine.

Electronic Engine Performance Control 
The use of engines that were, or are, equipped with electronic engine performance control is not recommended.  These components greatly and unnecessarily complicate the conversion process.  The electronic throttle controls pose the greatest difficulties, as the components needed to operate such a system are not present in the e30 chassis.  The complete wiring harness and throttle body would need to be exchanged in favor of a non-electronic version in order facilitate a 335i conversion.  These factors should be kept in mind when buying an engine for the conversion.

**Available in German Only

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