E28 Engine Wiring Conversion

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Courtesy of Hans Vaalund



This is a simple description on how to interface the engine wiring of an E28 engine to a E12 car. It is based on my experiences of adding the electrical fuel system from a E28 518i to my E12 520 with an M10 engine. I have also verified this with different wiring diagrams and think it will cover all E12 models and E28 with L-jetronic and Motoronic.

The electrical interface between the engine and the car is fairly simple. The fuse box connector contains all the signals between the engine and the car on the E12. The E28 engine wiring also has a small interface connector next to the ECU.

Step one, fuse box connector: Make sure to get an E28 engine with complete wiring. Keep the ignition unit and combo relay(for the fuel pump and injectors) from the E28 wiring. The fuse box connector will not fit the E12, cut it off! Cut all the wires close to the connector. Get a new E12 fuse box connector, or reuse the old one. I removed all the connector pins from the old connector and just used the connector housing. Then I mounted new 1/4" tab connectors (the flat type with crimp tube and melting glue) on the wires and put them into the connector housing(see picture). Note from Peter Florance: "You can purchase tab connectors with the locking tabs from Molex and Amp but they do require special 'roll crimper'." I secured them with melting glue, too be able to remove them later. The pin numbers are marked on the connector housing.

Click on all photos for larger view.

Here is the list of which wires to put where:

Pin number:   E28 wire color:   Function:
1   black/yellow   Starter solenoid voltage
2   note 1   Ignition voltage, fuse 1, to combo relay
3   blue   Charge warning light
4   green   Ignition voltage
5   note 2   Coil -, to tachometer
6   brown/white   Temp gauge sensor
7   brown/green   Oil pressure switch
8   note 3   Input power combo relay, fused


Note 1: This pin is not used. Ignition voltage on the E28 is supplied to the combo relay by the green wire on the small connector next to the ECU. I just cut open the wiring tube a little bit from the combo relay, cut the green wire(pin 15 on the combo relay), stripped the green wire on the fuse box connector(pin 4) a bit and soldered the wires together. I then isolated them and taped all the wires together again. The combo relay pin 15 can be connected otherways, but it needs to be connected to the ignition!

Note 2: The E28 gets the ignition signal to the tachometer from the ignition module and not directly from the coil. The E28 the ignition signal is connected to the instrument panel by the small wiring harness connector next to the ECU. If your E12 has a tachometer, you need to make a new connection from the coil to pin 5 on the fuse box connector. It can be connected directly to the negative pin of the coil or the center pin of the black wire on the diagnostic connector.

Note 3: On the E28 the input power on the combo relay is connected directly to the battery. Leave it like this! Pin 8 on the fuse box connector is not used.

Step two, ECU (electronic control unit): If the car already has electronical fuel injection, just replace the old ECU with the new from the E28 engine. If not, make a hole on the firewall (app. 50mm) to fit the large rubber grommet on the cable. Remove the glove box and mount the ECU under the dashboard.

Step three, combo relay: Mount the combo relay somewhere near the fuse box. The connector 87b with the green/violet wire is the power supply to the fuel pumps. Connect this to wire for the fuel pumps in the car. This is not located on the fusebox. I had to install in a new wire to the trunk, yellow wire on the pictures.

Step four, finish it: Use cable ties and secure all loose wires. Keep the diagnostic connector if you want. Not very useful, but nice for turning the starter when working with the engine(by connecting the black/yellow wire to battery +). Connect the combo relay to the battery connector. Connect the battery. The ignition can be checked without the ECU connected. Connect a strobe light, turn on the ignition and verify that you gets spark and the timing is correct. Verify that the fuel pumps are running 1-2 seconds after the ignition is turned on, and then stops. If everything else on the engine is in place, connect the ECU and try to start it up.

Good luck!

Disclaimer: I have not verified this modifications with US car wiring diagrams. There could be some variations that I am not aware of. Check the connections yourself with a multimeter to make sure everything is correct. See the connector pinout list (PDF file) if you want to learn more about the wiring on the different connectors.

Sources of information:
Haynes Owners Workshop Manual BMW 520i & 525e 1981-1988
My car, 520i 1975
EFI system from a E28 518i 1985

List of e12 and e28 connector pinouts (PDF file)

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