ECU wire mismatch. Need help with E car and I mot


I just installed my 89 I motor in my 86 E car and have come upon my first issue. My E car had a plug that went over to the ECU box from the drivers side. See photo 1 and 2. The new harness from the I engine has a different plug and wire colors. This is the separate plug that is connected right next to the ECU plug. See photos 3 and 4. Does anyone have a good solution for this?

Photo 1

Photo 2

Photo 3

Photo 4


That white plug is for the DME START signal, tachometer signal, and econometer signal. Early cars (through 8/87) had a pigtail off the engine harness near the DME connector for those signals. Later cars routed them through C101.

The fix is to graft the connector from the old engine harness to the 89 harness or to re-wire those through C101. Grafting the connector on is easier. Pin 20 of C101 may be grounded on your car. It serves a different function in later cars, isn’t used on your car, and will smoke your harness. Clip the wire to pin 20 on the body side of C101.


OK, I think I know what you are talking about on C101. That is the large round plug in the engine compartment. I would assume #20 is a red/yellow wire on one of the plugs and brown on the body side from what I have read while searching. I will cut the brown wire to keep the ground getting to the hot red/yellow wire. Do I need to add power to the Red/yellow wire to get power to the ABS system.

Not sure what to do with the three wires coming out of the DME from the engine harness and the three wires that come from the drivers side of the dash as in my first post. Can you explain how I would get all six of these wires routed? When you say graft, do you mean to splice the wires together? If so, what wire goes to what wire? They are all different colors.

Btw, thanks for your help!


I am still searching to find what the three wires that come from the engine harness go to and where to wire the three wires that come from the body harness that went to the original loose wires coming from the original E motor harness. I assume they need to connect to each other but could be completely off base. Car will turn over but not start. Any help or guidance would be greatly appreciated.


You can forget the wires in the pigtail off the engine harness. You don’t need those function in a race car. But you will have make the connections from the wires in the body side white connector to the DME. You don’t necessarily have to connect the econometer, but you probably do want the tach and you must have the START signal connected. That signal tells the DME to turn on.


After a full day of searches, I finally found what I was looking for. For future reference:

c104 plug from 1986 325e body harness has three wires:
1- black wire-engine speed
2-green wire-OBC fuel cut function
3-yellow/white wire-fuel rate
On the 89 I harness, splice or connect the following wires to the ones on the c104
Pin 3 from c104 to pin 8 on the c101 of the 89(yellow/white or white/white wire)
Pin 2 from c104, run a wire directly to the + terminal on the coil (green wire)
Pin 1 from c104 to pin 9 on the c101(black wire)

This is what I did this afternoon. Started right up!


Getting ready to do the same i motor to e car swap. Does anyone have any current pics of the c101 and c104 connectors? Having probs licating pics of these. Thanks in advance



Just trying to make sure i have this right. The original pics from this thread arent accessible. After further research on the interwebs, these are what i believe im looking for between the harnesses.

The 1st being c101
The 2nd being c104

As long as i have the connectors correct, following the rest of this thread is a breeze.

Again thanks in advance