Motronic-adding a check engine light

[Blaise]

I thought it would be good for us to have a thread regarding adding a check engine light to motronic converted E12s.

As you guys know, I have installed motronic 1.3, which uses the 179 ECU. This ECU has a grey wire which is noted as the check engine light. My understanding is that this light is meant to come on when the ignition is first turned on (engine not running) and then be off while the car is normally operated. If it is on during operation, then the computer has stored a fault code, indicating that something is amiss.

What I don't know yet, is how this is accomplished electrically. Is the ECU grey wire a positive or negative (ground) feed? Does the light check occur through the ECU or via separate route? I plan to research this, but if you know, let me know. I know that Anthony has already done this.

Further, I thought it would be great to make use of the existing lights in the center dash console. In my 1977 530i, there were three lights there (reactor service, fasten seatbelt and EGR service). I believe that the two service lights were controlled by a set mileage box that lives under the car, inline with the odometer cable. I think the seatbelt sign is just timed to stay illuminated for a while upon start up (or is there really a belt sensor).

I imagine, but don't recall, that when the ignition is turned on, these lights are illuminated as a check (much like standard dash lights).

Finally, my thought was to use the "Reactor Service" as my check engine light. Seems it would be a simple solution. Would it be as simple as swapping the mileage box wire to that bulb, with my new ECU grey wire?