M42 coil #3 not firing... coil pack is good

[Codym42]

I'm trying to get my 318i sedan roadworthy but it has no spark to cylinder #3. I initially thought I had a bad coil pack so I bought a used set of 4. I tried all 4 and none of them would put out spark. I connected these 4 coils to other connectors coming from the ecu and they all worked and I even swapped plug wires around. It seems as if whatever coil is plugged into the #3 plug from the engine harness, doesn't fire.

I tested the 3 pins at the coil plug. I have switched power, ground, and good continuity from the pin on the plug to the ecu connector. After that, I swapped in a used ECU from a friend and there is still no signal on coil #3.

I'm not sure what else to check. Any advice would be appreciated. Thanks.

[bmwman91]

My first thought was that the output transistor for #3 had taken a dump, but that should go away if you swapped ECUs. Could there be a short within the wire harness? Try testing continuity between the pins in the connector. They should all register as high impedance to one another.

[Codym42]

My first thought was that the output transistor for #3 had taken a dump, but that should go away if you swapped ECUs. Could there be a short within the wire harness? Try testing continuity between the pins in the connector. They should all register as high impedance to one another.

Test the resistance between the 3 pins at the #3 coil connector? I can do that. There almost has to be an issue with the harness unless something else weird is going on.

[Codym42]

It looks like there were 2 issues with my car. After swapping in the new ecu, I didn't even think to swap coils around again. Sure enough, the coil I had was bad. After swapping that out, the car runs great. The ecu was definitely the original problem though.

[bmwman91]

Nice, glad you figured it out. That sure beats trying to repair a damaged harness.