One common issue when swapping an LS engine into a BMW E9X (E90, E91, E92, or E93) is figuring out how to get the reverse lights working. From the factory, reverse lights are triggered through the BMW transmission and CAS/ECU system. Once that drivetrain is removed, this function is lost unless you wire it back in.
Fortunately, the reverse lights are controlled directly by the Footwell Module (FRM), and all it takes to activate them is a properly timed ground signal.
How It Works
The FRM will turn on the reverse lights when Pin 19 on connector X14261 sees a ground signal. This means all you need to do is figure out how to send a ground to that pin when the car is in reverse, and your reverse lights will function like stock.
Wiring the Reverse Light Trigger
To trigger the FRM:
- Run a wire from your transmission's reverse light switch to Pin 19 on connector X14261 of the FRM.
- When the car is in reverse, this wire should connect to ground — either directly through the reverse switch or via an ECU-controlled output.
Depending on which transmission you're using (T56, TR6060, or others), you may need to ground one side of the reverse switch and run the other side to the FRM so that it completes a ground circuit when the transmission is in reverse.
If you're running a standalone ECU like a Holley Terminator X, you can also use a programmable output to send a ground signal when reverse is selected. In this case, you don't need to use a mechanical switch — just configure an output in the ECU to ground Pin 19 when needed.
Ultimately, as long as you send a ground signal to Pin 19, the reverse lights will activate. It's up to you to figure out what fits your application best — whether that's wiring through a mechanical reverse switch, using a Holley ECU output, or integrating it into your shifter setup.
Adding a Pin to Connector X14261 (If Needed)
If your E9X was originally an automatic transmission car, Pin 19 on connector X14261 may not be populated. In that case, you'll need to add a pin to the connector.
The easiest way to do this is to pull an unused terminal from one of the DME connectors (X60004, X60005, or X60006), as these pins are compatible with the FRM connector. Once removed, insert the pin into slot 19 on X14261.
Coding the Car as Manual
For the FRM to properly respond to the reverse light trigger, the car must be coded as a manual transmission. If the vehicle is still coded as an automatic, the FRM may ignore the ground signal.
To update the coding:
- Remove the automatic transmission code ($205) from the vehicle order (VO).
- Recode the FRM, CAS, and any other relevant modules using Pro Tools, NCS Expert or equivalent BMW coding software.
Once this is done, the FRM will accept the ground trigger on Pin 19 and activate the reverse lights accordingly.
Final Thoughts
At the end of the day, as long as you get a ground to Pin 19 when the car is in reverse, the reverse lights will work. Whether that means using a mechanical reverse switch, a Holley ECU output, or a custom shifter-based solution is up to you and your specific transmission and setup.
