Odometer backlight stays on after locking - normal?

[StephenFord]

With the darker evening I noticed that after being parked up on driveway, car cold, when I remotely locked my car, the green odometer light then came on and stayed on for about 20 minutes. Is that normal? If so, why would they design that feature? If not what indication of impending doom is it a sign of?

[RJ414i]

Normal.

[Tdci-Peter]

2 hours ago, StephenFord said:

Is that normal?

Yes!, as above.

If it stays on all night, it is a symptom of something not shutting down, maybe an audio unit or other hi-tech add-on, and can indicate battery drain. But 20min or so is absolutely normal. Don't really understand why, though it can be handy to read the odometer with the ignition off, eg at a filling station.

[JW1982]

The display illumination is switched by CANbus activity. When the CANbus network is active the display is illuminated. 

Approximately 20 to 30 minutes after locking the car the traffic on the CANbus databus stops and all CANbus modules enter sleep mode. At this time the illumination of the display is also switched off. 

[StephenFord]

So, by that reckoning, the very action of simply locking the car remotely activates the CANbus system? I never knew that, how odd... 🧐

[Tdci-Peter]

4 hours ago, StephenFord said:

the very action of simply locking the car remotely activates the CANbus system?

On the Mk2/2a Focus there are two main CAN buses, called HS-CAN & MS-CAN. HS-CAN is for engine related stuff, it goes to the ECU, other under-bonnet modules like ABS, and the Instrument Cluster (IC).

MS-CAN is for body related stuff, it goes to the BCM (aka Passenger Fuse Box), door modules, park aid, Audio system, etc, and to the IC.. If you have front & rear electric windows, you will have electronic door modules, and all the central locking signals are sent over the MS-CAN bus from BCM to & from these modules. So CAN bus activity is essential.

If you have manual rear windows, the wiring from BCM to all the locking is conventional (lots of wires), but it is likely there will still be communication between BCM and IC on any locking operations.

Anything that keeps the MS-CAN bus active not only keeps the odometer light on, but can cause battery drain. Not the tiny light itself, but if major electronic modules are not going into sleep mode, they can draw appreciable current. So it is worth checking from time to time that this light does eventually go out.

The Mk2 Focus is a bit more electronic than many people think. Mk3 & Mk4 are even more stuffed with data buses of course!