Originally Posted by Tim Crump
Yes the BCU does all sorts of things that the ECU does not. Whilst not an accurate reflection, but as a guide, the fuses are split into two banks on the recent +4 Block A and Block B
Block A are things associated with the BCU and Block B the ECU

So the BCU gets involved with the immobiliser, lights, wipers, heated screen etc. It will always be drawing power for the immobiliser and maybe other functions.

You will have noticed that there are only three traditional relays on the +4 that is because four other relays are soldered directly into the BCU

[Linked Image]

Photo is from my 2018 +4 and if you look closely bottom right it is marked +4 suggesting different BCUs for other models.

Interesting picture, thanks. I'm guessing those BCU relays cover heated screen, heater motor (two possibly) and front fog lights, all triggered by switch/sense logic.

The 2014 onwards +4 and Roadster have the same two multi-pin connectors to BCU with 34 pins in A set and 26 pins in B set. The Roadster BCU would differ from it at least because it relies on BCU and PCM for its unique Mustang origin instrument cluster feed.

Items I have identified that the BCU is concerned with are Sidelights, Headlights, Front and rear fog lights, Indicators and Hazards, Instrument Cluster, Start switch, Interior lights, Heated screen, Heater fan control and Ignition. Not sure about wiper/washers without the help of a diagram unless its a spill over from an unimplemented rain sensing system maybe. MMC have used the CAN topology wiring diagram twice, once correctly but again under wiper/washer heading confused2

Thanfully there is no bulb monitoring so no issues fitting LED headlamps or sidelamps on these models.

Richard

2018 Roadster 3.7
1966 Land Rover S2a 88
2024 Royal Enfield Guerrilla 450
1945 Guzzi Airone