Originally Posted by Alan Karna

Hi guys! I just came across your question today, so sorry this isn't timely, but in case anyone else is searching.

Steve Cole is mostly right about what the BCM does. The exact functions of it depend on exactly what model and model year vehicle it is in, and I can't divulge technical details, but here's a quick high level overview of it and what it does:

The BCM was introduced on MY2011 Softails. As of 2014 it is on all Harleys except for Street and V-Rod. It is the successor to the TSM/TSSM (turn signal and security module).

There are six different varities of BCM in existence today on commerically available bikes (not counting changes from model year to model year). It comes in two "orientations". One orientation ("Y-Axis") is for softail, the other orientation ("X-Axis") is for all other vehicles. Then there are 3 security types. One without security/fob support, one with security/fob support for Japan only, and one with security/fob support for the rest of the world (odd Japan wireless frequency regulation no other country has - automotive typically has the same difference). So between the 2 orientations and the 3 security versions, there are 6 total. A BCM on softail cannot be used with any other family. Part numbers vary from model year to model year.

Steve, you said it's the "nerve center" and that's fairly accurate. It is the "network master" and tells the other modules when to talk and when to be silent.
It also controls the vehicle mode. Whether your vehicle has an ignition switch with IGN/OFF/ACCY right on it or is the "keyless" variety, the BCM tells everyone else what mode you're in - OFF, accessory, or ignition, and also communicates when the starter is cranking. It also cuts engine and pump power if the kill switch is pressed or it tips over.

It powers the starter solenoid, and as such controls the starter interlock - making sure you're not in gear (or the clutch is in).

It powers the fuel pump, and can cut of fuel in the event of a tip. It will prevent starting and disable the fuel pump if there is a security violation (NOTE: for safety reasons, it will not do this is the vehicle is already running and moving).

In combination with the ECM, it powers the engine coils and the fuel injectors, so it can cut power to the engine, but does not play a role in throttle control.

It DOES contain an internal accelerometer. It uses this both to detect if the vehicle is being disturbed for security, and to detect if the vehicle is tipped over. As noted, it DOES NOT work for Trike. On factory Trikes MY18 and prior, there is an additional Bank Angle Sensor (BAS) that tells the BCM if the trike is tipped or not. For MY19 and later factory trikes - it's actually the ABS module (which in turn is talking to an inertial measurment unit [IMU] ) that tells the BCM how much the bike is leaning. It is much more accurate than the internal sensor. I highly recommend against turning a vehicle into a trike or adding a sidecar for this reason. At the very least, if you do, you need to properly install a BAS and take it to a dealer and have them change the BCM settings to the trike/sidecar configuration.

It talks to the ABS module, but it does not intervene in any braking events. In some later model vehicles, it uses information from the ABS unit to light the brake lamp.

It controls all of the lighting on the vehicle (except aux/fog lights on older vehicles and ones installed via kits). There are no lighting fuses, the BCM detects if there's an issue and shuts off the power if there is.

It also controls the security system, talks to the fobs and siren (if installed) and will flash the turn signals, disable the fuel pump and coils if there is a security violation.

It also powers the accessory connector.

If you have a CVO MY16 or later, the BCM also controls the power locks.

Note that you cannot swap/replace a BCM without the dealer (or at least a dealer tool). The ECM knows exactly which BCM it is talking to and if it senses a different BCM, it will assume something on the bike is stolen and not allow the engine to run and will also tell the BCM to turn off the fuel pump and not power the coils/injectors. So do not swap a BCM on your own (unless you have and know how to use the dealer tool - and I'm not talking about the generic diagnostic tools you can get online, I'm talking the actual H-D dealer tool. The generic diagnostic tools can be used to read trouble codes but cannot be used for replacing and configuring electronic units). That being said, if you ever need to find a replacement BCM, a new BCM can replace an older BCM of the same type - but with few exceptions, you cannot replace a newer BCM with an older one. Additionally, each BCM has to be configured for the specific set of features on that bike. You cannot swap a softail BCM with a BCM from any other family and vice-versa.

So that's about it as far as vehicle functions it supports. This post is for information only. BCM replacement and/or configuration should only be done by a qualified technician. Neither Harley-Davidson nor I can be held liable for any work you do on your own!

Originally Posted by Alan Karna

I forgot about one other feature - tire pressure monitoring. If you have a CVO from MY14-20 or now some MY20 non-CVO touring bikes the BCM also talks wirelessly to the tire pressure sensors in the tires and flags low pressure to the instrument and sends the pressure values to the radio (if equipped)..

Originally Posted by Peter Quinn

I imagine it will take a Steve Cole to explain the full functions of this hidden cellphone sized module that also lets the E.C.M know where the bike is and what forces are acting upon it. it has links to the lean angle,G-forces,and speed (Amongst other things).
I understand that it interprets this data and can intervene directly with the throttle control and the A.B.S braking and moderate them.
This is important to know when you choose the A.B.S braking system option.
If you decide to convert your bike to a three wheeler it will shut you down when it detects that your cornering G-force is obviously too high for non existing lean angle.