Many modern vehicles utilise a computerised control unit for activation and deactivation of autoelectrical systems.
Various vehicle functions may operate via a body control module.
The BCM may be integrated with a central junction box containing fuses, circuit breakers and relays to provide switching and circuit protection.
The BCM is often linked to other control units as part of a vehicle network.
The vehicle body control module is often configured specially to specific vehicle specification and may be security code programmed requiring pin code access to complete vehicle identification coding such as VIN.
The body control module uses driver inputs from switches and modules (including CIM) via the vehicle controller area network CAN (CANbus) or Local interconnect Network LIN (LINbus).
Serial data is sent on the bus (CANbus or LINbus) as bits and bytes (Hexadecimal code).
Controller modules process information,
and provide electrical outputs to control vehicle body autoelectrical systems.
Typical body control module functions and fault codes may include:
Vehicle Alarm system activation of flashing lights and siren. Fault code 01134 Alarm Horn no signal
Central Door Locking (CDL) system to lock and unlock vehicle doors. Fault code 01331 ECU driver door J386 No signal,
Electric door module passenger 01553 Motor for central door lock passenger, electrical problem in circuit, 00097 Front passenger central locking motor v162 electrical problem in circuit.
Key programming to allow recognition of the correct drivers key and allow / prevent vehicle engine starting.
Sidelights and Number Plate Lights , 00984 Lefttail light, 00985 Right tail light
Headlights
Front and Rear Fog lamps
Direction Indicators : 03098 Left rear turn signal, 03099 Right rear turn signal
Hazard Warning Lights
Brake lights
Reversing Lights
Windscreen Wipers and Washers
Horn
Interior courtesy lighting including timed delay
Heating and Air conditioning
Battery monitoring and charge control
Speed monitoring and cruise control
Air suspension (if fitted)
Audio systems
The BCM may be integrate both fuses and relays to provide electrical switching and circuit protection.
There are various names given to the control module providing switchgear and control stalks for headlights, indicator stalk, and wiper switch stalk
including Column integrated module, CIM, Column interface module, etc.
The main function of the CIM is to provide a driver input control which is communicated to the body control module over the vehicle network,
many other functions may be incorporated in this programmed module.
Obsolescence becomes a problem as vehicles age and switchgear becomes worn, particularly at high mileage.
Problems we have encountered with CIM equipped vehicles have included headlights stuck on or coming on intermittently causing the battery to drain and consequent
failure of the vehicle to start.
Other problems have included CIM losing communication with other modules and preventing the vehicle starting resulting in immobilisation.
Replacement of the vehicle body control module may require specific configuration using diagnostic equipment and manufacturers information particularly where security access is required. This may mean a SAE J2534 pass thru device linked to a manufacturers service portal is required.
BCM inputs from switches may be checked by using vehicle diagnostic equipment serial data mode where voltages and logic can be monitored.
BCM output functions may be checked with specialised vehicle diagnostic test equipment using an actuator test function which will send commands to the vehicle computer telling it to switch on lights etc.
The BCM is likely to be part of a network of vehicle computer modules which communicate serial data via the controller area network bus (CANbus) system.
Diagnostic serial data may raise fault codes in other modules when communication faults with bcm exist.
Manufacturers may limit programming access to modules by use of digital locking requiring a security access code before certain levels of diagnostics and reprogramming are available.
Replacement of a BCM may require software is loaded on to the module to configure and gain functionality which can present problems for aftermarket repairers
BCM faults may prevent the vehicle from starting, allow to start and run for a few seconds, or provide strange effects with functions such as lighting, wipers, door locks, etc.
Many BCM faults are a result of water ingress causing corrosion and deterioration of circuit board electronics.
Replacement of BCM may require dealer level programming.
Vehicles using a BCM to control lighting functions may require that a logic relay eg 7 way bypass relay is fitted if wiring additional trailer electrics.
Vehicle specific trailer modules may be required by certain manufacturers.
The BCM may detect circuit loading so when additional lights are fitted to the vehicle lighting circuit the module detects extra load and sees this as a fault
subsequently logging error codes and denying power to the affected circuit stopping particular lighting functions.