What is a body control module
Understanding the Role of The Body Control Module (BCM) in Modern Vehicles
The Body Control Module (BCM) is a pivotal electronic control unit in modern vehicles. It manages various electronic accessories like lighting, instruments, power, and anti-theft features of a vehicle. Understanding the BCM’s roles and the common issues associated with it is crucial for both automotive enthusiasts and everyday drivers.
The BCM is a computer that receives requests from the driver, either directly or remotely, to control many electronic features. Car makers switched to this design for several reasons. First, it reduces overall vehicle weight and cost by reducing the size and quantity of copper wire. Second, it more efficiently monitors and controls power usage and allows for remote operation and diagnostics.
The body control module is what turns on the lights in a modern vehicle
Here’s an example of how carmakers use a body control module to reduce cost and weight and allow remote operation. In the past, carmakers ran heavy gauge wire from the fuse box to the headlight switch and from the switch to the headlights. If the driver left the vehicle with the headlights still on, the current draw would drain the battery.
In a BCM-controlled vehicle, the headlight switch is wired with a small gauge wire. The BCM supplies low voltage to the switch. Activating the switch connects the BCM power to the ground. The BCM sees the voltage drop, interprets it as a driver request to turn on the headlight, and activates the headlight relay, turning on the headlights. If your vehicle is equipped with remote operation through an app, the BCM bypasses the mechanical switch and activates the headlight relay.
Here’s an example of some of the components controlled by the BCM controls in late model vehicles
windshield washer
windshield wiper
turn signals
hazard signals
headlights
high beam
flash to pass
shift interlock solenoid
trunk release
power door locks
door open/closed switches
stop lights
hood switch
horn
courtesy lights
backup lights
instrument panel lights
trunk release
traction control on/off
The body control module communicates digitally with the ECM or PCM
In addition to receiving inputs from switches and commanding relay operations, the BCM also communicates with the ECM or PCM via a high or low speed data bus. For example, in some vehicles, the anti-theft system works partially through the BCM. The transponder ignition key communicates with the receiver located around the ignition lock cylinder. The receiver communicates the key code to the BCM. The body control module passes the key code to the PCM for confirmation. If the key code is correct, the PCM sends a verification signal back to the BCM, and the BCM supplies power to the control coil of the starter motor relay, activating power to the starter motor.
What goes wrong with a body control module?
It’s a solid-state computer, so any electrical surge can damage it. The BCM can also be damaged by a short in a relay or an induced voltage on the data lines.
How to fix a bad BCM
A bad BCM can prevent you from scanning for trouble codes, kill your digital instrument cluster and gas gauge, and prevent you from turning on lights or starting the engine. In the early days, replacing a bad body control module meant paying a small fortune for a new one from a dealer. That’s not the case anymore. Many companies offer rebuilding services to repair damaged board components and return your BCM back to you in working order for a fraction fo the price of a new unit.
Should you get a BCM body control module with a unit from a junkyard?
NO! First, a junkyard unit will most likely have the same manufacturing defects as the bad body control module body control module you’re trying to replace. Secondly, the body control module must be the same part number as the bad BCM you’re replacing. A rebuilt body control module will most likely include upgrades to prevent the types of failures that occur in the original design. Finally, you won’t save much money by buying a junkyard BCM.
Plus, the BCM must be programmed to your exact VIN code and, in many cases, must be synched with the PCM to allow engine startup.
Where can you get a replacement?
This is a typical rebuilt body control module BCM
Do an online search for body control module rebuilders. Then check the Company’s reviews before you send yours off to be fixed. Choose a rebuilder with full programming capabilities so yours is ready to plug and play.
Auto parts stores also sell rebuilt them. The largest rebuilder is Cardone and that’s the brand you’ll likely find at the auto parts store. Cardone is considered a high quality rebuilder of bad body control modules.
How to remove and re-install a BCM
• Disconnect the battery cables first.
• Disconnect each connector from the BCM and label where it goes.
• Wrap the BCM in tin foil to shield it from static electricity.
• Send it to the rebuilder.
• Reverse the process to re-install it
©, 2015 Rick Muscoplat
Posted on by Rick Muscoplat