What does a BCM body control module do?
A body control module (BCM) is a computer used to control electrical items in a car or truck which used to be controlled individually by switches. Car makers switched to this design for several reasons: to reduce overall vehicle weight by reducing the size and quantity of copper wire used, to monitor and control power usage, to allow for remote operation and diagnostics.
BCM can switch power to lights
For example, in non-computerized vehicles, the power for headlights was supplied by a headlight fuse. The power then flowed to the headlight switch, and from the headlight switch to each headlight. The wire used for the run from the fuse to the switch and from the switch to the headlights had to be a large enough gauge to handle the current load of the headlights. If the driver left the vehicle with the headlights still on, the current draw would drain the battery.
In a vehicle equipped with a BCM, headlight power runs through a headlight relay in the underhood fuse box and then to the headlights. The headlight relay is switched by the BCM, which in turn is commanded by the headlight switch. In this case, the headlight switch acts as an input to the BCM.
In most cases the wiring to the headlight switch is just a single small gauge wire. The BCM supplies low voltage to the headlight switch on this single wire and monitors the voltage. When the driver turns on the headlights, the headlight switch either connects that voltage to ground, causing a voltage drop, or open the circuit to ground. In either case, the BCM detects the change and energizes or de-energizes the control coil for the headlight relay.
This design saves considerable wiring weight and cost and allows the BCM to monitor whether the driver has left the vehicle with the headlights on. If so, the BCM can turn off the headlights to save battery power.
In more complicated and more recent designs, car makers run a single wire to the multi function switch to control headlights, high beams, parking lamps, turn signals and hazards. How can they accomplish all that with a single wire? Simple, the switch contains several resistors. The BCM monitors the voltage flowing through the resistors and, depending on how much voltage drop it see, determines whether the driver is calling for headlights, high beams, hazards, etc.
Here’s an example of some of the components controlled by the BCM controls in 2010 Chevrolet Impala
flash to pass
shift interlock solenoid
power door locks
door open/closed switches
instrument panel lights
traction control on/off
The BCM also communicates with the ECM or PCM
In addition to receiving inputs from switches and commanding relay operation, the BCM can also communicate with the ECM or PCM. For example, in some vehicles the anti-theft system works partially through the BCM. The transponder ignition key communicates with the receiver which then communicates the key code to the BCM. The body control module passes the key code on 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 on the starter motor relay.
What goes wrong with a BCM?
Any type of electrical surge can damage a body control module, so it’s wise to exercise extra care when jumping the vehicle. The BCM can also be damaged by a short in a relay or an induced voltage on the data lines. Keep in mind that the body control module is simply a computer with programming, inputs and outputs.
How to fix a bad BCM body control module
A bad BCM body control module 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. Lots of companies offer rebuilt units to replace your bad body control module BCM.
Should you get a BCM body control module with a unit from a junk yard?
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 exact 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 junk yard BCM.
Plus, the BCM must be programmed to your exact VIN code, and in many cases must be synched with the PCM in order to allow engine startup.
Where to get a body control module replacement?
Let’s use a 2002 Saturn L200 as an example. An GM parts site shows a part number of 19116648 with a retail price of $446.12.
That price doesn’t include programming. However, you can buy a body control module replacement unit 19116648 from autoecmstore.com for around $295 (once you return your old unit for a core charge credit).
A body control module rebuilder should perform complete body control module reprogramming in their price. Auto parts stores also sell rebuilt body control modules. 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.
When rebuilding a bad body control module, the rebuilder must first clean the entire circuit board and case. Next, they must remove the cleaning agent. If they don’t the cleaning residue can absorb moisture that can lead to intermittent problems.
Next, the rebuilder should resolder all the critical joints. After replacing the bad electrical components, the circuit board should be re-coated with a “conformal coating.” This is a clear liquid plastic that seals out moisture. Many rebuilders either skip this step or just spray the board with a can of aerosol clear coat from the hardware store. Quality rebuilders also replace the PROM cork, which is a stiffening part to prevent board flex when installing the PROM.
Finally, a quality rebuilder will print program the unit to your exact VIN number so you don’t have to pay the dealer to do a program update. Once it’s programmed, they’ll put a new part number label so the body control module can be properly identified in the future.
Remove and install a BCM body control module
All repair procedures tell you to disconnect the battery cables first. Well, this time they mean it. NEVER disconnect or connect a BCM body control module with the battery cables connected. You can damage other electronic components connected to the sensitive data bus and you can even blow out the new parts. The hardest part of the job, aside from finding it, is figuring out how to remove the electrical connector locking latches. Once you figure that out, just remove the connectors and remove the BCM.
©, 2015 Rick Muscoplat
Posted on by Rick Muscoplat
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