What is a Bodytrain Control Module (BCM)?

Engine Control Modules (ECMs) should be very familiar to you at this point. If you’ve read any of our other posts, then you should be comfortable with the basic concept of an ECM. Today we want to take a look at one of the potential areas of concern for the do-it-yourself mechanic when it comes to replacing electric components on the car.

The fact is that, unless we have a detailed schematic of our car’s engine bay in front of us, a lot of the components may look very similar! In modern cars, so much of the actual working engine parts of covered in cowlings and plastic coatings, to help keep the interior of the engine bay looking tidy, that even the most seasoned DIY mechanic may have a problem identifying critical engine components. That can be costly and even potentially damage your car’s electronic systems, if you try to plug the wrong replacement part into a connector port!

The two most frequently replaced electrical components on a modern car are the ECM and another component that looks very similar, the Body Control Module, or BCM. You can see them both pictured here:

Source: eBay

Upon first inspection, the BCM DOES look a lot like the ECM, which can lead to problems when it comes time to replace one or the other. You may even be thinking to yourself, what’s the big deal, a control module is a control module! Not true, and not safe! Both modules have very distinct jobs, and misidentifying them can be harmful to your car’s electronic systems.

So How do you make sure you’ll removing, and replacing, the right part? Well, in the example above, you can see that the BCM, pictured on the left, is both smaller and has different connector ports than the ECM, pictured at right. In this case, mistaking one part for the other would be fairly tricky, but some automakers have a tendency to make their control modules look much more similar than Infiniti (the manufacturer in the above photo).

While both components may appear to be little anonymous black boxes, nothing could be further from the truth, which is why we’re going to next discuss the importance of correctly diagnosing the symptoms of a bad BCM.

Engine control module trouble is characterized by a lack of regulation of the engine components. Since the function of the BCM is to control various portions of the car’s electronics dealing with the body of the car itself, you can expect to see symptoms of a failing BCM in the body components.

The BCM controls things like automatic headlights, door locks, windows, and the car alarm system. Failure of a BCM leads to sporadic and failing ability of these components to do their job. A bad BCM could mean that your car alarm goes off at 3 AM when you (and your neighbors) are sound asleep! A bad BCM could mean that you can’t remotely lock your vehicle, obviously not ideal for anyone who doesn’t want their car stolen! So, we can see that once the BCM starts to go bad, it’s not long before we need to think seriously about replacing it.

Since the BCM and ECM may look similar, and even have similar connectors in certain cars, the fact that they have such totally different functions means that it’s extremely important for you to carefully check any parts that you order or buy in a junk yard to be sure you’ve bought the correct part. Trying to plug a BCM into an ECM port and vice versa will wreak havoc with your cars electrical system, and definitely won’t solve your problems! To be sure you’ve got the right part, either keep the old part in hand when you go to the junk yard or, better yet, buy your part from a reputable dealer, to ensure that you’re getting a true OEM part, not some shady knock off or worse, the wrong part entirely!

In conclusion, while the BCM and ECM on your car may both look like nondescript plastic boxes, understanding and correctly identifying the symptoms of failure is extremely important to knowing what part to replace. Likewise, understanding how to spot the sometimes subtle differences in connectors, weight, size, and placement in the Engine Bay between an ECM and a BCM will prevent you from either purchasing the wrong part, or being ripped off by somebody trying to scam you into paying for one and receiving the other. BCMs are integral to a car’s function, just like ECMs, so it pays to know the difference, and to go with trusted sources of supply to get the parts you need to keep your car running in top form!