By Jim Kerr

They have been hiding beneath the upholstery for several years now, but we seldom give them any thought. Relax – its nothing dangerous. It’s your vehicle’s body computer. Over the past few years, vehicle electrical systems have been undergoing dramatic changes, even though we probably are not aware of it. The key to many of these changes is the body computer.

Body computers can go by many names, depending upon the vehicle manufacturer, but often they have very similar functions. They are in control of locks, lighting and alarms. The computer or module receives input signals from many switches or other modules and then either controls devices directly or sends a data signal to another module so that the other module can operate something. Sounds complex but it isn’t. Let me give you a few examples.

When you open the door of your vehicle, switches in the door latch or door jamb send a voltage change to the body computer module. The module then operates a relay to turn on the interior lights and turns them off again after a few seconds or when the key is turned on (another input to the module). This simple task was once handled by a small interior lamp module, but doing it with the body computer has several advantages, as we will see later.

Headlight operation is similar. In the past, the headlight switch was wired directly to the headlights. On many vehicles, the headlight switch is now wired only to the body computer. It is the body computer that turns on the lights.

The backup lights may also be operated by the body computer, but on some systems, the body computer sees the signal from the Park/Neutral switch for reverse gear selection and then sends a data signal to a module at the rear of the vehicle. That module turns on a relay to power the backup lights.

To offer all the features we have today on vehicles without using a body computer, we would have to have wiring harnesses the size of a coffee cup running the full circumference of the vehicle. A signal from each switch would need to be wired to every device that uses it. Instead, the wiring harness is much smaller because a switch only needs to be wired to one module, and that module can pass the information through data wires to many other modules on the vehicle. This allows many additional functions to be added with relative ease.

For example, with the body computer already controlling the lights, it is a simple matter to program it so that when it sees a door unlock signal from the keyless entry system, the headlights and backup lights turn on so you can see around your car. It also helps find it in a
crowded parking lot.

Program the body computer with an anti-theft program and the computer can flash the lights and honk the horn if someone opens a door without unlocking the door with the electronic key fob. The body computer already has the inputs for door opening and door locking and it already
controls lights, so adding horn control isn’t a big problem. In fact, many vehicles already have the horn switched on by a computer. The body computer can also send a signal to the engine computer so it won’t turn on the fuel injection. With the lights flashing, horn blowing and a vehicle that won’t start, a thief quickly departs the scene.

Another advantage of using body computers is that they allow personalization of the vehicle for different drivers. The body computer recognizes what key is being used and sends signals to different modules so they are adjusted for each driver. You may want the horn to chirp when you lock the car. I prefer the lights to flash. You may want the radio set to AM stations and I prefer FM. That’s personalization.

The body computer is like a master controller for all body functions. It allows us to add many additional convenience features without having to add a complex wiring harness, it allows us to customize the operation of many vehicles, and it actually makes diagnosis easier for technicians. Even the lowest priced vehicles on the market are starting to incorporate body computer control. That’s why many features only found on luxury cars in the past are making their way into vehicles than we can all enjoy.

Connect with