During the mid-1990s, auto manufacturers were faced with the increased demand for on-board electronic devices such as air bag, anti-theft, anti-lock braking, collision avoidance, vehicle stability control, lighting, and a myriad of convenience systems that increased the size and complexity of the vehicle’s electrical system. Today, more refined safety and convenience systems such as telematics communications systems have placed an even greater load on vehicle electrical systems.
CONVENTIONAL ELECTRICAL ARCHITECTURE
As a point of reference, let’s review how the architecture of vehicle electrical systems has evolved over the years. In the beginning, vehicles had no electrical systems because they used hand-cranked engines and were equipped with acetylene gas lighting. When self-starting was popularly introduced in the 1920s, a storage battery and generator were required to supply current to the starter motor. Electric exterior lighting controlled by a simple hand switch became standard during the 1920s. During the 1930s, radios and passenger compartment heaters with electric blower motors also became popular.
With the standardization of air conditioning, electric wiper motors, power windows, power seats and cruise controls during the 1960s, electrical architecture generally stabilized until Chrysler Corp. popularly introduced its Chrysler Collision Detection (CCD) system during the late 1980s. Up to this point, all body controls, such as lighting, windshield wiper, heating and air conditioning were controlled by manual switches. With the introduction of the Chrysler CCD system, many components became activated or controlled by a body control module (BCM) instead of a manual switch.
CHRYSLER CCD SYSTEMS
Let’s keep in mind that, because body control electronics generally fall outside of Environmental Protection Agency (EPA) emissions mandates, auto manufacturers aren’t legally required to standardize nomenclature or testing functions. Consequently, the architecture of most body control systems varies considerably among nameplates and vehicle models.
For that reason, I’ll use one of the first popular applications of body control, the 1990s Chrysler Collision Detection (CCD) multiplexed body control system, to generally illustrate the fundamentals of how these systems work.
Unlike the conventional architecture that lasted through the 1980s, Chrysler’s CCD system integrated the operations of most body electrical and electronic systems into a body control module (BCM). In this system, for example, a manual switch commands the BCM to activate the windshield wipers or other accessories. Because this system inserts a module or computer between the manual switch and the accessory, it requires a different diagnostic strategy.
To illustrate, electric wiper motors are generally diagnosed by establishing if the motor is being supplied with electricity by the manual switch. If the switch is working correctly, the assumption is that the wiper motor or the wiring is at fault. But, in body control systems, the electric wiper motor must be tested by using a scan tool to verify that the manual switch is commanding the BCM to activate the wiper motor and change the motor speeds according to command.
Many body control systems similarly monitor the amperages flowing to the various external lights. Using this information, the BCM or, in some cases, a lighting module illuminates a convenience center warning light when it “sees” zero electrical flow in a specific lighting circuit. The technician must then determine if the problem is caused by a failure like a burned-out light bulb or open wiring circuit.
Most body control systems store a B, C, or U-series diagnostic trouble code (DTC) when a critical circuit fails. The “B” code indicates “body” and the “C” code indicates “chassis,” which includes steering and anti-locking braking systems. The “U” codes indicate a communications failure between two or more modules.
Obviously, the more electrical devices that are added to a conventional electrical system, the larger the wiring bundle required to operate them. To reduce the size of the wiring bundles, auto manufacturers began using multiplexed wiring systems in their vehicles.
At their most basic level, multiplexed electrical systems use a communications wire and a power wire to operate complex electrical devices like power windows. In such systems, the vehicle’s metal body usually acts as the ground wire or the “second wire” needed to complete the electrical circuit with the battery negative terminal.
To illustrate, a conventional power window system must use a separate switch and wiring circuit to raise or lower each window. Each window switch raises or lowers the window by reversing the polarity of electrical flow through the power window motor. When combined with power door locks, the wiring bundle passing from the body into the door becomes quite large and highly susceptible to metal fatigue.
A multiplexed system, on the other hand, places an electronic control module inside the door to operate the power window motor and the power door locks. The “down” switch for the passenger side window would, for example, command the BCM to make the window go down. The BCM would re-issue that command through the communications wire to the door module. The door module would then reverse the polarity of electrical flow through the power window motor to make the window go up or down.
The upside of multiplexed systems is that they greatly simplify the electrical system by reducing the size of most wiring bundles. The downside, of course, is that they are, on a conceptual level, more complex to diagnose than a single mechanical switch connected to a single-wire circuit.
Unfortunately, many parts professionals and working technicians don’t realize that even the most basic current vehicle platforms usually incorporate at least four modules designed to control most of the vehicle’s body hardware functions. An “economy” vehicle platform would have on board, at the very least, a powertrain control module (PCM), anti-lock braking system (ABS) module, a supplemental restraint system (SRS) or air bag module, and an anti-theft or vehicle security module. Most vehicles would also have a heating, ventilation, and air conditioning (HVAC) module, an instrument cluster module, a lighting module and other modules, depending upon application.
Because these modules must share data with each other, they must also communicate with each other. For example, the ABS module generates a vehicle speed signal which is shared by the instrument cluster, automatic transmission, body control, air bag, and powertrain control modules. That data passes through the ABS module and is supplied to other modules by a “communications bus” that uses a low-voltage signal to transmit the speed signal to all applicable modules.
But another reality is that the air bag module, for example must have a faster communications speed and higher communications priority than other systems. Communications with low-priority accessories like heating and air conditioning don’t need to be as fast. Automakers responded around 2004 by incorporating the Controller Area Network or CAN system which allows various body control communications systems to operate at faster communication speeds, with one module, such as the powertrain control module, acting as a “master” module that prioritizes the various communications protocols.
WHAT BODY CONTROL MEANS TO YOU
As a parts professional, you’ll probably begin to see more warranty complaints caused by consumers and technicians not fully understanding how body control systems operate. A good example is a modern charging system in which an alternator becomes a load-responsive unit controlled by the PCM. Because some systems may not even produce charging voltage at idle as long as the battery maintains at least an 80 percent state of charge, a professional-level scan tool is required to evaluate alternator performance.
Similarly, modifying a vehicle’s wiring system or installing aftermarket accessories isn’t a good idea on modern vehicles. As mentioned above, lighting modules are programmed to recognize a specific amperage flow to all exterior lighting. Hacking into a modern wiring harness to attach trailer towing lights or other accessory lighting can cause problems with the vehicle’s lighting module simply because the module isn’t programmed to deal with the additional current flow.
Other accessories like remote starting systems are designed to defeat the vehicle’s anti-theft or vehicle security systems. If the remote starting system fails to consistently do that, the vehicle might develop complaints like intermittent starting and stalling or an outright cranking, no starting failure.
In a word, nothing regarding modern wiring systems is as simple as it might appear. Because a computer has been inserted between the switch and the accessory it controls, we could say that modern electrical systems certainly aren’t like those on your father’s Oldsmobile. But one caveat: General Motors no longer makes Oldsmobiles.
Gary Goms is a former educator and shop owner who remains active in the aftermarket service industry. Gary is an ASE-certified Master Automobile Technician (CMAT) and has earned the L1 advanced engine performance certification. He is also a graduate of Colorado State University and belongs to the Automotive Service Association (ASA) and the Society of Automotive Engineers (SAE).