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
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.
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
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.
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
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
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
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.
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
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
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
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.
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.
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).