Get any teenager their first car
and the first thing they want to do is modify it with new wheels, a different
exhaust, seat covers or, the most important and popular upgrade, the stereo.
You know, when they have to get to school with all their buddies in the car,
they’ve got to have those tunes. They can’t be seen around town with a dull
factory radio or wimpy stock wheels.
However, on today’s modern import
vehicles, adding on some accessories can leave them stranded in dad’s driveway.
When it comes to installing that new stereo system or video system, a lot of
“pre-thinking” needs to occur before reaching into that dash and ripping out
the old one. That’s because in many of today’s cars the stereo system is tied
into the CAN Bus line. This line allows communication to and from the Body
Control Module (BCM) and other modules.
Some manufacturers will use the
left front speaker as the warning chime speaker for all the necessary functions
such as “door open-lights on,” “key in the ignition,” etc. The BCM will tell
the stereo to produce the appropriate bell or “ding” to inform the driver of
the immediate problem of which they should be aware. Some will keep the radio
in check with the security systems; others incorporate the stereo and the
climate control into one unit and display them together on a screen in the
dash.
The problem comes down to the fact
that the communication between these units needs to be there, or at least in
some way recognized, so the CAN Bus line is not lost or shorted. It could lead
to a service light on the dash or, even worse, a no-start condition.
WHAT IS CAN?
Just like your cable TV, as a
signal is sent down the wire from one communication device, there needs to be
another at the other end that can “descramble” that information and turn it
into readable information. These “lines” are generally referred to as Bus
lines, or Data lines.
Most of the time they are in pairs
of wires that are twisted together (less radio frequency [RF] interference).
Some manufacturers use a two-speed CAN.
One line is for low-priority
information, such as radio, windows, etc. A second, faster speed line is for
things like the transmission, theft devices, etc. Both systems move along the
same wires at the same time.
Each of the “modules” on the Bus
line use the information that they are programmed to read, and any other
information on the Bus is ignored and not read by that particular module.
My advice when it comes to
diagnosing power windows, gauges, or, for that matter, just about anything
these days, is to get your scanner out and look for codes, look for a Class 2
serial data line, read the Mode $06 information, or whatever that particular
manufacturer is calling its CAN line information.
In 1983, Bosch Corporation
introduced the CAN system to the world as a precursor to what they saw as an
increase in the automotive electrical system advancements. In 1987, the first
CAN system was officially called “CAN,” but it wasn’t until 1992 that a Mercedes-Benz
CAN system was accepted as the first true CAN system. Early Saabs had a system
that could have been called CAN back in 1987, but the only references were to
call the lines “data lines.” It still worked about the same way, but wasn’t
diagnosed the same way as it is today.
In 1995, Saab introduced Class 2
serial data lines that run at a speed of 10.4 kbps. In 2004, GM went to its
next generation system called GMLAN (local area network) on Saab vehicles,
which had a two-speed system: low at 33.3 kbps and a high at 500 kbps.
Mercedes-Benz uses several Bus lines; on one car I counted five different CAN
speeds.
With the speed and flexibility of
these electronic systems that manufacturers can create in today’s cars, I can
only imagine how far all this information is going to go. At some point in
time, that wiring will also be a thing of the past. Everything in the car could
someday go completely wireless; modules will get smaller, faster and less
likely to fail. Scanning could be done without even seeing the car in a repair
shop. Just dial your cell phone to your shop of choice and a complete
diagnostics could be done right then.
JAMMING THE CAN
Here’s an example that happened at
my shop with a car that had a no-start condition. When checking the service
codes, I found the Class 2 serial data (Bus) line showed one to be shorted to
ground. Tracing down the shorting bar for the Bus line, you can then
individually test each Bus line for a grounded signal. One glance at the dash
and it wasn’t hard to tell it didn’t have the factory radio in place.
After tearing out the aftermarket
radio system and locating the Bus wiring, the “chase” was on. Whoever
installed the aftermarket stereo knew enough that the factory radio still
needed to be hooked up in order for the door chimes to work. The factory radio
was still wired in and was jammed under the back seat cushion, which is where I
found a crushed stereo system case caused by someone sitting in the back seat.
As soon as the aftermarket radio
was installed, there was still enough time for the vehicle owner to cruise down
the street to show off for all his friends with the boom box cranked to max,
and, of course, a car full of his buddies jamming to the music. Everything was
fine until they stopped at the local hangout to look cool next to another car
with the windows down and stereo still cranking out the latest tunes. When they
all piled back into their “concert on wheels,” it wouldn’t start. That’s when
it was towed to the shop where I had to find out what was wrong.
A stereo system is not the only
thing that can cause this problem. But it’s sure the most common culprit. Theft
systems that try to “piggy-back” the factory alarm system, GPS units, van
conversions and aftermarket add-ons of all types can cause a problem when it
comes to CAN systems.
These data information screens
will give you the clues as to what to look for. The next stop is to go to your
PC and look up the wiring diagrams. Codes are only a starting point. Remember,
you still have to diagnose the cause of that code and what it means.
Be sure to read, follow and
understand the circuits before you attempt to change or add something to these
types of systems. And, if you get a no-start in the shop, don’t be afraid to
ask the question…“So when did you install the stereo? It might be the “bling”
in your diagnostics.
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You Can CAN!
As great as multiplexing is, it
can cause all kinds of strange problems when communications break down. If the
system detects a loss of communication with a module, it will usually set a
Diagnostic Trouble Code (DTC) to help you identify and isolate the fault.
However, intermittent problems may or may not set a DTC, depending on their
severity and duration.
A worse situation is when
communications are distorted because of unwanted noise on the data Bus or
because a module is generating a bad signal. This may cause other modules to
ignore or misinterpret their instructions with unpredictable results. The
vehicle may not start, it may stall for no apparent reason, lights may turn on
or off, and accessories may operate intermittently. These types of problems
may not set a DTC and could act as though the electrical system was possessed.
Spark plug wires routed too close
to the wiring harness can sometimes generate noise. A defective alternator
could create enough voltage variation to upset the operation of the system. Low
battery voltage, loose or corroded connectors, poor grounds, shorts and opens
will destroy the integrity of the data Bus circuit.
Tech Tip: If you have a vehicle
with no serial Bus communications for any module on multiple serial Bus
networks, look in the vehicle’s wiring diagram and/or theory of operation for a
module that has a function known as a “gateway.” Simply put, if there is a
module on more than one Bus and more than one Bus has problems, disconnect that
module to see if the problem goes away.
Tech Tip: You can measure for the
presence (or absence) of the 60 ohm resistors that terminate each end of a
two-wire CAN Bus. Simply connect an ohmmeter between pins six and 14 of the
DLC. Make sure the ignition is off and the Bus is not active (if in doubt,
disconnect the negative battery cable) to ensure your ohmmeter can accurately
measure resistance.
Tech Tip: The key to diagnosing
multiplex electrical problems is to understand the system you’re working on,
test for any data on the data Bus, measure the data Bus for voltage within
specifications and isolate the fault by unplugging or swapping modules one by
one until the fault is eliminated.
On connecting the oscilloscope to
a data Bus wire and displaying the signal, you’ll see a digital wave pattern
that changes in pulse width. The waveform confirms there is data on the Bus,
however, the data can be meaningless because it’s in a binary code. If there’s
no data on the Bus (a flat line), there is no communication and the module that
generates the bias signal is most likely dead. Voltage and grounds for the
device should be tested.
A good data Bus signal should have
nice sharp corners, flat tops and bottoms, and vertical sides. Peaks, rounded
corners, spikes and sloping lines are all indications of trouble because they
distort the signal waveform. This can result from internal or external noise,
poor connections, excessive resistance and voltage variations. The height of
the waveform can be measured to test for proper voltage. It should be within
the specified voltage range (typically 2.5 volts or 7.75 volts, depending on
the application).
The same basic test can be made
with a DMM, although due to the DMM’s speed of sampling a Bus waveform may
cause voltage to vary greatly or only slightly depending on the meter.
A flat line on a lab scope
checking a data Bus may indicate a shorted Bus. However, not every flat line
scope pattern is a problem.
Delphi offers a four-hour training
course for intermediate to advanced technicians, which will show you how to
troubleshoot stubborn communications problems with electronic control modules.
The course provides practical tips on the uses of meters, scopes, and factory
and aftermarket scan tools are combined with theory and real-world case studies
to demonstrate how to diagnose everything from U-codes to power mode masters to
serial bus gateways.
Courtesy of Delphi Product & Service Solutions
About the Author
Scott “Gonzo” Weaver
Gonzosae@aol.com
Scott “Gonzo” Weaver is the owner of Superior Auto Electric in Tulsa, Okla. and has owned the shop for 27 years. He was given his trademark nickname “Gonzo” while serving in the USMC. He is the author of the book “Hey Look! I Found the Loose Nut”, that can be purchased online at Amazon.com or at www.gonzostoolbox.com.
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