October 2023 - 34

Some manufacturers do an excellent job
of locating network junction points which
can be extremely useful for diagnostics such
as shown in Figure 2.
MODULE FAILURE
Verification of bucket #1 is an absolute must
before condemning a module and possessing
a solid understanding of how a module
is powered and commanded to wake up is
essential. Additionally, understanding where
relevant messages originate, which modules
are complaining, and when their complaints
surface will greatly assist the technician in
assessing the situation.
EXTERNAL INFLUENCE
External influence is always a possibility, and
this is why performing visual inspections is a
must. I recommend following this pathway so
that your initial visual inspection isn't influenced
by any specific fault. I find myself picking
up potentially useful information when
taking a quick look under the hood, under
the dash, seats, and cargo areas, especially
when looking for any add-on communication
devices. Additionally, I like to walk around
the vehicle inspecting for evidence of collision
repair or other service events that may
have introduced problems. This provides
a global view of the vehicle I can use during
the next phases of diagnostics, greatly
enhancing the decision-making processes to
come. Your front office staff should always
be questioning your clients for clues that
might prove to be useful. I have an example
to share with you later in this article about
a scan tool that induced a communication
fault (U0235) that stayed active even after
the tool was disconnected.
p Figure 3 - Network error reporting
Photo from CANCapture
p Figure 4 - Normal network operation
Photo from CANCapture
HSCAN CIRCUIT BASICS
Not all HSCAN networks are the same, but they all have terminating
resistance. Terminating resistance is used in high-speed
networks to minimize signal reflections that result from rapidly
changing voltage levels. Many HSCAN systems utilize two terminating
resistors which are typically located at the ends of the
network. There are many network construction guidelines manufacturers
follow regarding the overall length of the wiring, number
of modules, and network message load. It's quite common to see
two 120-ohm terminating resistors used at the end of each network
backbone but it isn't always the case. General Motors typically uses
34 PTEN OCTOBER 2023 www.VehicleServicePros.com
passthrough wiring for its two-wire CANBUS systems. This allows
the technician to troubleshoot network wiring integrity more effectively
than other systems that use stub harnesses to link modules
to the network. With GM's circuit, a technician can truly assess
total wiring integrity by leveraging terminating resistors where
some other manufacturers are different.
For example, Volkswagen Audi Group (VAG) will utilize a single
module to serve as the primary termination point and this is usually
done by using a resistance value of about 60 ohms. Additionally,
the other modules will also be equipped with a higher level of termination
between CANH and CANL, typically 2k-3k ohms. The
design of the network is usually driven by the manufacturer and
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October 2023

Table of Contents for the Digital Edition of October 2023
