MACS Service Reports - 2015 - JUL3

Dave Hobbs

gram it correctly and get that telltale
light for the option that didn't exist to turn off. Not only can't you
fix what isn't broke, you can't fix
what's not there!

Some Buses "Not Ready for
Prime Time"
You will find on many vehicles
that simple data buses don't run
from a string of modules to the
DLC. An example of this would
be the LIN bus (Local Interconnect Network) commonly seen in GM and Chrysler vehicles running
between a set of door switches to a BCM or door
module. Some simple buses may use a pair of wires
utilizing one as a clock pulse (measured on a scope
or DVOM with a frequency counter as a consistent
frequency) and the other wire with a unique square
wave that contains the data signal being sent.
In some CAN bus applications such as GM vehicles with stability controls, smart sensors (Yaw, Lateral Acceleration, Steering Angle) may be on the HS
(High Speed) CAN bus but they are not, however,
programmed to communicate with a scan tool. They
are, on the other hand, programmed to communicate with the ABS / Stability Control Module. That
module may then communicate what it learned
from those sensors to your scan tool.
You may also run into a serial bus from time to
time that is only readable with the factory scan tool.
One example is the Ford ACP bus. The Audio Corporate Protocol will only communicate with the
Ford IDS. Finally there is Mercedes and Chrysler.
Both are known to utilize one serial bus for scan
tool communications and another serial bus utilized
for module to module and / or programming communications. Older Chrysler / Dodge vehicles with
the PCI bus also used an SCI (Serial Communication
Interface) bus. The SCI bus is not bidirectional; one
SCI circuit transmits while the other receives.

Figure 3: Class A buses can communicate at up to 10 Kbps. Next up the ladder is Class B, with speeds of
between 10 and 125 Kbps, followed by HS CAN which starts at 500 Kbps and goes up. Newer buses like
Flexray and MOST can run at even higher speeds.

modules are literally connected together: one wire
circuit, two wire circuit, fiber optic, etc. Software
layer is a term used to describe how the software is
coded for tasks like master / slave relationships or
arbitration (modules deciding who talks and who
listens) and handling communications problems.

The Nodes Know
Another term you'll likely see on a scan tool or in
a DTC is Node, which is simply a module (or scan
tool) that is recognized on the serial bus. When programming a BCM incorrectly for example, there
may be a DTC stating "too many nodes on the bus."
Not only can erratic communications cause UCodes (communications codes), a missing or extra
module can cause U-Code as well. Learned nodes
are modules that other modules have been programmed to look for. Depending on the particular
bus and OEM, there will usually be some kind of a
system of checks and balances for modules to keep
track of who is there and who is missing. This is accomplished on a J1850 GM Class 2 or J1850 Chrysler PCI bus with periodic SOH (State of Health) bus
messages from each module, which act as a kind of
roll call. It's also done via the virtual networks arrangement used in CAN data buses for modules to
have their own private communications within their
sub groups.
Knowing this fact about the familiarity of modules on a serial data bus regarding who belongs and
who is missing, always make sure you know all the
options on a vehicle when programming or flashing to avoid problems with missing or too many
nodes. I once assumed that a late model GM vehicle on which I was programming a BCM had ABS.
It was loaded with every option imaginable, so it
had to have ABS too, right? Turns out it didn't. The
first thing that happened after the BCM flash was
an ABS light on the dash. I went to retrieve DTCs
from the ABS module in order to diagnose the ABS
light and discovered my mistake. My scan tool said
"No Communications" when I tried to pull DTCs
from the ABS, because the car simply didn't have
an ABS module! Luckily for me I was able to repro-

July 2015

Bus Topology: Knowing How It's Wired
The layout of a bus system, sometimes referred
to as topology, is important to know for diagnostics.
Wiring configurations range from star (Figure 4)
with a shorting bar to tie each module to a common
point (Figure 5), to a loop configuration (Figure 6),
which requires each single wire bus module to have
2 circuits per module or each dual wire bus module
to have 4 circuits per module.

Getting a Look:
The DLC Break Out Box
Probing into the female terminals from the front
of the DLC is a connection problem waiting to happen,

MACS Service Reports

Table of Contents for the Digital Edition of MACS Service Reports - 2015