MACS Service Reports - 2015 - JUL2

Dave Hobbs

per second), which wasn't even as fast as a computer 's dial up modem. Chrysler came out with a
two wire bus called CCD (Chrysler Collision Detection) prior to OBD-II. Once that came onto the scene
in 1996, US domestic carmakers adapted an SAE
standardized J1850 serial bus, while Asian car makers focused on improving a bus called "keyword"
and the Europeans adapted to a bus developed by
Bosch called CAN (Controller Area Network). GM's
and Chrysler 's version of the J1850 standard was
a single wire in pin # 2 of the new standardized
16 pin OBD-II diagnostic connector (GM Class 2
and Chrysler PCI) while Ford used pins 2 and 10
for their SCP (Standard Corporate Protocol) bus. A
quick look at the DLC tells you a lot about the particular serial buses which the vehicle you are working on may have (Figure 1).

Figure 2: Some of the pins in the J1962 OBD-II DLC are set in stone by
federal regulations, while others are OEM discretionary. For example, you
might find a HS CAN expansion bus in pins 12 and 13 on a vehicle that has
excessive nodes on the bus. The expansion bus, along with another gateway
module (usually the ABS module), add capacity to the HS CAN bus without
adversely effecting its operation.

the bus that keeps modules communicating, much
like one might call a language that keeps people
communicating.
A high speed CAN bus is a serial bus protocol
that has been required for vehicles sold in the US
since 2008 for any system that has to do with powertrain (for emission reasons) or chassis systems
(for safety reasons). Using a pair of wires that are
twisted and / or shielded for EMI / RFI reasons,
this serial bus is more expensive to produce. Therefore it's often only used where it is required, such
as for powertrain and chassis. Other modules on
the vehicle that are not required by federal law to
be on the CAN bus are often on lower or medium
speed buses, which means using a different protocol. Just as in the case where two groups of people
speaking different languages need someone who
speaks BOTH languages to translate between the
two groups, your customer 's vehicle may have a
translator to keep the different protocols communicating with each other. In the world of serial buses,
this module is typically called a Gateway module.
Some OEMs like Toyota actually call this multilingual module a gateway, while other OEMs leave
you guessing.

Geek Terms: Learning a Few Will Actually Help!

Dave Hobbs

Geeky sounding words aren't just for the books
that engineers read. They are often wielded around
in the service manual or listed on scan tool screens,

Figure 1: This standardized J1962 OBD-II DLC has the following pins:
1 (GM LAN Low Speed), 4 & 5 (Signal and Power Ground), 6 & 14
(standard placement for HS CAN circuits), 16 (B+ or 12 volts). Many
techs are familiar with DLC adapters, which connect to the power and
ground pins of the DLC on one side and have a 12 volt accessory plug
on the other. They're used to connect a boost box as a method to maintain
keep alive memory on the 12 volt side of the vehicle while changing out
the battery to avoid time consuming resets and relearns.

Higher Letters = Higher Speeds
Buses are classified into letters that increment up
as bus communication speeds increase. See Figure
3. You may see the term CAN B or CAN C in a service manual or on a scan tool. This alerts you to
the speed. If you see the term CAN C Diagnostic,
this bus will likely run at 500 Kbps and only connect between the gateway module and the DLC. It's
just for the scan tool and reflash tool. Mercedes and
Chrysler are well known for having the vehicle's
various modules connect to a gateway module, and
then on from the gateway module to the DLC with a
completely different data bus circuit called CAN C
Diagnostics. Therefore, if you wish to diagnose another bus on the vehicle, you'll need to find a module on that other bus to back probe and not focus
on the DLC. The term physical layer describes how

so we'll have to make sure we know the key terms.
First off, ISO (International Standards Organization) and SAE (Society of Automotive Engineers)
assign names to data buses. Typically you see the
SAE label applied to domestic vehicles and the ISO
title applied to the imports. CAN is the exception
to that rule due to the fact that you see it on all vehicles now. See Figure 2. You may see the title for an
ISO or SAE standard as your scan tool runs down
the list of modules and data buses during a full vehicle system scan. Bus protocol is a term used to describe the general unique communications style of

July 2015

MACS Service Reports

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