MACS Service Reports - 2015 - JUL1

Total Vehicle Climate and Thermal ManagementTM

July 2015

By Dave Hobbs, MACS Technical Correspondent
F.R.E.D. Takes the FASTER Bus
The customer is hot and you find the compressor in his 2012 Honda Civic won't come on. Your
scan tool says No Communications, or you might get
a mysterious U-Code for a lack of proper communications. Maybe you shout: "Blasted Frustrating
Ridiculous Electronic Devices!" Or perhaps you use
some less wholesome vocabulary? You've had a few
classes on the subject of serial communications buses (a.k.a. multiplexing networks), but the vehicle
you are working on has a new bus which you've
never encountered. Don't worry! This MACS Service Report will help you get up to date so you can
diagnose the problem, regardless of how complicated it may initially appear!

repeating the famous line from the classic Steven
McQueen movie Cool Hand Luke, "What we've got
here is...failure to communicate."
Communications failures can range from scan
tools that can't read PIDs and modules that set
DTCs for no or poor communications, to A/C compressors that won't engage and even engine no start
conditions. When today's vehicles with multiple serial bus networks are working correctly, the entire
vehicle comes to life with benefits such as flawless
drivability, lower emissions and fantastic consumer
functions that keep us comfortable and connected
to the world around us.

An "Alphabet Soup" of Acronyms
No matter how similar the OEMs make their technologies or how long they've been around, they still
appear to be more complicated than they actually
are. They sure love their unique names: CCD, UART,
BEAN, SCP, GMLAN, LIN, SPI, CAN, KW2000,
MOST, etc. This alphabet soup of acronyms abounds
in the auto repair industry, including ALDL, the old
name for the diagnostic connector.
Younger techs may only know about the DLC, or
Diagnostic Link Connector, as in the days of post
OBD-II vehicles equipped with an SAE standardized J1962 connector with 16 pins under the dash.
ALDL (Assembly Line Data Link) was the old connector located in various places and in various configurations, some of which did not even include the
helpful but sometimes troublesome serial data bus
line. Helpful even in the old OBD-I days using the
older (and slow) UART (Universal Asynchronous
Receive & Transmit) bus, allowing HVAC techs to
see data PIDs (Parameter ID) such as A/C Request
and A/C High Side Pressure along with DTCs. The
troublesome adjective comes into play when the serial data bus won't do its job and has the technician

Before heading straight into where bus communications are now, let's take a moment to review
where the industry has taken us. In the early computer days (ECMs debuted on GM's 1980-½ models), there was only a slow UART single wire bus
between the ECM and the ALDL that would communicate with a scan tool at the snail's pace baud
rate of 160 ones and zeroes (digital communications) per second. The line (in pins L & M of the
old GM 1982 and up ALDL) didn't even have usable
communications from the ECM until a 10K resistor
(internal to the older scan tools) was put in between
pins A and B of those old 12 pin connectors. In the
early 1980's, most other OEMs didn't even have serial communications. Only flash code diagnostics
when you flipped the ignition on and off a certain
way (Chrysler) or shorted some wires in their ALDLs or connected an analog voltmeter to one of the
wires to get flash code diagnostics via a movement
of the voltmeter needle (Ford, Mitsubishi).
Just prior to OBD-II, many OEMs began to play
ball with this new gadget called a scan tool, and
communications took off in all directions. GM sped
up their UART line to a baud rate of 8190 bps (bits

MACS Service Reports is the official technical publication of the
Mobile Air Conditioning Society Worldwide, Inc., P.O. Box 88,
Lansdale, PA 19446. The material published in MACS Service
Reports expresses the views of the contributors and not necessarily that of MACS. Every attempt has been made to ensure

the accuracy of the content of MACS Service Reports. MACS,
however, will not be responsible for the accuracy of the information published nor will MACS Worldwide be liable in any way for
injury, labor, parts or other expenses resulting from the use of
information appearing in MACS Service Reports.

July 2015

Serial Bus Review

MACS Service Reports

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