MACS Service Reports - 2015 - JUL8

Gauge Control Module (Tach). That also has a HS two
wire F-CAN bus leading out of it into the PCM. So without any further research, we know the Gauge Control
Module is working as a gateway to get that HVAC request from your customer's fingertips at the HVAC head
to the PCM, which is talking on a different bus. If the
message isn't getting through, make sure you have no
U-Codes and / or open circuits anywhere in the trail just
described. If more than one bus is having issues, suspect
the Gauge Control Module. If just the CAN B bus is having fits, besides the head itself there are other modules
on that bus that could be causing a fault, including that
interesting MICU (Multiplexing Interface Control Unit).

cle even has that module on it first. Then, look to see if there
are other modules on the same bus also not answering the scan
tool to determine if further testing is warranted. Factory tools
like the GM Tech 2 will run a module status test to determine
which module is awake or asleep on GM vehicles with the older
Class 2 bus. The Chrysler DRB-III could do a very quick CCD
bus and J1850 PCI (Programmable Communication Interface)
bus check and the Ford IDS can do a nice job of determining
module status on CAN bus vehicles. Chrysler's WiTech is probably the most impressive however, giving you the scoop on
which module is on what bus and whether it is active or not, all
in a well laid out bus topology diagram. See Figure 15.

Pulling It All Together

Looking Ahead: What's Next in Serial Buses?

Before we started down this deep path of 1's and 0's
spitting out at the end of a DLC terminal, we had a compressor that wouldn't engage. All the usual suspects

General Motors

Fiat Chrysler Automobiles

In addition to CAN expansion buses, we've been seeing the fiber optic MOST (Media Oriented Systems Transport) busses come out with the European cars for several
years now, but mostly (no pun intended) on infotainment

Figure 15: This Chrysler vehicle's entire arrangement of CAN B and
CAN C / Diagnostic CAN C networks is shown under the "Bus Topology" option, available in the DRB-III factory scan tool replacements,
dedicated to CAN bus equipped Chrysler products. The key on the left
shows what each color means (module active, module non responsive, etc.)
along with bus descriptions. This very useful feature is on the Star Scanner, Star Mobile (both now obsolete) and the Wi-Tech factory tools. If you
own a J2534 universal pass through device (flasher) you can also (with a
short term subscription) activate this same feature on your laptop without
using the factory tool.

Figure 16: GM's MOST bus uses a wired physical layer as opposed to
the European method of fiber optics. Diagnosing these systems is entirely
different than normal methods (the bad news) but can be easy to do with
a scan tool from the driver's seat.

must of course be tracked down, such as blown fuses,
bad relays, a bad compressor clutch, inputs into the PCM
preventing the clutch from operating and the like. Next
on the obvious list are the powers and grounds to all the
modules involved. The best communications systems in
the world can't compensate for F.R.E.D. being dead due
to a lack of power or ground.
If by this point you are justified in going down the data
bus diagnostic path, you'll see there is a CAN B (medium
speed) two wire bus leading from the HVAC head to the

systems. Those systems commonly found on BMWs and
the like are diagnosed with a tool nicknamed lollipop due
to its shape. It bypasses each MOST bus module one
at a time to provide a process of elimination on which
one is causing a problem. GM started using MOST on
the Chevy Silverado and Impala models in 2014 (Figure
16). Diagnostics on this new system, which uses a wired
physical layer instead of fiber optics, has some strange
but efficient methods as well. ■

MACS Service Reports is published monthly by the Mobile Air Conditioning Society Worldwide. It is distributed to members of MACS Worldwide
and is intended for the educational use of members of the automotive air
conditioning service and repair industry. Suggestions for articles will be
considered for publication, however, MACS Worldwide reserves the right
to choose and edit all submissions.

Editors:
Elvis Hoffpauir, Steve Schaeber
Production Designer:
Laina Casey
Manager of Service Training:
Steve Schaeber
Mobile Air Conditioning Society Worldwide
P.O. Box 88, Lansdale, PA 19446
Phone: (215) 631-7020 * Fax: (215) 631-7017
Email: membership@macsw.org * Website: www.macsw.org

Unless otherwise noted, all photos/art by author.

July 2015

MACS Service Reports

http://www.macsw.org

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