MACS Monthly Newsletter - 2017 - MAR3

Dave Hobbs

the Chevrolet Impala. Every data bus has a
hardware layer (how the bus is wired) and a
software layer (how the software works). Unlike European cars which utilize a fiber optic
hardware layer for their M.O.S.T. bus, General Motors has given their M.O.S.T. bus a copper wire physical layer.
Figure 4: This scan tool PID capture shows the ambient temp sensor as
a key PID. On some vehicles, this sensor may not illuminate a MIL
but might illuminate another warning indicator - the stability controls
/ ABS light due to there being an engine management related fault that
could potentially effect torque management required for stability controls.
Whether the sensor is measuring ambient air or coolant temperature, the
circuitry is almost always the same.

M.O.S.T. Overview
Since a late model GM vehicle's outside / ambient air temp reading on the radio may now involve
various module and data busses including the new
M.O.S.T. bus it would be a good idea to mention a
bit about what makes it tick. Ken Zanders covered
the topic of GM M.O.S.T. in a Motor Age article last
year and AC Delco released a Tech Connect Newsletter three years ago summarizing GM serial busses
and common problems. For more information check
out:
http://www.searchautoparts.com/automechanika-chicago/commitment-training/getting-most-out-most and
https://www.acdelcotechconnect.com/pdf/Newsletter_
TechConnect_MarApr_2013.pdf

add a MAF (Mass Air Flow) sensor into the air inlet
with a built in IAT (Intake Air Temp) while others
skip the MAF and simply pop in a standalone IAT.
In either case that PID (Parameter ID) on a scan
tool will be found under the Powertrain data menu
(Fig. 4) and will likely be termed "IAT 1". Another
IAT can be found after the turbocharger 's induction
cooler and EGR valve. Another air temp sensor is
the one HVAC techs have been looking at for years
- the OAT / AAT behind the grill. Three sensors for
three unique air temperatures located in three different locations.

M.O.S.T. is primarily for multimedia data transmissions however (USB, video and audio) requires
very high speeds of communications to work correctly. The C.A.N. bus is too slow with its 500 Kb/s
(Kilobits per second) baud rate. M.O.S.T. can send
data at baud rates up to 500 Mb/s (Megabits per
second). GM's physical layer for this protocol is
2-wire twisted pair. This bus is much like the original GM UART buses from the 80's in respect to the
use of 'master ' and 'slave' modules. Modules on the
bus (usually referred to as 'nodes') on the M.O.S.T.
bus are referred to as 'frames.' The bus master (the
radio) is the module that is required to initiate communications on the M.O.S.T. bus. The radio also
sends wake up messages that are used in on board
diagnostics in order to diagnose bus failures. (Fig.
5)

O.A.T.S. and M.O.S.T.?
How the ambient temperature reading is sent
to the various modules on the vehicle varies from
OEM to OEM and even model to model within a
single manufacturer. For an example on some 20152017 GM full size trucks and SUVs the Ambient
Air Temperature Sensor is hard wired to the ECM
where it becomes a high Speed GMLAN (2-wire HS
CAN) message and sent on to the BCM. The BCM
then sends the AAT info on to the HVAC head and
radio tuner via a low speed GMLAN bus (single
wire LS CAN) and finally to the Integrated Center
Stack Display via a special video harness called a
LVDS (Low Voltage Differential Signaling) cable.
However, on select up-level GM radio options there
is a component called an HMI module (Human Machine Interface Module) in the loop between the radio tuner module and the Center Stack Display. The
HMI is basically an infotainment integrated OnStar
module. All the bells and whistles of internet from
an optional onboard WiFi router, blue tooth devices, USB devices, etc. are handled by the HMI module. The radio tuner module sends audio and video
information on to the HMI module via the M.O.S.T.
(Media Oriented Systems Transport) bus. The HMI
then passes that information on to the Center Stack
Display via a LVDS cable. The same equipment and
data routing strategy is used in numerous other
GM vehicles including various Cadillac models and

March 2017

Hard and Soft failures, Diagnostics and Known
Problems
Any thermistor can fail. The wiring circuits to the
thermistor can have an open or high resistance connection, be completely open, or become shorted to
another circuit (with 12-volts or ground). The sensor itself can electrically go open circuit, internally
short out or simply become inaccurate / skewed.
Regardless of thermistor function or location, the
primary module responsible for dealing with the
OAT / AAT sensor watches for an analog voltage on
the sensor circuit and makes a decision accordingly.

MACS Service Reports

http://www.searchautoparts.com/automechanika-chicago/commitment-training/getting-most-out-most http://www.searchautoparts.com/automechanika-chicago/commitment-training/getting-most-out-most https://www.acdelcotechconnect.com/pdf/Newsletter_TechConnect_MarApr_2013.pdf

Table of Contents for the Digital Edition of MACS Monthly Newsletter - 2017