Using Code Set Order To Diagnose
Posted to Technical Discussion Forum on 7/4/2017
13 Replies
I just repaired a 2010 Equinox with a complaint of ABS light
on and Service Stabilitrack message displayed. The vehicle
started and ran normally driving into the shop and the light
and the message were displayed as in the complaint. My first
test was to check for codes set. I did a vehicle code check
of all modules and saw only these codes set in the EBCM.
[2010 Chevrolet Equinox LT, ECM/Inputs/Outputs Scan
Data]
The problem seems to be limited to the HS Chassis Expansion
Bus, although the EBCM is also on the main HS bus. That
calls for a fast look at the main HS bus first.
[2010 Chevrolet Equinox LT, ECM/Inputs/Outputs Scan
Data]
The voltages are normal. Let's see if the EBCM is present on
this bus...
[2010 Chevrolet Equinox LT, ECM/Inputs/Outputs Scan
Data]
OK, it's present on the HS bus, how about the expansion bus?
[2010 Chevrolet Equinox LT, ECM/Inputs/Outputs Scan
Data]
Well, no modules are detected and the bus is shown as CAN
Bus open. Let's see if it is...
[2010 Chevrolet Equinox LT, ECM/Inputs/Outputs Scan
Data]
It sure looks like it is either open or shorted to ground.
With key off, I checked the expansion bus resistance and saw
61.2 ohms and normal resistance to ground on CAN A and B.
So, it's neither open or shorted to ground with key off but
low voltage with key on. Why? There are other possibilities.
With key on, engine off, I cleared codes and noted that the
codes did not reset immediately. Monitoring the codes set,
this was the first...
[2010 Chevrolet Equinox LT, ECM/Inputs/Outputs Scan
Data]
After 20 seconds more, this was set...
[2010 Chevrolet Equinox LT, ECM/Inputs/Outputs Scan
Data]
And 30 seconds later...
[2010 Chevrolet Equinox LT, ECM/Inputs/Outputs Scan
Data]
Time to analyze data collected. The first code set was by
the PSCM for invalid data from the SAS. Since the PSCM is
not on the expansion bus, this data must be transferred
between buses by the EBCM. Next codes set were from the EBCM
showing Can Bus B off and a loss of communication with the
MAASM. And the third code set was from the EBCM losing
communication with the SAS. What does all this mean?
The first problem detected was invalid data from the SAS. If
the EBCM sees invalid data from either the SAS or the MAASM,
it can shut down the CAN Bus and substitute values for these
modules. Once the Bus is open, the other two modules cannot
communicate with the EBCM and codes were set for loss of
communication. What's the best way to prove this theory as
we cannot read any data on this bus to help us. I unplugged
the SAS and jumpered the 4 com lines and rechecked the
expansion bus voltages, seeing this...
[2010 Chevrolet Equinox LT, ECM/Inputs/Outputs Scan
Data]
OK, we now have an active expansion bus again. I installed a
new SAS and rechecked the bus for modules present.
[2010 Chevrolet Equinox LT, ECM/Inputs/Outputs Scan
Data]
Everybody is present and talking together now and the lights
and messages are gone. A little thought analyzing the
failure can go a long way.
James from Florida
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