Intermittent LAN Failure Clues
Posted to Technical Discussion Forum on 2/27/2017
8 Replies
We see a lot of intermittent failures in the dealership.
Most are either LAN or infotainment related. In either case,
although the problem may not be current in the shop, there
are usually clues to follow. How you follow these clues will
usually mean success or failure in your repair attempt.
A 2016 Colorado came in today with an intermittent failure.
It has 28,000 miles and the customer states that the ABS and
SIR lights come on once a week or so for a few seconds and
then go off. The truck ran normally and no lights were
illuminated when I drove it around the block.
Doing a full module scan with GDS2, I saw codes set only in
three modules, the EBCM, PSCM and SDM. This is clue #1 - all
3 modules are the only modules on the Chassis Expansion Bus.
Let's see who is active on that bus.
[2016 Chevrolet Colorado WT, ECM/Inputs/Outputs Scan
Data]
OK, all 3 modules are present and currently communicating.
Now, what codes are set?
[2016 Chevrolet Colorado WT, ECM/Inputs/Outputs Scan
Data]
There's obviously a communication issue on the Expansion
Bus. All 3 modules set U0077. This is clue #2 - When a
modules is unable to communicate over the bus for 3 or more
attempts, it will set U0077. At this point, it will suspend
all message transmissions and will use default values for
all parameters received over the bus. This will cease when
communication is restored. So, all 3 modules lost
communication at the same time. This points to a total LAN
failure, not an individual module. How's the LAN voltages
look?
[2016 Chevrolet Colorado WT, ECM/Inputs/Outputs Scan
Data]
Well, we know that the problem is intermittent. The Chassis
Expansion Bus looks normal right now. What can cause the
problem? An open is unlikely as all three modules are
equally affected. This leaves a short to power or ground. An
intermittent short to power is very unlikely and I would
expect a grounded LAN harness. Since we're dealing with only
3 modules in a LAN, it shouldn't be too hard to locate. With
the LAN voltages on the screen, I started at the EBCM, then
the PSCM and gently wiggled and pulled slightly on the
harness with no change. The harness then goes up and under
the UBEC before leading inside the truck to the SDM. When I
shook the upper part of the harness, I saw this...
[2016 Chevrolet Colorado WT, ECM/Inputs/Outputs Scan
Data]
There's our ground out. Both CAN+ and CAN- are being pulled
down. Following the harness past the UBEC, I saw it lying on
an A/C line with the conduit worn in a concave fashion
against the aluminum tubing. I pushed it against the tubing
and saw this...
[2016 Chevrolet Colorado WT, ECM/Inputs/Outputs Scan
Data]
Well, that's pretty obvious, isn't it? Repairing the rubbed
through wiring, installing new conduit and routing the
harness away from the tubing solved the problem. With an
intermittent, just follow the clues and proceed in an
orderly, systematic fashion and you will be more successful
with these type of failures.
James from Florida
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