Elementary Communication Diagnosis
Posted to Technical Discussion Forum on 7/5/2017
12 Replies
I've posted many LAN diagnostics on iATN over the years.
Since I specialize in LAN and electrical diagnostics, I do a
lot of them. It's not uncommon for me to work on 2 or 3 a
day. As few techs want to work on them, most are shuttled to
me. I don't really mind as I've done so many of them that it
has become very easy to me.
Because of this, most of the ones that I post are uncommonly
difficult or unusual. I've been receiving e-mails from some
members that are still cutting their diagnostic teeth on LAN
diagnosis and who want to see some of the more elementary
diagnostics. You need to walk before you can run, right? So,
this post is a common, fairly easy LAN diagnosis for all the
lurking newbies out there.
This 2011 Equinox came in today with a complaint of
intermittently shutting down and not starting. The customer
says that he drives down the road and all the dash warning
lights come on and the engine shuts down. Then, it will not
crank until he sits for and hour or so, then the cycle
begins again.
The vehicle started normally and drove in the shop normally
and I parked it and shut it off while I finished another
car. An hour later, when I went to start it, all the dash
lamps illuminated normally but the vehicle would not
crank....no click, nothing. From the owner's description, I
anticipated a module or LAN problem. Now that it won't
start, the diagnosis becomes much easier than chasing an
intermittent failure.
I did'nt even check codes first. I just looked at the HSLAN
voltages.
[2011 Chevrolet Equinox LT, ECM/Inputs/Outputs Scan
Data]
It's pretty obvious that we have a problem. look at how high
terminal 6 voltage is. Just to make sure, I looked at the
LSLAN next.
[2011 Chevrolet Equinox LT, ECM/Inputs/Outputs Scan
Data]
Completly normal. How about the Chassis Expansion bus?
[2011 Chevrolet Equinox LT, ECM/Inputs/Outputs Scan
Data]
Normal also. Good, we can concentrate on the high Speed LAN.
Let's see who's present on the LAN.
[2011 Chevrolet Equinox LT, ECM/Inputs/Outputs Scan
Data]
OK We're missing the ECM and the TCM. It also says CAN BUS
open. The ECM is the last module on one side of the LAN and
holds the terminating resistor. The module before it is the
TCM. Before the TCM is the PSCM. Since we can communicate
with the PSCM but not the TCM and ECM, our problem must be
before the TCM, right? It could be caused by a module or a
wiring problem. My first place to check is the connectors
and pin fits at the PSCM and TCM.
The TCM is easier to get to so I unplugged it first and
checked the LAN connectors between the PSCM and the TCM for
pin fits and corrosion. All seemed OK. Next, to access the
PSCM, we need to raise the vehicle and unplug the connector
from the top of the steering rack. This connector is
difficult to release as the tab is above the connector and
hard to depress. I saw that part of the connector was broken
off. I guess somebody replaced the rack before and just
gomered the connector off. The wire standoff was unhooked
also. After removing the connector I pulled gently on the
wiring and the LAN wiring from terminal 6 stretched out like
a rubber band. It was obviously mostly broken inside the
insulation., A new connector spliced in restored our
communication for a fast, easy fix. Of course, if the
problem wasn't easily evident, I would have needed to do
some voltage and resistance testing but not needed in this
case.
James from Florida
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