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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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car Vehicle Data

2011 Chevrolet Equinox LS 2.4L

Engine2.4 L
Trans6-speed Automatic (Electronic)

car Vehicle Data

2011 Chevrolet Equinox LS 2.4L

Engine2.4 L
Trans6-speed Automatic (Electronic)