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Doing the CAN CAN again
Posted to Technical Discussion Forum on 6/25/2011 65 Replies

Hi all. I have followed with interest some of the posts in the last year or so, where CAN based communication case studies have been presented for our interest and discussion. In particular, more recently James Avery posted a High Speed GM LAN issue and as I observed the ensuing thread unfold, it became apparent that there was plenty of interest in this topic.

While CAN systems have been around for a long time, we have seen CAN "C" brought into the forefront since the 2008 MY. What I notice is that the case studies are great, but participation is somewhat limited to a few "die hards" who either understand the system function, failure and diagnostics, or conversely, quite openly admit that they lack knowledge of the CAN systems and yearn for more.

So, the purpose of my post is to contribute some numbers and a few thoughts to the cause. While I no longer actively wrench as a technician, I must also learn each new technology and system as it is introduced, on a wider range of vehicle systems than when I was a technician full-time. Apprenticeship, vehicle manufacturer and dealership needs of the shop and technicians, dictate that we train students in specific areas and network communications is a clearly defined apprenticeship topic in third level.

While my work may not quite assimilate your "real world" experiences, it is my "real world", where not only installed bugs, mishaps, damage and other problems happen to challenge us along the way. During my week, there is some time set aside without students present, for me to attend to "Functions 3 to 8" in our collective agreement. This covers a wide range of program maintenance needs, processing marks, prepping labs and taking care of ensuring that everything is up and running.

This particular Friday began with me scheduling marks processing and prep work, but first I needed to move a few vehicles around for the next series of activities. One car, a 2009 Chevrolet Mailbu 2.4L, was sitting in the shop. That needed to go outside, but it didn't crank. The dome lamps and IP were lit with doors open and key on, exterior lamps turned on, but nothing happend when the key was turned to crank. Without getting sidetracked, there were a few other systems that didn't function, such as the electric trunk release, but all that I wanted at this point was to shift the beast outside. I'm on my own, all alone with nobody with me, so pushing is out.

Darn, I'd better fix it myself, methinks. Besides, this is a bit more interesting than paperwork anyway. Work boots, and PPE at the ready, what's the next step going to be? I know where iATN's resident litigation consultant would have us head, word by word, but GM created something known as "Diagnostic Strategies" a few years ago, that provides GM technicians with more diagnostic freedom to use their knowledge, expertise and experience to either use some very basic methods, or if a novice, to do the step by step dance.

I manage my novices closely enough to develop and install some critical thinking skills and to understand that they need to be able to manage their diagnostic skills and logic within reasonable relams of the corporate process to be sure to be paid. Once they have developed a wider range of skills, their wings will be set free to soar, but for now they're "clipped" to keep them from wandering too far from the beaten path. Since I routinely lead students into developing a logical diagnostic approach that may assimilate, but not necessarily 100% duplicate the manufacturer steps, based on needs and critical thinking abilities, much of the stuff is ingrained in my "noggin." Besides, I'm not on flat rate any longer and I am my only audience, with the freedom to stray, wander and experiment today, as I see fit.

So, let's gather a little info. What do I know of the vehicle? It is a 2009 Malibu, donated to the program by GM Canada, with a couple of scrapes/mild dents in the roof from transit damage. It has performed flawlessly to date, used for various topics this term from body electrical, network communications, engine management and emissions. I don't know and really don't care at this point, how it became non-functional. I'm just a tech today with a broken car in my work bay that needs to be fixed ASAP.

Now, you already know that as I write this, that the vehicle may well be fixed and it is, but when I spent a minute or so getting my diagnostic direction figured, it became apparent that this may be useful in a post. To that end, I elected to snap a few photos of scan data, DMM display values etc along the way. Since taking photos of a DMM display with overhead lights, trouble lights and other effect on camera angle, can produce inconsistent quality photos, I snapped some photos of the Fluke 87 V photo, but for clarity, created some text boxes for better viewing on this wet Saturday afternoon.

So, what's the approach? Preliminaries, looked okayish and while I might have tested every fuse in some random fashion like many do, which simply wastes time, I'd rather home in on what's working or not working in the major sense, to get a better grasp. Besides, if get a schematic, it will show the appropriate fuses and save a bunch of time not checking those that don't need it.

While a few other things may or may not work, my primary concern was a no crank, no click, no nothing response to rotating the ignition key. Keep in mind, preliminaries include visual, bulletins and more, depending on need, so I'll keep that in mind. In my ex-role as a GM technician, I'd use my "gut" feeling diagnostic approach to gain a sense of direction, willing to trade 5 minutes or so using what I know, versus following the "path of righteousness". No sense of where to go, get on track with published info, not much time lost.

Anyway, I know where the Diagnostic Starting Point et al, will lead, so don't need to read it for the ten thousandth time just yet. So, with Tech 2 and CANdi in hand, vehicle info entered, the results of selecting "Vehicle Control Systems" and waiting what seems like an eternity, renders some modules with no communications and some with DTCs, on Low Speed and High Speed GM LANs (HS & LS).

Ah, the path to take. Info is readily captured and kept in mind for future needs. All this happened at one instant in time, so in my mind, one problem is likely presenting multiple DTCs and symptoms. I note that the BCM (on LS and HS LAN) is present and showing 3 DTCs U2105 00, U 2106 00 and U2108 00, all were communication lost with other module DTCs. There's also U 2109 00 (PSCM) and B2455 04 (VCIM) listed.

So, we have problems communicating with various modules and since the BCM and VCIM reside on both HS and LS LANs, some stuff that may be useful later. Go ahead if you wish, check bulletins and find the one that lists four of the five DTCs and jump to conclusions that will not net a successful diagnosis. I just log this stuff into long term memory in case actual diagnostics don't yield a fix.

Now, I am already well aware from my background and training, of the fact that some" big player" modules that some of the DTCs represent only communicate on the HS LAN, while some of the lower priority modules reside on the LS LAN. Now, "Diagnostic Tragedies" as we so fondly call it, allows us some freedom, but for those of you unfamiliar with GM LAN schematics, you can use the menu path or a keyword search to seek out some visual aids.

I didn't need to take this path, because I'm already homing in on the HS LAN to do some basic tests, but then again, I'm familiar enough to go ahead at this point. Still, looking at the schematics can render some useful information such as where to access the system for diagnostic tests beyond the DLC.

Using the menu file path, head over to Power and Signal Distribution, Data Communications, Schematics and Routing Diagrams, Data Schematics, to arrive at Power, Ground and Serial Data, where HS LAN is available with or without HP7 (hybrid) and LS LAN is also listed. This one is not equipped with HP7. A quick review of both LAN schematics and noting connector locations may be useful while here. Clicking on the "LOC" button on a schematic (not available once in the magnified view), will net the Master Electrical Component page, where all components, connectors and harness routing diagrams should be located.

If you want to have both windows, when clicking on the LOC button, use the right click, open in a new window " method. Real world, I rarely would bother. If I have an interesting window open, I print it and take it to the vehicle.

If I already know the connector ID, I don't waste time with file paths. Keywording "Master" from any search box and dragging the slider rapidly to the very bottom of the page will net "Master Electrical Component List" at or just above the very bottom. The path above is for you if you wish, but based on my knowledge, I'm going to do some quick tests at the DLC.

Since the BCM is online, but nothing else, I expect that voltages on the HS LAN should be as expected. I did make one notation when I used the schematic, to aid my testing and that was the location of the two 120 ohm termination resistors in the HS LAN circuit. One is in the BCM which is one stop downstream from the DLC and the second is internal to the ECM, which is at the far end of the system, not communicating.

This is the value obtained [2009 Chevrolet Malibu LS, ECM/Inputs/Outputs Drawing]

That makes me happy, since if the resistance is as I expected for a system with a data line problem that I predicted from scan data and system funcitonality, I'm probably looking for a simple open in the circuit. Don't forget though, any time you make resistance measurements, the system should be powered down for the values to make sense. I caught a couple of students last week, diagnosing an '08 Aveo CKP failure, measuring a ground circuit with the ignition on, DRL (headlamps) shining brightly. They were wondering while their measure valued was a really high ohms value, but did net the same result on another ground. I just happened to beat them to the ignition switch! VBG.

To get a mental picture, this HS LAN begins with the DLC and strings modules along one after another and culminates at the ECM. Two wires (Bus + and Bus -) enter and leave each module. Looking at the schematics with the choices of RPO applications can give you a headache and a couple of modules are hung off the side of the chain and also have LS LAN communications. If you need, it can be simpler to draw a block diagram.

In this case I suspected that I would find around 120ohms resistance when I measure the HS LAN across Bus Hi and Bus Lo terminals 6 and 14 and net 121 ohms. If functioning as designed, the value should be close to 60 ohms Don't forget though, modules/networks need three things to function, power, ground and a healthy data line. I have a scan tool that functions, BCM is active and my focus is on physical HS LAN Data Bus wiring.

Nothing beyond the BCM is communicating on the HS LAN. The BCM is "conveniently" located in the centre console, where each side panel is released by gently pulling to release the fastener clips and Velcro. Access is pretty simple. I don't look too deeply, but the BCM connections appear to not have been disturbed by student hands, wiring all seems to be in place securely.

Next stop along the trail is connector X206, sorry forgot to snap a pic of these locations. It is very conveniently located behind the removable access panel at the left end of the dash, with the driver's door opened. Yippee, pick me, pick me, I found it, stumbled right onto a problem!!! There is a lever lock style connector that also has locking tabs, but it is not fully seated. I'm not so sure that the terminals can even make connection properly unless the lever mechanism is snapped home, but I'm a "happy camper" (or will be if this is the only thing I find wrong). I'll investigate later whether this is a student installed bug or whatever, but for now, I just want the vehicle out of the shop, so that I can complete some paperwork and prep for next week.

With the ignition off, I pull the connector halves apart and inspect for anything that might prevent them from proper intercourse and find nothing. All terminals of this hybrid (two sizes of terminals in one connector body) are fine and undamaged. With the connector reconnected, the engine now cranks and runs fine.

So, since I mentioned that I'd record some HS LAN measurements for reference, I will disclose the values measured from the DLC, prior to me chasing down the open circuit. When testing the HS LAN, there are some very basic tests to be completed and not all are necessary to complete the diagnosis (See Diagnostic Strategies) if the fault is found. The status of the HS LAN can be checked using a DMM or a DSO if you so wish, but it needs to be fast enough to capture the data accurately enough for comparison to specs. Throw away that cheapo $25 meter and ante up for a professional DMM. GM recommends the Fluke 87 V and since I have one, I used it to record the following measurements. I did also use a Fluke 87 III, which while a capable meter and did capture reasonably accurate results, was just a skip behind the 87 and I did have to retake a couple of measurements. I noticed that the high and low values on peak min max were not quite as high or low when using the 87 III, but they were still valid enough to make sense.

What's to be measured? Well, the simplest is to check the resistance across HS LAN terminals 6 and 14 at the DLC. Here's the measurement after the system has been repaired [2009 Chevrolet Malibu LS, ECM/Inputs/Outputs Photo]

On a healthy HS LAN, this should be approximately 60 ohms, but on a system where the module with the second terminating resistor is missing, 120 ohms will be measured if the data Bus is open. Remember, we still need power, ground, a healthy pair of data lines and modules for everything to function normally.

Here is the resistance check value again, of this Malibu with no communication on the HS LAN data lines beyond the BCM. [2009 Chevrolet Malibu LS, ECM/Inputs/Outputs Drawing]

Additionally, there is a photo of the external termination resistor from a current model GMT900 Chevy/GMC pickup truck, showing the same value as achieved on the Malibu. [2008 Chevrolet Silverado 1500 LT, ECM/Inputs/Outputs Photo] This resistor resides on the left frame rail close to the spare tire.

The resistors provide a load on the circuit and also are very useful diagnostic aids as James, Bob and others have already eluded. I would suggest that if you wish to learn a little about GM HS LAN diagnosis, that these trucks are plentiful and easy to access. I'll let you be creative and learn for yourself, what happens when you remove this resistor and leave the circuit open or jumper the lines together, or ground either one or both. This is your homework project! You will learn when you do it for yourself.

Measure voltage using a high quality meter capable of making the following measurements: Measure terminals 6 (Bus Hi) to (5 Ground) with a DMM on volts. [2009 Chevrolet Malibu LS, ECM/Inputs/Outputs Photo] Note the measurement is in basic DC volts and also autoranging. it was simply turned on with voltage selected set up to make basic voltage measurements, but regardless of whether a fixed range is selected the meter just isn't fast enough for testing serial data communication circuits accurately this way.

You can try the Fluke 87 variants on min max, where the meter is not auto ranging, but will need to depress the "speaker" button to select Peak Min Max for the reading to be accurate enough. Since the 87 V is four times faster at 250 microseconds in peak min max, as opposed to the 87 III mS capability, I'm going to use the 87 V for these measurements.

What do we know about the GM LAN? It utilizes a twisted pair of data communication lines aka Data Busses. They are stacked at the DLC at terminals 6 and 14, which are directly above and below one another, since its hard to have twisted pairs anyplace but next to one another. Here's the DLC terminal pin assignment [2009 Chevrolet Malibu LS, ECM/Inputs/Outputs Photo]

HS is at terminal #6 and the wire is tan with a brown tracer. LS is at terminal 14 and the wire is tan in colour. When we make measurements from terminal 6 to 5 (ground), we expect to find a nominal 2.5 volts when measured on a DCV scale. In reality, the value for terminal 6 will be a few points of a volt higher than for terminal 14. Remember, this is when performing a basic voltage measurement.

When we select the peak min max function, the values will be quite different. E.g. we should see approximately 3.5V MAX measured at 6 to 5 and 2.4v MIN. This verifies our understanding that HS Bus HI moves up approximately 1 volt during activity.

Similarly, when making the same peak min max measurements from terminal 14 - 5, we will see approximately 2.5v MAX and 1.5v MIN values displayed if all is well, with the same 1 volt differential from rest. If voltages do not fall into these ranges, problems exist that require diagnosis, such as any short to voltage, open or ground condition will cause.

When measuring voltage across terminals 6-14, we will measure somewhere in the millivolt range between 50 and 500 millivolts. This one measured [2009 Chevrolet Malibu LS, ECM/Inputs/Outputs Drawing]

However, when we repeat the same measurements using the peak min max feature of the Fluke 87 V, the results will be in the neighbourhood of 2.4V MAX and 0.25V MIN. Here are the actual measurements [2009 Chevrolet Malibu LS, ECM/Inputs/Outputs Drawing] and [55184] This reading is simply the differential voltage on BUS and the only way to see it is using a meter or scope that is capable of recording accurately.

I happened to have a couple of logic probes handy along with a few other "quick and dirty" tools for basic system quick checks, but prefer the peak min max on the DMM because the numbers mean more to me. "Scopeaholics" can capture very accurate waveforms and analyze them to "death." Personally, I use the "KISS" approach when diagnosing vehicles and use the most basic tools first. If I am not satisfied or looking for something beyond, I have scopes close by for back up.

Bear in mind that some vehicles utilize more than one HS LAN, simply to limitations of the CAN BUS. In those cases, we might find a chassis expansion bus or hybrid expansion bus, piggy-backed to the system, much like you can add an electrical subpanel to your home electrical system.

So, when James and other post voltages, we need to understand or at least ask how the measurement was made, if we are to be able to compare their measurements to good known values. If the method of recording the voltages isn't good enough, the numbers won't provide proper diagnostic value.

Here are actual photos and a series of text boxes showing Volts and Ohms Basic tests follow:

KOEO Terminal 6-5 = [2009 Chevrolet Malibu LS, ECM/Inputs/Outputs Photo]

KOEO Terminal 6-5 peak min max -- MIN = [2009 Chevrolet Malibu LS, ECM/Inputs/Outputs Drawing] KOEO Terminal 6-5 peak min max - MAX = [2009 Chevrolet Malibu LS, ECM/Inputs/Outputs Drawing]

KOEO Terminal 14-5 = [2009 Chevrolet Malibu LS, ECM/Inputs/Outputs Drawing] KOEO Terminal 14-5 peak min max -- MAX = [2009 Chevrolet Malibu LS, ECM/Inputs/Outputs Drawing] KOEO Terminal 14-5 peak min max -- MIN = [2009 Chevrolet Malibu LS, ECM/Inputs/Outputs Drawing]

KOEO Terminal 6-14 = 0.314v DC [2009 Chevrolet Malibu LS, ECM/Inputs/Outputs Drawing] KOEO Terminal 6-14 peak min max- MAX = [2009 Chevrolet Malibu LS, ECM/Inputs/Outputs Drawing] KOEO Terminal 6-14 peak min max -- MIN = [2009 Chevrolet Malibu LS, ECM/Inputs/Outputs Drawing]

Powered down Terminal 6-14 ohms ( in spec) = [2009 Chevrolet Malibu LS, ECM/Inputs/Outputs Drawing] Powered down Terminal 6-14 ohms (out of spec) = [2009 Chevrolet Malibu LS, ECM/Inputs/Outputs Drawing]

Here's a few of the other Fluke 87 V photos. {File55169] [2009 Chevrolet Malibu LS, ECM/Inputs/Outputs Photo] [2009 Chevrolet Malibu LS, ECM/Inputs/Outputs Photo] [2009 Chevrolet Malibu LS, ECM/Inputs/Outputs Photo]

The basics of the GM HS LAN is that at rest, the voltages are close to 2.5v DC on both Bus Hi and Bus Lo and considered to be "recessive transmitted logic", while the Bus + (HI) will climb approximately 1 volt and the Bus - (LO) will drop approximately 1 volt in the "dominant" state. In the recessive state there is close to 0 volts differential, but in the dominant state, the voltage differential is approximately 2 volts.

The BCM is the Power Mode Master for wake up of HS LAN modules. If the modules are present on the list when scanned, but receive no wake up signal, they will not become active.

If you take the number presented here and do the math, the measured values support the theory behind how the system functions.

So, do we need to run off to investigate service information every time we are faced with a high speed GM LAN failure? It Depends on our abilities and needs. If we can remember some rest voltages and peak min max values of up and down 1 volt for a 2 volt differential, along with what 60 ohms and 120 ohms represent, or anything in between, we may only need to ustilze information for connector access and the basic LAN layout, to get our heads around where the LAN may be broken. You can pretty much guess what the ohm value will be if modules downstream are lost due to a data line failure.


Martin from British Columbia

Files Referenced:

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car Vehicle Data

2009 Chevrolet Malibu LS 2.4L

Engine2.4 L
Trans4-speed Automatic (Electronic)

car Vehicle Data

2009 Chevrolet Malibu LS 2.4L

Engine2.4 L
Trans4-speed Automatic (Electronic)