344: GM MOST Diagnostic

Welcome to the Automotive Diagnostic Podcast.

We're going to explore ways to sharpen our diagnostic skills, find learning resources,

and hear from experts in the automotive field.

Hey, have you ever been faced with the challenge of sourcing, installing, and programming a used

control module in a vehicle? I know a lot of us have. It seems to be happening more and more

often today with the volume of control modules on vehicles, the cost of some new ones, or even

the availability of new control modules. In some cases used may be the only option. So what do you

do here? I strongly recommend checking out SJ Auto Solutions and Tommy Oliva. Tommy offers a cloning

service for used control modules to make these things plug and play for the vehicle that you're

working on. In a lot of cases, he is also able to source the control modules if you're unable to

locate one for the vehicle that you're working on. But once you get connected with Tommy,

he's going to offer fantastic support from start to finish to make sure that that control module

is going to work in your application. He's also got tech support that he offers through his website,

along with some free resources there as well on information about used control module programming.

So make sure to check out SJ Auto Solutions. I can't recommend that enough.

Hey, what's going on Automotive World? Welcome to another episode of the Automotive Diagnostic

Podcast. My name is Sean Tipping. I'll be your host once again for this week's episode. Thank you

so much for joining me. This week on the show, I'm going to share a case study on a 2017 Cadillac

CT6. And the issue with this vehicle was no audio from the or response at all from the infotainment

system. And you know, the obvious symptoms that were customer facing was that there was no audio,

but then also there was nothing on the display screen for the center of the dash. So the shop

that I was working with on this vehicle, they had actually replaced a couple components and we

were initially called in just for programming. So that was the initial touch on the vehicle. And

actually, I had first dealt with this vehicle a few months ago, when they just called us into

program use radio, which that can get tricky on some of the newer GMs. This is a global A vehicle

and the radio is one of the modules that has the environmental identifier, which is a pin code

essentially has to match all the other modules, ECM, TCM, BCM, SRS, cluster, airbag, I said SRS.

Anyways, there's a number of modules that SMS match up to. So that's not the super important

part about this. But just so you're aware, if you're going to try to do a used radio on one of

these, it may or may not work with the standard TLC method. Anyways, when we got there to program

this radio for them, it programmed it communicated, but it didn't change their situation. They still

had no display on the panel, the infotainment display, and no audio. Now there were some codes

in there, but they did not have us dig into it like a full diagnostic. I did mention to them

that, hey, there is a internal fault code in the HMI, which is human machine interface module.

And I said that could be, you know, where the actual fault is any of the modules that fail

on these particular type of GM infotainment systems can cause the whole thing to go down.

And we'll talk more about the specific system, but this is prior to me really jumping in to any

sort of diagnostic on this thing. I was just, hey, your radio didn't fix your problem.

And then this is, you know, the module that's setting a code is internal fault code with the

HMI. So if you guys want to look at that or have us come die, I get and I didn't hear anything else

about the car for several months. So I didn't know what happened to it. I just kind of forgot

about it honestly, because they paid us for the radio programming. And that was that.

And then last week calls me said, Hey, we finally got a used HMI module for this thing.

Could you come program this? Okay, cool. So I get into it. And again, doing the HMI is used,

changing the VIN number can be a little bit tricky. There are ways to do it. Anyways, again, not the

main part of this episode, but I did have some struggles getting the HMI program correctly

and getting the VIN in there, even using the methods that I have proven to work in the past.

But it was at this point, after I'm trying to program it, that I'm starting to realize I'm

like, I think there's other problems with this thing. You know, was the internal fault code of

the HMI a problem? I don't know. Maybe I can figure that out with the original. But I think

there's more going on here. So I talked to him, I said, Hey, I think we need to do an actual

diagnostic on this thing. Do you want me to dig in and figure out what's going on? And then at

that time, he also told me that he tried replacing the screen. I think they were just chucking parts

at this thing, hoping they would get it to work. And I said, let's, let's do a diagnostic and we'll

figure out what's going on. And this car proceeded to kind of kick my butt. So I'll tell you about

that. But I'm going to just explain the system briefly, if you're not familiar, for the infotainment

system on this General Motors product and others, the newer vehicles like 2020 plus use more of a

ethernet based system for the infotainment. And then if you go back prior to like global a stuff,

you don't see any of this. But from that like 2013 ish to 2020 ish range, this is where you're

seeing the GM most system. Now, you've probably heard of most or dealt with it on European models

where it uses a fiber OPTIQ cable in order to transfer information for the infotainment systems.

You see this on Mercedes vehicles, I think BMWs as well. And they that that's not

the same as this, although the system name is the same, it's media oriented systems transport.

And it's just a communication network between infotainment modules. But when we're talking about

transferring audio and specifically video information, we need something that can

communicate very fast and transfer large amounts of data very quickly. And that's where they usually

rely on these most systems for the infotainment. Now, GM's most system is a little bit different

than the fiber OPTIQ setup. I think the idea of communication is the same between the two.

But GM uses a copper wire setup in order to link the modules together. Now,

that's why this is different than the fiber OPTIQ one. And the testing is a little bit different,

because with the fiber OPTIQ one, you can't exactly like scope it. And there, you know,

there's ways to check it. And you can actually just look for a light at the end of the fiber OPTIQ

cable. This one's going to be electrical signals, similar to a can network. But part of the reason

I wanted to talk about it is there are definitely some differences between a most system and a can

system. Okay, so surprisingly enough, I had not really dealt with one of these system failures

before, like I've worked on a lot of GM vehicles, and I fixed problems in the infotainment system,

like I've touched it, I've looked at it. But usually what it's come down to for me, in my

experience, fixing these is it just ends up being there's a bad module, right? I've had the human

interface modules just not communicate. And of course, that causes an issue with the most bus,

we'll talk about that. But you can't communicate with the scan tool. And so you're just going

after that module, you're replacing it. Same thing for the radio, I've seen that happen as well,

where the radio is the failure point, you replace it, and that's it. And I've never really dove in

to doing circuit diagnostics involved with this copper wire most system in GMs. Now, I did take

a class from John Thornton back in 2019. And the reason I know that is I actually have the

training manual, and I dug that out of the boxes that I had, and I found it and I went through it.

And it's one of those things where like, I remember taking the class, I remember learning

about the system, that was the first time that I'd really sat down and had someone explain to me

how this system works. But I never applied it in real life. So like I had the idea of the ring style

communication that they use. And the radio is like the it's the central hub, it's the first module,

it is the module you're going to use for diagnostics. And then it sends out communication

in a ring style, and then comes back to the radio. So whatever modules are involved on

this most system, the radio starts the communication. And if you're looking at the diagram,

there's actually little arrows that point the direction of normal communication, which looking

on the diagram is going to be in a counterclockwise direction. But follow the arrows on the diagram

just in case there's one that doesn't set up that way. But you can see the area, the arrows,

and it's different than can in the fact that communication normally is only going to go in

one direction and then loop back to the radio. And it's a ring of communication. And so if you

have a break somewhere in that system, you have either, you know, an actual circuit issue,

or you have a module offline. And that's the other important part is when if you have a module fail,

like let's say it loses power, it loses ground, the modules just junk, it's not turning on,

it is not like a can system where when you if you think of a GM daisy, daisy chained can system,

right, you have two can lines that go to the module, you could say in and then to the go out.

Again, can's different because it's not moving in one direction. But even if that module didn't

empower it and have ground, and I should be careful what I say there, I'll make an asterisk

there. But let's just say the modules offline, it's not talking that module is still part of

the physical layer of the network. And whatever is on the other side of that network could be

talked to by the rest of the network, because those can lines go in, and they just pass through

the module, and then the can transceivers connected internally. But if you took the

oh meter and went across the two terminals, you would see continuity, you know, on can high to

can high, can low to can low. Okay, now the asterisk there is yes, you can have a module that loses

ground and it corrupts the network and nobody talks. That's a different story. But what I'm

saying is as far as the physical layer goes on those style, you have connection through the module.

That's not true on the most system. So if you have a module that's offline, it essentially

becomes a break in the system. Okay, you actually truly do have an in and an out on these most

circuits. And if that module is offline, even though physically connected with the connector,

it is now a break in the system. Okay, now, GM has a way of determining where this break is

in the system. And this is where I recall the class. And I recall the method in order to determine,

you know, where this break is using a scan tool. And this is the other part that I definitely want

to make clear, because you're not looking at a diagram, this most system, you're not actually

directly connecting to it with a scan tool. And this is just going to be between the modules involved

with the infotainment system, you are going to talk to the radio, and you can talk to these other

modules as well. And you can look for codes. But the radio is going to be the best place for you

to go in order to diagnose what's actually going on. And you're talking to the radio through the

low speed GM LAN in most cases. So the scan tool is coming in on a communication network that is

hopefully complete. And then you are going to be diagnosing what the problem is with this most

circuit, but through the radio, right? So you're not directly connected to it. So something to

consider there. Now, again, I had remembered taking the class from John and that there was a method

to determine where the break was in the system. If there is a break and there's a code, it's a

use 0028 that will set for the most bus system. And you can go into either data stream of the

module, or there's a special function, which is most bus circuit diagnostic starting point

in the radio itself. And they actually updated this somewhere along the way. I think when they

first came out, this didn't exist within the most system. But there is a data parameter on some of

the newer options where you can see this. But the old school way is, well, this still happens

within the system. When there's a fault, the communication will actually reverse direction.

So if you think about it going in a counterclockwise direction, when there is a noticed fault within

the system, the communication then moves the other way, right? Because again, it loops back to the

radio. It starts and starts and ends at the radio. So you have four communication lines going in.

Okay. Now, it is going to reverse direction and go out the radio. And then it will see what's the

last module that it can hit going the opposite direction. And this will be the surrogate most

master is what they call it a replacement for the master node, because each module sets itself up

as a node on this network. Okay. And you could count the numbers and then figure out where the

break was based on the scan tool data that you have, who was the surrogate most master. And

I had never actually utilized this knowledge out in the field. I kind of had

the basis of it like, okay, you're supposed to, you know, count backwards on the diagram. And then

that's the, you know, that's the module that the communication could reach, meaning that the break

has to be, you know, somewhere between here and here on the diagram. Okay. And if that didn't

make a whole lot of sense, just audio, I'm going to give you some references here, where you can

see some good visuals on this system. And honestly, looking at the factory wiring diagram, don't even

bother with the after markets, looking at the factory wiring diagram with these really does help.

It's a very nice visual. And then you can also utilize the diagrams and the RPO codes to figure

out what module it has, right? But again, I'm sitting there with this car, and I'm trying to

figure out what's going on with it, because I've no infotainment, you know, options at all. And I

heard a good quote this week is, you can rent, you can rent wisdom for free. But if you want to

purchase it, you have to do it through pain. And I, that was, it's not my quote, and it didn't have

anything to do with automotive. But I thought about that while I was working on this car. And I'm

like, it's so true of automotive, right? We can go to classes and stuff, and we can learn this stuff.

But and you can have the basic knowledge about it, you can get yourself ahead. But man, if you

really want to have that like ingrained in an actual skill set that you can just hop in and use

it on a car, you usually have to go through it yourself, you usually have to actually apply it,

maybe get your butt kicked a little bit, struggle, and have to figure something out in order for that

to really be yours and a piece of your diagnostic or automotive skill set. And it was definitely

the case with this, because I was fumbling through it from my knowledge of the class granted, this

was this episode is brought to you by L one automotive training and Keith Perkins. If you're

looking for education on module programming, J 2534, eprom work, key and immobilizer, electrical

diagnostics or drivability diagnostics, Keith has a website L one training calm that's got over

60 hours of training videos on all those subjects and more. When I first started out doing mobile,

I utilized Keith videos on module programming and J 2534 in order to get my head wrapped around

what I would need for the tooling, the computers, the software setups, you know,

what kind of obstacles I would be up against when I'm out there programming modules on cars.

And it was a huge benefit to me and I continue to use the training videos that he has on his

website. So I strongly recommend checking out L one training calm. The link is in the show notes.

You know, seven years ago that I took it. And I've seen things here and there. There's been

some really good posts about this. But again, just never applied it. I never had to dive into a

circuit issue with the most bus. So on this car, all the modules that were involved did communicate.

Okay. And there was quite a few this thing was fully loaded. It has like the DVD or the video

screens that come up from the back seats and everything. So it's got the radio, the human

interface control module, the there's a infotainment touchpad, which is actually on the center

console, the media disc player, which is in the back of the center console, the instrument cluster

is actually part of this most bus as well. And then it also has the amplifier in the trunk and

back to the radio. And I think I may have done a couple modules out of order, but that's pretty

close to the order of the circuit on this one. Now, here's the thing. If you start replacing

components on one of these most systems with a broken network with a problem, it is going to make

that diagnosis of what you're doing more difficult. Now, I mentioned you can go in and you can use

this most bus diagnostic starting point. Now, they've added a data PID and I don't know exactly when

it happened. But I know, you know, initially, you had to do that count backwards thing to figure

out the point in the network where it's broke. Now there's a data PID that just tells you,

okay. And so the, the nodes are numbered. And you can see that on the data stream one, two,

three, four, five, six, right? The radio or the radio is going to be zero in some cases, but

it'll have a number applying to each one of the modules on there. And then it will say, hey,

there's a break between here and here between one and two between three and four. And that data PID

kind of does the hard work for you of, okay, I need to look between this module and this module

in order to find out, you know, where my break is. And again, it could be the module itself.

That's definitely a possibility, but it could be a wiring issue as well. And I'll talk a little bit

about the wiring. But when you introduce, especially a new radio, or in this case, a used radio into

a system that's broken, it's going to make things much more confusing for you. So here's my suggestion.

If you've replaced a module or somebody replaced a module and didn't fix the problem,

I would highly suggest going back to the original module, because it's actually going to have the

information from which modules were on there, and it will be able to give you a much better

starting point. So I wasted some time with the used radio and the used HMI in there,

trying to figure out what was going on. And I initially I eventually went back to putting

the original components back in. Now, the HMI did have an internal fault code in it still. So we

weren't sure if we're going to have to replace that or not. But what I really want to do here is

start fresh. And so the only thing that was in there was the infotainment screen, which by the

way, is not even on the most bus on this one. It communicates on a separate network. But it was

blank. There was nothing there. You could get the reverse camera to come up. So I didn't know the

screen worked, but none of the infotainments, you didn't get any options through the screen,

none of that stuff. But anyways, that part of it doesn't matter, because it's not part of this.

But once I got the original radio back in, I was able to utilize the where the break was in the

system. Now, it was kind of confusing again, because I really hadn't been through this before.

And I had to go back into the training and read up on it that when the communication fault is

recognized within this most system, the communication then goes clockwise, the opposite

direction. Okay. So again, it was radio, HMI, touchpad, media player, cluster, amplifier,

back to radio. That was the order of this, this circuit and the modules in line. And what it was

telling me was that there was a break immediately, basically between the radio and the amplifier.

And it took me a while to figure that out again. I struggled with that and I had to really understand

the nodes and how they are numbered and then how it reacts when there is a fault. And there's a lot

of literature on this. You can look it up in GM service information. I'm going to give you a link

to a post on Diagnostic Network that a gentleman from British Columbia did like an extremely

good write up on GM most systems. He's got screenshots and diagrams. If you want to really

learn the nitty gritty, which would be pretty tough to do over audio, I'd suggest going to check

this out. I don't know if John still does his class. It was in a GM diagnostics class where he

had the stuff on the most bus. I don't know if he does that one anymore, but if that's available,

check that one out as well. Anyways, I had to take some time and read and I wasted time on the system

before I actually went to read up on like, okay, this is how this works. And once I understood

how it works, and then I use the datapids and the scan tool and the radio, and it basically just

tells me, hey, the break's right here. You look at the diagram and say, okay, it was on basically the

backside, the last section of the network between the amplifier and the radio where

normal communication direction going counterclockwise, it's coming back to the radio and the break was

there. And could that be the amplifier itself? Definitely possibly it's in the trunk, it's in an

area where it could definitely get wet. But that's where I need to go basically is get to the amplifier.

Okay. And there is a toolkit and you'll see this again in the literature where GM has some jump

connectors that will bypass a module essentially. Again, because once that module is offline,

it essentially is going to open the network. Or if it's a problem, you know, you would want to

bypass it and the rest of the system can turn on and operate without a particular component. Now,

without the amplifier, are you going to have sound at all? Probably not. But could I get my screen

back potentially? Yeah, maybe, you know, if the amplifier is the problem, but could be a circuit

issue as well. Now, when I got to the amplifier, and I disconnected it, I saw what I was pretty

sure was going to be my problem is someone had been in there before to do some wiring repair,

and they did a pretty poor job of putting a series of butt connectors in, just like the

heat shrink butt connectors. And it was actually on both of the most bus communication wires.

Now the connector had like some remnants of green stuff in there, but it also looked like it'd been

cleaned up. But now what I have to determine is the break in the, you know, these butt connectors and

circuits. Is it the connector itself? Is it the amplifier? Like I feel like I'm, you know,

this is definitely the area that I have to assess. And I did some circuit checks here to try to figure

out what's going on. And I want to touch on a few things here in case anybody's hopping in,

trying to do the circuit side of things on these most bus networks. And this was a big learning

point for me as well. I wasted some time here could have done this easier, or I could have

made this more simple and figured it out. And honestly, what I should have done as soon as I

saw those butt connectors and like, Hey, this is not a quality repair. It's just done the quality

repair myself. And then see, does that take care of my problem? Also, I could just bypass it by

passing it did not work in this case, kind of proving that it was a circuit issue rather than

the amplifier. But anyways, I did try to do some scoping of the network to see if I could,

you know, manipulate the circuit and get things to happen. Now, a couple of things here that you

should probably know. And again, John had some great stuff. Once I looked at the training manual

on scope patterns with the Pico, you're only going to see about 300 millivolts, either in a positive

or negative direction on each side of the network. And it says within the literature, you see about

a 1.2 volt total difference in voltage fluctuation. If you're to look at the peaks and valleys,

but it's a very fast moving signal. And so you're not going to see data packets like you would

be used to, it's just kind of noise on the circuit. And you can look at both sides. But

one thing I did find is if you take two channels from a scope, and obviously the black lead is

going to be grounded, and you go across the network, this will actually take the network down.

And that was using a Pico scope of 4425. I don't know if a different scope impedance would be

different. But that was enough, like I still saw a signal, but it was enough to basically kill audio.

And I proved this once I had the problem fixed. And I did that, I was just like, I want to see,

you know, with no issues in the system, if I hook up my scope to it, is this going to be an issue?

Is it going to actually take the network down? And it did. And so that's something you would want to

know. Because if you're trying to diagnose like an intermittent issue, doing it while a scope is

connected to each side with a couple of channels, that can take the network down. And I had some

issues with my use scope as well, when I went across the network, just with a single lead,

where that was enough to actually disable the audio in the system. So keep that in mind is this

communications network is very sensitive. And what you should be using, in my opinion,

is actually the scan tool data pids. And there's a data pit that says the amount of most bus breaks.

And you can actually watch this in real time, if you're trying to find an intermittent connection,

wiggle wires, watch the scan tool, that's the way to do it. Now, could you scope it? Sure.

You get a signal, just be aware that you can disable the actual system. Now, I was able to

check one side at a time with my scope, didn't seem to be an issue. It was still able to produce

audio through it. Here's the other thing. And again, I struggled on this one a little bit. I

went in circles before I actually sat down and came up with a plan and read the stuff that I

needed to read to get through this. But I was actually doing some home checks on this circuit.

And one thing I was looking for was, you know, or is the network shorted together at any point?

Okay. And the thing about these most bus networks that this is, again, is very different than what

you would find in a CAN bus network is that if you are to take your own meter and to go across

the two sides and measure the resistance, I'll tell you, it's not a useful measurement. Don't

bother doing it because you will measure continuity. You will measure no resistance between these two

sides when they're connected to other modules. And that is inside of each module. They're basically,

if you go home check, they're shorted together according to the own meter. Now, I don't think

they actually are because if you short them together, that will disable the audio as well.

I'm sure it has something to do with the transmitter receiver function

of the internal circuitry. I don't know exactly what it is, but if you own it, I know it will show

you shorted. And that threw me for a loop for a little while too. Again, because if you actually

do short the wires together, that's a problem that will take out that network. And I did it and

touched them together. And okay, this takes out the network. So that would be a problem. But

if you're looking for two most bus wires that are shorted together, you are going to have to

disconnect them from all modules, right? So let's say we're working between the instrument cluster

and the amp. And I suspect that the wires are shorted together because it's totally possible.

That could happen if, you know, a harness is smashed somehow or another, I would have to

disconnect the instrument cluster and disconnect the amplifier and then test for a short together.

If any of the modules are still connected and you do an ohm track, you will see

shorted together. But that's how those are set up. And that is normal. Okay, now that was part of

my checks I was doing on this one as I was trying to work through it. That one definitely

cost me some ways to waste some time. But now I know that about these modules that are involved

on this most network, and it is going in the notes for diagnostics on these. And I wanted to tell

you guys too, in case you're doing circuit checks, because, and I'm just using a fluke, a volt,

you know, ohmeter in order to do my checks there, you could see shorted together, but that's not

actually a problem. But again, going back to the simplest way to do this is use your scan tool,

look for the number of most ring breaks, and you can actually figure out the location to

this one showed, you know, is between the amplifier and the radio using the number of the nodes.

And it was in one of these butt connectors that was at the amplifier connector. And I was able

to wiggle it, watch the scan tool. Okay, it's right here. And I just need to, you know, cut out the

bad section of the wiring to a proper repair. And then this system is all good to go. Even with the

original human interface module, I still had a code in it, but it was functional. So we still

actually have this in our shop right now. So we'll see which one they want us to put in. But

right now, the audio works, everything functions the way it's supposed to. It's just some

reassembly that we got to do before you get it out the door. But anyways, I just wanted to go

through that with you. I'm going to put some information up with the show notes so that you

have some references for information on these. Again, there's a post on dia.net from a guy.

I'll put his name in the show notes. I think it's Martin. Excellent write up if you want to learn

about these systems. And then again, I don't know if John still has a class available out there,

but he had some good stuff. You can also read this in service information as well. And GM has

a lot of information there too. So that's going to do it for today. Hopefully you found that

interesting. And maybe you run into one of these systems and you can be a little bit more prepared

than I was in the moment. But thank you for listening. I really appreciate it. Let's get out

344: GM MOST Diagnostic

About this episode

used control module

SJ Auto Solutions

Tommy Oliva

2017 Cadillac CT6

infotainment system

environmental identifier

TCM

ECM

infotainment display

internal fault code

HMI

VIN

fiber OPTIQ cable

Mercedes

CAN network

MOST bus

radio

module offline

oh meter

master node

factory wiring diagram

RPO codes

electrical diagnostics

module communication

datapids

module is offline, it essentially is going to open the network

butt connectors

oscilloscope

wiggle wires

disconnect the instrument cluster and disconnect the amplifier

Request an Explanation

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