The instrument cluster is the dashboard with the gauges and warning lights. If it’s replaced, the car usually needs to “learn” it and verify it’s the right part before it will work correctly.
They’re talking about using a different type of connection—Ethernet instead of USB—to talk to the car’s computers. Some cars won’t communicate properly unless you use the right connection type.
They tried connecting the diagnostic tool with a USB cable. When switching to Ethernet didn’t help either, it suggests the issue isn’t just the connection type.
A service bulletin is like the factory’s “known problem” memo. It tells mechanics what’s going wrong and what to do to fix it, usually first for dealerships.
Aftermarket service information refers to third-party repair databases and manuals used by independent shops. Some OEM-only documents (like internal technical notes) may never make it into these resources.
An OE subscription is paid access to the carmaker’s official diagnostic and service info. Some internal documents may still be difficult to locate even with that access.
General Motors is the car company behind the document being discussed. The takeaway is that GM’s internal diagnostic info can include steps that aren’t widely available elsewhere.
Wiring diagrams are maps of how the car’s wires connect. For troubleshooting, they help you figure out where power, ground, and signals should go when something isn’t working.
LIN is a type of communication wiring the car uses to let modules talk to small devices. If a sensor isn’t working, the problem can be power/ground—or the communication line like LIN.
Service mode is a hidden diagnostic mode for the car. It lets a technician look deeper into what the car is doing and why something like charging might be failing.
The quarter panel is part of the car’s outer body near the rear wheel. If it’s replaced after an accident, it can also disturb nearby parts and wiring that the car depends on.
Concept
large language model
They’re using an AI tool to help read and ask questions about the service instructions. The goal is to get answers faster while they’re figuring out what’s wrong.
The charge port is the opening where you plug in the charger. If it’s faulty or the connector isn’t sitting right, the car may refuse to start charging.
The “high voltage section” refers to the part of the Tesla diagnostic interface focused on the EV’s high-voltage system. It’s where the technician can view interlock-related data like HVIL to determine why charging is disabled.
The safety loop (HVIL) uses low voltage—about 12 volts—even though it’s related to the high-voltage system. So the fault you’re chasing is about the safety wiring/contacts, not the main battery power.
A terminating resistor is like a built-in “signal conditioner” in a circuit. The car looks for the right resistance value to know the safety loop is connected correctly.
CAN is the “conversation system” between car computers. Here, the speaker is saying the issue they’re diagnosing isn’t happening through that normal data conversation.
EVs still use a 12-volt battery for controls and electronics. The speaker is pointing out that this part of the diagnostic/safety setup uses a 12V feed.
Voltage drop is the “loss of electrical pressure” as electricity passes through parts. If the drop isn’t where it should be, you can suspect a bad connection or component.
Ground is the reference point the car uses for electrical circuits. If the resistance to ground isn’t what it should be, it can mean the circuit is open or not connected correctly.
They’re focusing on the wiring between the big battery and the rear power components. If something in that path is wrong, the car may not be able to charge or may refuse to operate safely.
The speaker stresses that high-voltage work should only be done after completing proper training. This is important because EV safety systems (like interlocks and contactors) can still present hazards even when protections are in place.
High-voltage diagnostics are the steps techs use to figure out why an EV’s battery system won’t charge or turn on. It’s basically troubleshooting the big electrical parts safely.
LIVE
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.
This episode is brought to you by L1 Automotive Training and Keith Perkins. If you're looking
for education on module programming, J2534, eProm work, key and immobilizer, electrical
over 60 hours of training videos on all those subjects and more.
When I first started out doing mobile, I utilized Keith's videos on module programming and J2534
in order to get my head wrapped around what I would need for the tooling, the computers,
the software setups, 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 L1Training.com. The link is in the
show notes. 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. It is just me on the show this week, once again, coming at you
with another case study. Actually, we're going to wrap up the GMC Yukon that I was talking about
last week. So if you didn't listen to that episode, go back one week. I talk about Global B VIP
architecture in the newer GM vehicles and the things that you might want to be aware of there.
Now, when I ended the episode, we still had to go back to that vehicle one more time to program a
brand new instrument cluster. So again, if you want more details on that specific vehicle, you can
check out the episode. But we went back there on Monday, because they got an instrument cluster,
and we were not able to program it, or I shouldn't say we weren't able to program it. We programmed
it. But then once we got to the S-stack, the security handshake, it aired out again for the
instrument cluster. And that was the issue that we had ended up with after attempting to program
another control module, the serial gateway module. But we ran in the same thing, aired out on the
cluster P16 as the ECU at the air for the S-stack, and we're right back in the same boat. So
we actually called GM Tech Support on this because we're following things to the letter as the
Service Bolton and Service Information suggests on these GM Global Bs and what you're supposed to
do if you get this air, right? And we had removed the cluster from the network and bypassed the
network wires, and it passed without the cluster. This was the original one. And according to GM
documentation, if it passes without said module, well, you replace that module after checking it
for updates. That's what we did. And this is a brand new instrument cluster from GM. So I was
a little stuck and we called GM Tech Support, which they attempted to help us, but didn't really
get us anywhere. One suggestion they made was to use Ethernet connection to the MDI2 rather than
a USB cable. And I think I mentioned in the episode last week that I've talked to people that
with the front camera, you might need to be using Ethernet connection, but I haven't actually run
into it. We tried it, didn't change anything, still failed. And again, you could remove the cluster
from everything and then it would pass no problem. So kind of stuck here. And I had actually sent
one of my techs out to do that part. We ran out of time and ideas. So I was like, well, I'm going
to research it a little bit more. We'll come back and we'll keep trying because we want to get it
figured out for this customer. Obviously, I must have missed something or not understanding something.
Not sure what, but got to research. So in the meantime, and actually this was earlier on
Monday, a gentleman had reached out to me. His name is Josh Hutchins and he has a GM
world-class technician. And he was just commenting on the podcast episode that I did and gave me
some other tips and advice on it, which is greatly appreciated because obviously I don't work in the
dealership. I don't know that stuff firsthand like someone in a dealer is going to be dealing with it
every single day and probably has training on it from people who are at the engineering level.
So anyways, I reached out back to him at this point and said, hey, do you have any tips for me
because here's my situation. We went through everything on this vehicle. We followed the
procedure and we're still ending up with the same error. And he actually was able to come back to
me with some information on how to proceed and fix this vehicle, which was huge because I don't
know that I would have been able to figure this out myself. So there was an internal document
within GM's internal system and I don't know what it takes for something to go from something like
this to a TSP. But I've seen some similar like that with like Star cases in Chrysler vehicles
where it's not a service bulletin and you might not be able to find it in aftermarket
service information, but it's there and it's, you know, within the dealership or at least
if you're paying for a OE subscription, I don't even know that I would have been able to find
this General Motors document because it looked like it was through some sort of internal technician
forum or something like that. Anyways, the technician had been experiencing almost the
exact same scenario as me. Now, they hadn't replaced the serial gateway module, but they had
a new telematics and a new cluster in this thing and it was still failing for the cluster. What
they ended up doing was disconnecting one of the connectors on the serial gateway module
and then running the SDAC and then plugging it back in and then going in and finishing the setup
of the cluster and it was all good to go. Everybody was happy and you didn't have to disconnect the
cluster to get it to pass this time. And the connector that is on the gateway module, it has
some ethernet connections on it, but not the ones that actually go to the cluster. So the cluster
is on for the handshake and I'm not exactly sure what's happening. It looked like it was suggested
by engineering that the gateway is reaching out to or onto networks to get information about a
security handshake that it shouldn't be. I'm not entirely sure. I didn't really pursue it anymore
than that because it worked. Now, the note in the document and this makes sense, if you would
attempt to replace another module and do an SDAC, you're probably going to run into the same issue.
So will this eventually turn into an actual service bulletin, known information out there?
Very possibly. I don't know how common it is. I don't know how lucky I am that I was the one
that ran into this exact problem, but since I just put out the episode on Global BE and I was saying,
hey, you might run into it where you have to replace a module and that could be true. That
could still happen. There are going to be some hurdles and challenges as we get into these things
because like I did with this one, I replaced the module that I'm supposed to replace based on what's
going on and still had the issue and I had to do something with this series of unplugging and
plugging back in while running the SDAC. I don't know that I would have figured that out on my
owner. It would have taken me a very long time to do. So anyways, just filling everybody in on that.
So you know, the whole story, I want to be transparent about the stuff I talk about on the
show. So anyways, big thank you to Josh because again, that would have taken me a very long time
on my own. It's all about who you know in this industry for sure. All right, now into the case
study for today's episode and a little bit of information on the system is on a 2021 Tesla Model
Y and it seems to be lately we're getting all kinds of really fun cars to deal with and this one
was no exception. This one, the actual problem that we found, nothing crazy. It wasn't anything like,
oh, wow, that's a crazy fix. But I have not dealt with a whole lot of Tesla diagnostics. Now,
I've gotten my hands into a few of them, done some stuff here and there, but it hasn't been anything
real involved, right? Like, oh, there's a parking sensor that's not working. It's power, ground,
and lin. There's a runk actuator that's not operating. It's power and grounding. You know,
you can get through that stuff and I've learned a little bit about the wiring diagrams with Tesla,
the service information, and the tooling, right? You can connect to the car to get information
in a few different ways on these Teslas. The Autel has a series of connectors and one of them,
specifically with this Model Y, you take a connector apart on the back of the center console
and you put this jumper harness in between. The jumper harness has no bd2 port. You plug your
Autel into that and then you can communicate with modules on the car. And I've done that a couple
times. You can also just get Tesla Toolbox and it's expensive. I think it's like $100 a day or
something like that. I've only actually made the purchase a couple times where we needed it. Everything
is an option and it's going to connect to the car in a different spot than what you're doing with
your Autel. But there's some cables that you can buy a set of on eBay for relatively cheap.
The other option you have, and I've found this to actually be really helpful, I'm sure there's
limitations to it, but putting the car into service mode and just using the infotainment pad
on the center of the dash to get the information that you need. And you can get the trouble codes
and you can actually get into some of the systems and figure out what's going on with the car.
And that was my experience with this one. So let me back up and tell you what's going on with this
car. This is at a body shop and I guess this thing has been sitting for at least six months
after an accident and there was some body work done to a quarter panel. There was a subframe
that was replaced that the shop did not tell me about. First I had to find that part of my own,
but there was a subframe that was replaced along with the quarter panel and they said
when the vehicle came to them, this is the story I got up front, when the vehicle came to them it
was charging fine, it was just smashed out from the accident. They performed the work to the vehicle
and then after the fact it would not charge anymore. So you plug the charger in, I think it
blinks blue at first and then it goes to red. It basically airs out and you can see on the
information pad inside that it's not charging and this thing's at zero percent. It's been sitting
for six months and anyways they tried replacing the actual like charger connection which is a unit
on the left rear quarter panel there that didn't change any forum. They had gotten an actual Tesla
charger because this thing's sitting in a shop and they thought maybe it was their charger
that they had and same thing still not charging. So anyways they call us to come in and take a
look at it and see what's going on and basically what I end up finding, well first let me actually
tell you about putting the car into service mode. So there's videos out there where you can actually
if you just go to service.tesla.com there's a ton of information on this but you can watch videos
on how to use it. There's a PDF that's just the entire like operation of service mode on Tesla
and one of the things I like doing this is a little bit separate is if I have a bit long PDF,
sure I can read through it definitely but I'll throw it into a large language model and then
as I'm working I can just ask that language model questions about the PDF as I'm trying to
figure stuff out and that's kind of what I was doing here to guide me through like finding what
I would want to find within service mode or how I would operate service mode to get to where I want
to go and in order to do it it's pretty straightforward you just pull up the menu and then you go to
software you'll see the actual like name of the vehicle written out in text you hold that with
your finger for a few seconds let go type in service there's going to be a window that pops up where
you with a keyboard you type in service you hit enter you agree and then it goes into service
mode and you can tell it's in service mode because there's a big red banner all the way around the
info pad the infotainment pad so you can tell like obviously okay this is in service mode
and then you have a menu of a whole bunch of stuff that you can do and I've not explored all of it
but again there's a instructional on all the details and all the things you can go through
but one of the areas that you can look at on these tesla vehicles is both the hv battery the
charging system and something called the hv il and this is where I ended up getting to
with this vehicle and I should say I hooked up the autel and I scan it for codes I actually
didn't get anything really useful out of the the autel when I was connected to it like I didn't see
anything you know obvious of why this thing's not charging or any errors there was a couple
codes in there and also the 12 volt had been dead for some time as well so we jumped that and put a
charger on it but I didn't really get anything useful from the scan tool side which is why I went
to the service mode side and it was basically my plan was to see what I could get through service
mode and then if I needed to make the purchase uh for you know the actual tesla toolbox I'll do that
but like I say I found a number of things you can actually figure out just using either the
the aftermarket scan tool or using the service mode right you know that that doesn't cost you
anything you don't even need a tool you just need to enter service mode so again one of the things
I can look at is the high voltage system on this which is kind of the direction I'm going because
it's not charging right so why would it not be charging is it something with the charger
is the high voltage system disabled for some reason let me see what I can find out here
and it is pretty obvious once you're into the high voltage section of service mode that it is
disabled and the contactors are open this show is brought to you by auto rescue tools and Isaac
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ready to tackle any make or model I've got one of these laptops myself and I can say that it is
outstanding and it really streamlines the process by having everything you need in one device
so if you're looking for something like that I highly recommend checking out auto rescue tools
dot com you'll also find scan tools diagnostic equipment key cutting equipment and much more
check out the link in the show notes I highly recommend it now it does show the batteries at
zero percent but it is not closing the contactors which is not going to allow you to charge the
battery even if the charger is plugged in which at that point I'm like okay well I understand
why it's not charging now I have to understand okay why is it not allowing the contactors to
close is there an air somewhere that it's seeing within the high voltage system that's just saying
hey I'm not going to do anything until you fix this um and like I said they had actually replaced
the charge port itself and I did do a visual on that just to make sure okay as everything
plugged in and the connector was a little bit wonky on there you could tell they'd been messing
around but I didn't see anything that was definitely out of place there um and looking through the
menus within the service mode and within the high voltage section there was an option to view the H
VIL which is high voltage interlock loop on the vehicle and this is something that I hadn't dealt
with before in a Tesla and wasn't really familiar with but kind of the reason I wanted to talk about
it because I thought it was actually pretty interesting and once you understand it and what
it is um it's actually pretty straightforward and pretty informative onto what's going on and how
you would find a fault but what this H VIL circuit is it is a loop of a 12 volt wire and connections
and some resistors but it's a 12 volt circuit okay so this is not a high voltage circuit but it is
meant to monitor the high voltage connections at the high voltage components okay so it leaves
the high voltage battery which is a battery control unit um it leaves on a 12 volt connection
right so this is just a regular connector with a lock lever um this is not one of the big orange
ones just um regular connector and it leaves on a 12 volt small gauge wire and it runs a loop
all the way around the vehicle through some of the high voltage components uh now on this particular
Tesla there were other interlock loops that just went to one component and not through others
but this main H VIL loop goes from the battery on this Tesla through the rear drive unit and then
up to the front drive unit and then back to the battery and it completes a loop now there are
three resistors within the components there's going to be one in the high voltage battery
there's going to be one in the rear drive unit and there's going to be one in the front drive
unit they're all 60 ohms so kind of reminded me of CAN you know but there are 120 in CAN 60 total
but anyways there's 60 ohms in each one of these components so what it does and this is the part
that I didn't really get until I actually read up on it but there is a technical service bulletin
in which you can google from Tesla on training on the H VIL circuit and it's just you know it's free
you can find it read up on it and everything made a little bit more sense there because I did kind
of chase my tail a little bit here like I recognized okay this is there's something going on here
and maybe I should stop and explain how I made that determination when you get to the screen
in service mode where it shows the H VIL you can it shows that it's off at least for me it showed
that it was just off nothing's happening and there's a button says H VIL on and it allows you to turn
this on if it's off and what happens is the high voltage battery unit actually outputs a voltage
onto this circuit now I found out if it's open circuit you're going to have about 8.8 8.9 volts
being output onto this circuit okay but it shows you right on the screen what it expects to see
on this circuit and it is not 8.8 volts but you can actually force it on for a period of time
it'll stay on for about 60 seconds at least on this one and you could see not only what the
voltage was but it shows you a physical loop of it going around the car and what components it
connects to and once I actually pulled up the H VIL training document I'll put that in the show notes
by the way um so that you have that again it's free you can just find it on google um but it
actually shows you exactly how this circuit works not in precise detail but once you understand
what's going on with it shows you everything and I I was looking at it that's the funny part of
like educating yourself kind of on the fly and then after the fact so you can talk about on a podcast
is you look at the components you're like oh it was showing me exactly how this thing
worked and where it went I just didn't have a full understanding so let me explain to you and
I'll see if I can post this picture in the facebook group too because the picture really
kind of does it justice and I think it's cool that it's just built into the car like I'm not on a
scan tool here I'm not you know in-service information for this it is on the screen on the car I'm
working on and it shows me exactly how it's laid out so high voltage battery we're going to output
8.8 volts and then through the components and it shows you on the screen which components
have terminating resistors or I keep saying terminating resistors they're just resistors
okay they're they're this is not a can network so um but for whatever reason unless he's 60 ohms
I think cans but it shows you which ones have the 60 ohm resistors within them okay and you could
actually look and see what it's showing as a representation as a resistor it's not the little
zigzag line that I'd be used to on a diagram but once I understand that oh yeah there is one there
it makes sense and it says right there 60 ohms so okay it's in that component and it shows you
the direction that it leaves out of the high voltage battery and then to the rear unit now
the rear drive inverter unit it shows you going in on a 12 volt connection to the rear drive unit
okay and it actually shows a little connector a 12 volt inline connector as well um I'll get back
to that but on the drive unit it shows that going into a 12 volt connector into the module across a
resistor and then here's the part that I took me a little while to understand this it actually goes
through the high voltage connector itself okay so that's the big orange one and that high voltage
connector has a metal conductive piece that goes through it and then on the unit itself there are
two pins that are kind of on the outside of the actual connection right this isn't the high voltage
part this is a 12 volt part and when that connector is fully seated and only when it's fully seated
do those contacts come together right so the connector itself completes the circuit and
that's the whole point of this right if one of these connectors are not connected correctly
then it will open up the circuit and how does it know the circuit's open well if you understand
you know series electrical or series electrical circuit you know after each
resistor in series you're going to have a voltage drop and again what's cool here
is you can look at the training material and it breaks it down like you should see this much
voltage here and this much voltage here and this much voltage here and you it's going to say you
should have this much resistance to the ground here you should have this much resistance to ground
here uh because the circuit actually does end connected to ground in the battery on the other
or the battery unit on the other end but it shows you on the screen what the expected voltage is
at each point and what it actually is so that's really helpful right you can look at this screen
and tell where your break is or where the section of the network the network i can't stop saying it
because this i'm thinking about this like a network but the section of the circuit where
your break is and again i understand this better now after dealing with the car and
after reading up on it and like going through the material i was kind of doing it on the fly
and it took me longer than it should have to understand this but now that i do like if i
get one another one of these this is relatively simple to figure out where you're going on this
circuit but again it shows it to you pretty clearly in the picture on the infotainment screen
once you understand how this works so what i was seeing was out of the hvc or high voltage control
unit i had 8.8 volts but that is not what it wanted it expected 3.6 volts at that point okay so what
that means to me is there's basically no terminating resistors connected to this and i need to look
between the high voltage battery and the rear drive units it's the first thing in line okay
and this is where okay i started looking and i got to the 12 volt connector and i had my
voltage there when i forced it on then my 8.8 volts so i know all the way from the battery
to the drive unit i have 8.8 volts then when i was down underneath and we did have to lift this up
this is when i found out that the subframe had been replaced by the way because i was like this
looks brand new oh yeah we did that i'm like why don't you tell me that when i asked you everything
that you did on this car um well we didn't think that mattered or was important or whatever the
usual stuff that we get um anyways i'm looking at the high voltage connector and you could tell
like it was latched down it was in place but it was not fully seated you could tell the clip wasn't
where it needed to be and so uh we got it seated correctly connected everything back up
and the hvil immediately started working the car started charging uh you had a little green light
at on the little charge port in the back and you could see on the screen the correct voltages
right so it expected 3.6 um and that's what it saw and then on the other side of the front drive
unit it was expecting 1.2 that's what it saw and so you can follow it around the car
and then it turns it green instead of red so hey everything's working and it's charging
and that was it so again once i understood what was going on and i knew it at check this really
wasn't you know real difficult to get through maybe a little bit daunting at first and only
because i didn't understand the system but now that i do i understand the system there's training
material out there it's just a pretty simple 12 volt circuit with some resistors but it allows you
to tell where the brake is and that i could do it through the service mode on the car uh makes
this actually do simple like i'll take on another tesla high voltage diagnostics now i'm sure there's
way more way more to it than that right i got a simple one but i mean that part of it really
not that bad now if you're gonna go work on high voltage stuff do make sure that you've taken the
training and that you're careful and you know because there is a potential to hurt yourself
yes there's a lot of safeties built into the system like this right if there's a
break in the system shuts everything down won't turn on the contactors but it doesn't mean you
can't you know hurt yourself if you're being stupid so um do be careful there um one final note
that i wanted to mention um on the service information for tesla for the actual wiring
diagrams and component locations you can use all data and they do have tesla information
and there's nothing wrong with that works great you can also just go to service dot tesla dot
com and go into the service information and you can find wiring diagrams part locations
our procedures for any model um and you don't even have to sign in it's all free like i don't
pay a subscription for that so um that's all out there um again you can actually use get
some training material that's on there as well um ton of information on these things now the one
thing i will say is that the component locations and the wiring diagrams were both wrong on this
and i looked up in preparation for this podcast every single different build of this vehicle
because when you go to select or you're looking at information for a particular model it's going
to ask you what plant that it was built at and there's several different options and i did find
the one that this one was actually built at and i used that but i checked all the other ones
and the component locations are just wrong for some of the connectors the inline connectors
and then the diagram was just completely wrong uh on another portion as far as where the wire went to
for that hvio loop so i don't know if that was just me being unlucky on this particular one
or if that's something that you're going to run into more often on teslas but i'm using the factory
diagram there and all data just has the factory diagram all data had the same information that i
found um on the tesla website so just approach that with caution if you're getting into these things
but um i guess i'm just here to say they're not that scary it's not that difficult and
with a little bit of training you can figure these things out so that's when i wrap this one
up today um hey just like with josh and gm if you are you know a tesla technician or you've
got a lot of experience with them hey reach out um we are we are all smarter together 100
percent so um like thank everybody for listening both that out of the way let's get out there
start fixing the world one car at a time
About this episode
Sean Tipping wraps up a GMC Yukon case tied to GM Global B security-handshake failures during instrument cluster programming, where GM Tech Support and Ethernet/MDI2 suggestions didn’t help. An internal GM document shared by GM world-class tech Josh Hutchins led to a workaround: disconnecting a connector on the serial gateway module, running SDAC, then reconnecting before finishing cluster setup. The episode then pivots to a 2021 Tesla Model Y charging-disabled diagnosis at a body shop, using Tesla service mode to inspect HVIL status. The root cause was an HV connector not fully seated after repairs; reseating it restored HVIL readings and charging. Service mode, HVIL theory, and cautions around high-voltage work are emphasized.
This week on the show I follow up on the 2021 GMC Yukon from last week's episode along with a diagnostic on a 2021 Tesla Model Y that's not charging. See the links below for more details.
