212: Say That Louder For The Module In The Back

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Hey, what's going on?

Automotive World.

Welcome to another episode of the Automotive Diagnostic Podcast.

My name is Sean Tipping and I'll be your host once again for today's episode.

Thank you for joining me Today on the show.

I am going to share with you a case study on a 2009 Chevy Suburban.

I know that's real exciting and it is a communication case study.

Oh, I've done a couple of those before too.

The reason I'm sharing it is to keep with the theme that I mentioned several episodes ago is the case studies that I share.

Obviously, I'll put the interesting stuff out there too when I run into new and interesting problems, but I want to share the ones that I struggle with, because that definitely happens time and time and I think we learn the most from the vehicles that challenge us the most.

And in this case, it's kind of different, because GM, and particularly GM, can bus networks kind of my bread butter.

I mean, I feel very confident walking into a communication issue.

If it's a hard fault on a General Motors vehicle, I mean even class two stuff but can bus stuff, I know it very well and I feel that I can assess the system very well.

Well, there's always a car out there to prove you wrong.

I did fix this one and it didn't end up being anything earth shattering, but I wasted about an hour of time.

And on a busy day when you got a lot to do, that is a lot of time.

And then, especially when you get to the conclusion, you're like, wow, I did not need to spend that additional hour here on this vehicle.

But that happens, it happens to all of us.

I know it happens to me.

And I think the important part here again is I'm sharing this with you.

But it's beneficial for me to go back over where I struggled so that I can identify that for the future, correct for that in the future, figure out exactly where I went wrong, what my mindset was in the moment that put me down the wrong path, and how to correct for it in the future.

So, anyways, that's why I wanted to share this.

This is a 2009 Chevy Suburban.

The shop actually called us to program a fuel pump control module.

My employee, steve, showed up out there to do so, immediately found out can't talk to the fuel pump control module, okay, so this thing was outside and we had a busy day, and so we told them hey, we don't have time for a diagnosis today, but that's what it needs.

Do you want us to come back and look at it?

And they're like well, we'll look at it.

You know, because we determined that the fuel pump control module was bad, we'll look at it, we'll see what we can do.

So they call me back the next day and say hey, we looked at this vehicle again and we determined the fuel pump control module that we put in there was an aftermarket one, and that was the problem.

We have now put in a GM, an OE fuel pump module per the shop.

So they that's what they tell me and we're good to go now.

Could you come out and program this thing?

And they said do you have communication with it?

They're like yeah, okay, cool.

So this time I actually showed up at the vehicle is just how our scheduled worked out the next day and I hook up to it again for programming and find the exact same thing that Steve did I can't talk to a fuel pump control module.

Okay.

So I go back in say, hey, you know, I'll come show you, but I can't talk to this thing and so I can't program it if I can't talk to it.

That's true in most Module scenarios and I know for a fact a fuel pump control module on an O9 suburban should definitely talk to my scanner or my Programming device in order to program it.

So we need to figure out what's going on.

So, hey, do you want me to dig into this?

And I have found doing this for a while now that when shops are at this point, you know and they've called somebody out a couple times, potentially, and they've gone through a couple different modules they're just like, yeah, just just look at it, just figure it out.

And that's a service we provide.

We'll get you the answer one way or another here.

And so I'm gonna dig in again.

I feel real confident in GM can bust stuff, and I've talked about it on the show a ton and I've given examples of how I assess it and how I think about it and Anyways, again, this one kind of threw me for a little bit of a loop.

So my two Big assessments I'm gonna make of this system and I already kind of did one is a full vehicle scan and really paying attention to the modules that are on the can bus.

And you can figure that out Sometimes within your scan tool, depending on the topology setup or a diagram to figure out who's on the can bus.

And I'm gonna go through and see who I can talk to and who I can't talk to.

I already know I can't talk to my fuel pump control module, but are there other modules that I can't talk to?

And this becomes Increasingly important when we're talking about a GM can bus system because of the fact that the network is daisy-chained, meaning there's a In I say this in quotations and in and out to the network on each module connector.

Now I realize the network traffic is.

It's not an in and out.

That's not how the actual communication works.

But if we think of the network Like a stream and I say that intentionally because when I'm referencing modules on a, on a diagram, a lot of the time I will say upstream, downstream, then we start thinking of it is potentially an in and out right and again.

It's kind of like the water analogy with electrical it works until it doesn't.

We know it's not perfect, but it's a good way to think about it.

Right, and I kind of take that into GM can bus stuff too, in and out, and what I usually consider upstream is my, my DLC right six and fourteen can bus lines at the DLC.

That's the starting point for me.

Now there's a ton of different variations in network construction and sometimes the DLC can sort of be placed in the middle of the stream, if you will.

But I Still consider it upstream because that's where my scan tool is and going to be connected to the network and it's also where I'm gonna probably start my testing.

Okay, and what I mean by actual electrical testing, I'm probably gonna start it there.

So again, full system scan.

What can't I talk to?

What can I talk to?

Based on my initial scan, the only thing that I for sure know that I can't talk to is a fuel pump control module, and I say that because I can't, and I know it has one on the car and there's other modules, the engine control module, specifically Indicating that, hey, I cannot talk to the fuel pump module as well.

Right, and the truck doesn't start, but the ECM says I can't talk to fuel pump control module.

There's a code in there indicating the same thing that I'm seeing.

Now We'll get into the construction of the network, but there are some modules that could potentially be Downstream from the fuel pump control module.

I Can't talk to those, but I don't know if everything's equipped on this vehicle.

And again, I'll get into the network construction.

But this is just my initial scan and what I see Definitely can't talk to fuel pump control module, not identifying anything for sure that I can't talk to.

But I'm not 100% until I look at a diagram.

So here's what I'm gonna do, even before I pull out a diagram.

My very next step is I'm going to unplug my scan tool, I'm gonna turn the key off, and the reason I'm turning the key off is I want the voltage on the can bus to drop down to zero and I'm gonna pull out my meter and the first thing I do is I will actually Verify that the voltage on the can bus lines drops down to zero, and on an 09 Chevy That'll take maybe a minute or two If it's something that

isn't dropping down to zero or a vehicle that just takes a really long time to go to sleep.

Maybe it won't go to sleep with the door open.

Well then you could disconnect the battery and that's for sure gonna drop the voltage down to zero.

Now, why am I doing this?

I'm gonna own check the network from the DLC.

I do that on pretty much every no-com scenario on a GM to see what's my network integrity.

Do I have an open somewhere in the network?

Because on a GM, if you have an open, because of the way the network is constructed it's daisy-chained, right or stream, if you think of the ECM is Almost always gonna be on one end, and correct me if I'm wrong, but just about every GM can bus that.

I see a terminating resistor of 120 ohms is on one end and Then there's another module at the other end or a resistor by itself on the other end of that network.

Right, and so that's our stream, if you will, and we're tapped into it at the DLC and If everything is plugged in and all the wires are good the whole way through, we should have 60 ohms there, right, we know this.

This isn't big news.

And If there's an open, a module unplugged or a wire broken or however, that network opens up.

Well, then we go to 120 ohms or potentially OL.

You know short, right, we'd have zero ohms.

But I'm really looking for an open here and, and I found it, I had 120 ohms at DLC, right, okay, so that's, I got a module I can't talk to him, and I've got an open network, from what I can see.

All right, cool, so I'm on the right track.

I can find this thing.

Now I really want to understand the network construction Specifically what modules are where, who should I be able to talk to, so on and so forth.

Now this is where I'd recommend using an OE diagram for GM specifically Because and in a Chevy suburban scenario, even more so because a suburban or big SUV is gonna have a lot of different options For modules, things that could be on the vehicle but might not be a quick equipped.

On this particular application, if you use the redrawn diagram, it's gonna show you all the modules and it may not be that great at indicating what is present on all and what is present on some, and and how do you find out the difference?

The redrons, in my opinion, aren't always great on that, but the factory OE diagrams are very good, and they will show you all the modules that could be on that network, right, optional modules that may or may not be equipped electronic suspension, trailer brake control module right, and I'm telling you those specifically because that's what I'm gonna be looking at for this vehicle.

But within the diagram they're gonna use RPO codes, right, and if you listen to an episode a couple episodes back and I mentioned a site that you can find out some GM RPO stuff and I had a ton of people message me about that, so I know people were listening you can use that website to do the VIN attributes and reference your RPO codes.

So I'll have a couple windows open.

I'll have my VIN attributes right there as I'm scrolling through the diagram.

Okay, so I put in the VIN for this truck.

I'm looking at the diagram and I want to know does the electronics suspension Control module exist?

Nope, it doesn't.

Does the trailer brake control module exists?

Nope, it doesn't.

Why am I looking at those two modules?

And I could be looking at others, but specifically I'm looking at those two because I'm looking at my network construction and again, if you think of a stream and it helps that I'm using this reference because this is audio only as we move down the line.

We don't really know if our open is on the engine control module side of the network it could be, but I can communicate to it, so it's probably not or If it's on the fuel pump control module side, because that's on the other end.

If you will right the Imagine in your mind that ECM's over on the left and our fuel pump control modules gonna be off to the right of our Stream and we've got multiple modules in between.

And so what I can do is I can go down the list from the DLC and look at all the modules and again I have to figure out are they equipped on the vehicle or not.

But I can do a couple things here.

I can use the scan tool, make sure I can talk to them if they are in fact equipped on this vehicle, and I know that I can't talk to my fuel pump control module.

So at that point the network appears to be open either before well, it would have to be before that fuel pump control module.

But I also just want to see what else is past it.

Am I missing communication with any other modules?

Is there anything in between the fuel pump control module and the last module that I can talk to Right, and I'll tell you, the ABS control module is the last thing that I can talk to on that network.

Now, after that it goes fuel pump control module, trailer brake control module and electronic suspension control module.

And then finally there's a data link resistor which lives all by itself.

It's a little two pin connector up on top of the cross member, the very rear, most cross member, on the passenger side.

It's stuck up on the top and has a little plug in.

That's where this guy lives.

He's the end of the network.

It's not in a module, it's 120 ohm resistor.

It's just sitting there up on top of the frame.

But again it goes ABS, fuel pump control, trailer brake control module, electronic suspension control module.

Now, that is, if the RPO codes indicate that the vehicle has it.

This one does not have electronic suspension.

You could look underneath and tell that too.

This also does not have a trailer brake control module.

Again, you could look at the dash and figure that out, but I always like to use the RPO codes to verify that I'm not missing something.

Okay, now there's even an option on this vehicle, depending on the engine, that it may not even have a fuel pump control module.

But this one definitely does.

There's a module down there.

It's got a plug in and the truck doesn't run.

It has no fuel pressure.

So we know that there should be a fuel pump control module on there.

But you got to pay attention to those RPOs because there is an option for the Ly6 where it doesn't use a fuel pump control module.

So, anyways, the last module I can talk to is ABS, and the only other thing downstream of that is our fuel pump control module and our data link resistor.

Okay Now, my assumption here is that the open is between the electronic brake controller and the fuel pump control module, and the reason I say that or think that, is that the open was after the fuel pump control module.

We get the same resistance reading for the network, right, let's say.

Let's say somebody unplugged the data link resistor and left the rest of the network.

Fine, in that case I should be able to still talk to my fuel pump control module.

I would think right, just because we've taken some terminating resistance away does not mean that network can't function at all.

We should still be able to talk to all of the modules.

Maybe there'll be some erroneous errors, I don't know.

It's hard to predict exactly how each different network is going to react when we change that terminating resistance.

But where I'm going with this is I don't believe the open is after the fuel pump control module.

I'm going to focus on the stretch of wire or the connections to the ABS and the fuel pump control module and, as luck would have it, they're about a foot apart from each other underneath this truck, so I did have to get it up in the air, but they're right there, and so that's what I'm going to be looking at is I have an open network.

I can't talk to my fuel pump control module.

Let's start looking at these wires.

So I get under there and I get the fuel pump control module disconnected and there's a big connector on there, and one of the things I'm looking for right away is pin fitment and also the way that the module seats when the lever is completely latched.

All right.

So the first thing is for pin fitment.

Big deal on GMs, a big deal on connectors that live underneath vehicles like this fuel pump control module, and we can have a female terminal that spreads out and doesn't make proper contact.

I've seen that happen, so I'm going to be looking for that.

I'm going to do a pin drag test Very easy to do once I find out where my can lines are.

Now the other thing I've seen on these these modules use a big lever, like a lot of modules do, just to actually like draw the electrical connector into the module and lock it into place.

Okay, this and a lot of other control modules of, even of other brands, has issues.

When we get sand and dirt build up and also the ceiling, the rubber, weather packing seal within the connector itself can kind of become swollen and actually become an obstruction to this connector fully seating Right.

And what will happen see it a lot is these levers will latch all the way and they'll click.

But the connector didn't fully see and if you look at it you can see it's either a cockeyed or it's just not all down all the way for one reason or another.

Again, the there's dirt and debris in there and maybe the lever actually broke because of that.

It's not moving the connector Right.

Chrysler has that on their PCMs.

Their slides don't go all the way in.

The lever comes down at latches but the slides didn't go in, so the connectors don't actually make contact.

But again, I seen this with the seal that's in there.

So I'm going to be looking for that and that all seems to be okay.

And I want to look for the same thing on the ABS right, because we've got can lines that go from the ABS to the fuel pump control module.

So I had a little bit of trouble here and this came down to the diagram, the pinout, and I actually wish I had the vehicle in front of me right now.

When I was working on it I got screwed up on which can lines went which direction from the fuel pump control module Right.

So we have two that come from the ABS unit and we have two that go way back to the end of the vehicle with data link resistor and somehow or another.

I did get those screwed up and I looked at the pinout and I was really certain on which one I was on.

But I eventually did figure out the correct pins.

But just, I guess, take caution on that, because I think the pinout is incorrect, at least the one I looked up on identifix.

Anyways, I ended up determining that the wires between the ABS module and the fuel pump control module were intact.

The pin fitment was good, everything was there that I needed right, and I tried reseeding these connectors a couple times, making sure that they're making contact with the actual pins that they're supposed to, and I still got my same reading.

I was checking up at the DLC at this point and still, getting 120 ohms open network, still can't talk to the fuel pump control module.

Man, this is weird, you know.

And again, I'm not super concerned about what's going on downstream just yet.

I'm really focused on between those two modules because in my mind that's what I thought I was going to find and nothing I was doing was making any difference.

But I still couldn't talk to my fuel pump control module.

So now that I'm down there at the connector, I'm like, okay, well, I'm right here, let me just check powers and grounds to this thing right.

We obviously need those for communication and I don't want to overlook that, and I've got the connector off already.

This is a great time just to verify that I've got everything to this module that I need for it to talk, right.

Because now I'm starting to think, well, maybe I got, you know, open downstream of this thing.

So I do that and I have a power feed missing, the main power feed to this control module.

It's a red white wire that comes from a fuse in the under hood fuse block and I'm using a test light and won't light up.

Right, I have my wake up, I have my, I have my ignition, I have the CAN bus lines, from what I can find, but I don't have any power on the red white wire.

And I check to make sure.

Okay, yep, keys on batteries charged.

Okay, I should have power there.

Where's this come from?

Under hood fuse box there's a 20 amp fuse.

I checked this with my test light.

It has power on both sides.

But when I pull the cover off of this fuse box, it looked like this thing was like growing green crusties out of the little empty holes where fuses and relays could be, and so I yanked this fuse out of there and one leg of the fuse was actually corroded, like inside where the metal part of the leg goes into the plastic part.

It was all full of corrosion and so there was power on both sides of the fuse, but not going all the way through it, so this was easy.

So I swapped in another fuse.

Right Now, this is where I kind of got screwed up.

This is where where I wasted more time because I fixed the problem with that fuse and I verified that I was like, oh okay, cool, I got power back here.

I plug the fuel pump control module back in.

Now what I should have done here hindsight, future Sean talking here I should have tried to start this stupid truck and it would have started at this point.

I should have checked fuel pressure.

I would have had fuel pressure had I cycled the key.

I don't think I cycled the key here in the midst of it, but I should have done that and this thing would have started.

Okay, but I didn't know this past.

Sean has no idea that that's the case.

He's just like I should be able to talk to this control module now.

So I've got my top down plugged in and I do want to make a point that I was using my top down, the Phoenix smart on this one and I go, when I click on the fuel pump control module, no communication.

Okay, that's weird.

And I do my vehicle scan.

I could talk to everything else.

Now I did not look closely at what codes were present in other modules.

What I should have done is look to see hey, is the ECM still upset about not being able to talk to fuel pump control module.

That's a little bit of a stretch, but that would have been verification.

But at this point I can't talk to the scantel.

I couldn't talk to it before.

I don't think that I changed anything.

Plus, I still know in the back of my head that I do have an open in the network and I'm like, well, that's really strange that there, I mean, there was 100% no power feed to this control module.

Now I'm thinking I'm like I got to look at this, this open in this network.

I feel like maybe I missed something, because I was already thrown off by the pin locations on the fuel pump control module.

I know that I have an open like it should be fixed before this thing goes down the road.

And this must be causing the no calm with this module.

I click on it all day along with my top down just says no communication to it.

All right, well, let's, let's dig in and let's figure this out.

So I am finding out that there was an open going past or downstream from the fuel pump control module, and this took me a little while to really verify because my thought in my head at this point was I still have an open and it's between the ABS and the fuel pump module and it wasn't.

And I went through and I checked it three, four times and I'm like, okay, I should be able to talk to this thing again, even if there is an open.

But the open was, for sure, downstream and I started looking and I was looking through, the harness kind of went up over the tank and I wasn't about to drop the tank or really get into it.

This thing was all full of dirt and stuff.

So every time you grab something, you had dirt falling on you and I'm thinking I'm like, do I really want to tear apart this harness to find this open?

And is that really what's causing my issue?

Because at this point and I've wasted some time here for sure I'm really debating whether I want to tear into this further because I don't know that it's going to change anything about this.

I feel like I'm missing something here.

So I took a moment and I thought about okay, well, what can I do here before I start tearing into this harness to find this open after this module?

I don't think it's going to change anything.

Anyways, I plugged in my breakout box to the DLC and I have some resistors in my van and I took a 120 ohm resistor and I put it across 6 and 14 at the DLC.

Okay so the breakout box I got a couple banana jacks I can plug into and I just go across with a 120 ohm resistor across 6 and 14, while my scan tool is teed in to the breakout box and I didn't do anything else different.

I hit that button and I can talk to that module and I'm like what the heck I was like so, wait a second.

So this thing was online and it was after I fixed the fuse, which at this point in time this was like 45 minutes after the point that I had fixed this fuse.

When I fixed that fuse I brought the module back to life.

I could not talk to that module with the particular tool I was using, which was top down launch.

I have had problems with this before on GM trucks With the terminating resistor not there, with 120 ohms of resistance, terminating resistance the network was full of some garbage.

I scoped it, I put my u-scope on it, I looked at it and then I put that 120 ohm resistor across there.

So now we have a total of 60.

It cleaned up the network significantly.

The rest of the modules on the car on the truck didn't care that open between the fuel pump control module and data link resistor at the end of the vehicle had been there for who knows how long they've been driving this thing around like that with no issues.

The only reason that it stopped working is because this fuse opened up in the fuse panel and so this thing had a pre-existing problem.

If you want to call it there, I would call it that.

Right, it could still cause some erroneous network issues, but guaranteed, the guy has been driving it around with no issues there.

But when I put that extra resistor across there, now my scan tool can talk to it.

Now why my scan tool could only not talk to the fuel pump control module?

Because I could talk to the ABS, I definitely tried.

I could talk to the ECM, I definitely tried.

I could talk to the BCM, I definitely tried.

All of those while the network was open, no problem.

But the fuel pump control module it wouldn't.

Is it because it's at the end of the existing network?

Does that have something to do with it?

Is there something about its messages that are just a little bit more scrambled without that resistance?

And my tool that I was using at the time just wasn't up to snuff.

Now I could say this in the past with launch tools where I've had issues communicating to GM trucks.

I kind of been through this before.

I don't use my my launch Diagon that I used to love and I didn't use it on GM trucks of this era because I had all kinds of communication issues that didn't exist.

The tool and built it in the hotel did a really good job here of not caring about the fact that there was an open here and I tried it and the auto would communicate with it.

So of course I sell top down tools, by the way, if you'd like to buy one.

In this case, in that specific circumstance, you know, I just kept hitting that button.

It wouldn't talk.

In that pre-existing condition really threw me for a loop, really had me second guessing myself.

But all that's all.

That being said, this thing needed a fuse, and I mean probably a fuse box eventually on this thing too.

But it used to wake up that fuel pump control module and then it also had an open between the fuel pump control module and the data link resistor.

But that really wasn't causing any issues for the truck, at least not for it to start, but explain it all the shop.

They were very understanding.

You know.

Obviously the fuse portion is fixed and they said they might look into the data link resistor.

I said there's nothing else on this network.

You could wire up that data link resistor a few inches from the fuel pump control module and you'd be all good if you wanted to do that or run new wires, whatever.

I was like, should it be fixed?

Yeah, I think so.

You could have random issues, but it doesn't have to be, because it's probably been like that for a while.

So I don't know what they did with it.

But that's my story, that's my fun on it and, like I said, I wasted about an hour of time on something that I feel like I'm really confident in.

So that's sort of stuff that happens out there and I wanted to share with everybody.

So, hey, if you got a story of you getting your butt kicked, I want to hear about it.

Right, interesting case studies are cool, like the really cool stuff.

I love that too.

But let's hear about what you get in your butt kicked and how did you get through it.

What did you learn from it?

That's the kind of stuff I want to know.

So reach out and put in the Facebook group.

There's a link in the show notes, but other than that, that's all I got for you.

So let's get out there, start fixing the world, one car at a time.

Chevrolet Suburban

Lucid Air

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