This episode is about a 2018 BMW 640i. It’s a modern BMW with a lot of electronic systems, and the problem they’re diagnosing is that the car won’t crank/start.
The BMW 6 Series is a luxury car designed for comfortable long-distance driving while still feeling sporty. A 2018 640i is one version of that model, and it has a lot of modern systems that can sometimes need troubleshooting. That’s why a mechanic or diagnostic show might talk about it.
“No crank” means the engine doesn’t even start turning over when you try to start the car. It usually comes down to the car’s starting system not engaging.
The starter is the part that tries to spin the engine when you start the car. If replacing it doesn’t fix the problem, something else is likely preventing it from being commanded to run.
The start button is how you tell the car to begin starting. If the car seems to wake up but won’t crank, the problem is likely not just the button—it’s something in the starting command system.
“Key up” means the car recognizes the key and turns on its electronics. If it powers up but still won’t crank, the issue is likely later in the starting process.
The engine control module (ECM/engine computer) manages engine operation and coordinates starting-related conditions. In this case, the hosts note that the ECM isn’t directly in charge of operating the starter, but it still plays a role in the overall start sequence.
Transmission range is the gear position the vehicle reports to its computers (for example, Park vs Neutral). Many cars use this input to prevent cranking unless the shifter is in Park or Neutral, so an incorrect range signal can stop the starter command.
The anti-theft module (immobilizer/key authorization system) verifies that the correct key is present before allowing starting. The hosts connect it to the starting circuit by explaining that it has key/mobilizer data and can directly control starter power.
Term
mobilizer data
“Mobilizer data” refers to immobilizer/key-authentication information used by the anti-theft system to decide whether the car is allowed to start. If this data doesn’t match what the car expects, the immobilizer can prevent starter activation.
Data pins are the electrical connection points on a module connector used to read or test signals with diagnostic tools. Checking data pins helps determine whether a module is trying to command a function (like starter activation) or whether the expected signals aren’t present.
No-crank means the engine doesn’t spin when you try to start it. Mechanics work step-by-step to see if the car is asking the starter to run, if the wiring can deliver the signal, and if the sensors are telling the computer the right things.
An output circuit is the car’s wiring that carries the “do this” signal from the computer to a part. If that wiring is broken or loose, the starter won’t get the message even if the computer is trying.
Inputs are the signals the car’s computer gets from sensors and other systems. If the computer isn’t telling the starter to run, checking the inputs helps find what’s wrong upstream.
BDC means the car’s “body computer.” It watches inputs like your key and brake/start button and decides whether the car is allowed to start. If the BDC isn’t switching the right start-related signal, the engine may not crank.
The immobilizer is the car’s anti-theft system. It checks whether your key is allowed to start the car; if it doesn’t recognize the key properly, the engine won’t crank.
Terminal 50 is the signal that tells the starter system to engage. The diagnostic goal here is to confirm that, when you press the start button (with the brake pressed), the car actually turns on the starter-command signal—otherwise the engine won’t crank.
A “datapid” is a specific piece of information your scan tool pulls from the car’s computer. If the number looks impossible, it might be a real wiring/electrical problem—or sometimes the scan tool is misreading it.
“Terminal 30” is a wiring label for a constant power supply coming straight from the battery. On a 12-volt car, the voltage there shouldn’t jump to something like 28–29 volts, so it suggests a weird electrical situation or a bad/odd scan reading.
Sometimes a scan tool can show weird numbers that don’t actually match what’s happening in the car. The host is saying you should consider whether the tool is the problem before assuming the wiring is truly failing.
“Power and grounds” are the basic electrical supply points for a module: power is the voltage feed, and grounds are the return paths to the battery/chassis. Many no-crank, no-communication, or weird-voltage issues come from missing power, a high-resistance ground, or corrosion—so checking them early is a key diagnostic step.
A “voltage reading” is the measured electrical potential at a connector, wire, or module pin using a multimeter. In diagnostics, the exact voltage value (and whether it changes with key-on, cranking, or commands) helps pinpoint whether the issue is supply-related, ground-related, or a control/output problem.
GDI stands for gasoline direct injection, where fuel is sprayed directly into the combustion chamber instead of the intake port. “GDI diagnostics” refers to troubleshooting the specific sensors, fuel delivery, and injector-related faults that are common to direct-injection systems.
A “body domain controller” (BDC) is a central computer that manages many body-related functions—things like lighting, locks, wipers, and other comfort/convenience electronics. Because it ties together lots of circuits, water intrusion or a power/ground fault at the BDC can lead to broad electrical symptoms, including crank/no-crank conditions.
“Visual inspection” is a diagnostic method where the technician physically checks components and wiring for obvious issues like corrosion, water intrusion, loose connectors, or damaged insulation. It’s often fast and can immediately reveal the root cause before deeper electrical testing.
Corrosion is what happens when metal gets damaged by moisture over time. In a control module, it can mess up the electrical connections and cause problems.
Silicone here is a sealant used to help keep water out of an electronics box. If it’s not sealed well, fluid can get inside and damage the circuit board.
Wicking means the liquid creeps along tiny spaces in the wiring materials. So coolant can start at the reservoir and then spread into the car’s electronics.
When you replace a computer module in the car, you usually have to “program” it so the car recognizes it. Think of it like getting the new module’s settings and permissions matched to the vehicle.
Term
module 38
“Module 38” is the name of a specific part of the ACDP tool lineup. It’s basically the right tool/license needed to work with a particular BMW module version.
ACDP is a brand of aftermarket diagnostic tools used to communicate with and program certain BMW computer modules. The speaker says you may need the right tool version/license for the exact module generation.
A body control module is the car’s computer that runs a lot of the convenience electronics—things like lights and locks. If it isn’t set up correctly for the specific car, it can cause weird electrical problems.
When you replace a car computer, you can’t just grab any similar one—you need the exact correct version. The “part number” is how you make sure it’s the right computer for that car.
“All keys loss” means you’ve lost every key for the car. Without an authorized key, the car may need extra steps (and sometimes extra software access) to get the immobilizer working again.
Cloning means copying the car’s settings from the old computer to the replacement one. “One for one” means it’s set up to match the original module closely enough that the car can recognize it.
and a ton more. And once you purchase one of these training seminars, you can access this
at any time that you want through your account at automotive seminars.com. You can rewatch the
training a year, two years later, which makes this training even more valuable. So I highly
recommend you check this out. The link is in the show notes. Not a problem. I don't want to go too
crazy. But I have a feeling I'm going to be testing at this body domain controller anyways,
regardless if I find a power or a ground issue, right? I could also test the output to the starter
directly from there. I can test any other inputs, not exactly sure what those are going to be yet.
Because again, all the inputs seem fine based on the scandal, but I'm going I'm getting to that.
I'm going to put eyes on it. Sometimes a visual inspection is the winner. When you know what
components you're dealing with, take a look at them, inspect them visually, and you can jump
ahead to the fix a lot of the time. So anyways, do that, pull carpet back, pull panel off, get to
the BDC. And I'm going to start checking powers and grounds, which means I got to take connectors
off. And I do that. And I start pulling connectors off of this thing. And like I said, with a visual
inspection, sometimes you get lucky. And on connector to be, which if you're looking at the
body domain controller, you're facing it, it's the lower right connector on this BDC. This one is wet.
And like, more wet than a connector should be. I'm like, Oh, okay, maybe there's water leaking
down on this thing. And I look over the module, I don't see anything coming down. I don't see any
other liquid, except for like, inside of this connector, on the bottom right, this to be connector.
And I wiped the liquid off my finger, and it's not water, it's something that's got a little bit
more, it's a little bit more viscous, almost kind of sticky a little bit, not like grease or anything
like that. But it's not water, it's got a different consistency. And I was thinking, well,
what would be in here? And now this is where I'm starting to think, okay, is this fluid coming
not from the sunroof? Is it fluid coming in through the harness? So what I do is I take a look at all
the other connectors, and there's like nine connectors on this BDC, all the rest of them are dry,
except for this one to be. And again, it's got this kind of almost slimy fluid on it. And I
look, and I'm like, okay, is it it's on the connectors, it in the BDC as well. And I look,
and in that connector, you can definitely see fluid that's pulled up in the corner of it. So
now I'm like, okay, well, I'm definitely onto something. I don't know for sure what this is yet,
but that shouldn't be there. And it's probably inside of this module too, most likely. So I'm
right there. I'm going to yank this thing out. And I'm going to take the cover off. I'm going to
inspect the inside of the module, and look at the board. I do that often. When I get to a component,
and I'm like, okay, I need to look and see the board, you can find your solution so often.
When you've gotten to that point where you've got things pointing to that control module,
again, take it apart. If it's doable, if it's all sealed up silicone together,
little tougher and less likely to have corrosion inside, but this one's not like you can see through
the holes in the plastic casing and see the control board. So obviously fluid can get in,
it's not meant to be in a position where it's exposed to fluid. So pull it out, pull it apart,
a few screws, a couple of clips, take it apart. It has definitely been wet inside from whatever
this fluid is. You can see it, it's pulled up kind of in the corner. The board has remnants and is
actually still wet from whatever this fluid is, but I got to figure out what this fluid is. Okay,
so I'm pretty sure BDC is going to be the issue here. I can do some verification, but this is
not a good sign for it. And I can see a little bit of corrosion on the board too.
But let's look at this connector and see what's actually on these circuits. Now,
this does happen to be the connector that has the output wire for the starter.
Let's pin one, the black wire goes right to the starter. Okay, what else is on this connector?
And I see on pin, let's see what pin was that. Pin 24 was the washer pump for the
headlight system. And I'm like, well, maybe it's washer fluid actually wicking back into this.
But I'm looking through the rest of the pins to see if there's anything else. And I see pin 10.
And the reason I kept looking because I'm like, I really don't feel like this is washer fluid.
I don't feel like it's water. It definitely could be coolant though. And pin 10 is the
coolant level sensor. And that's in the reservoir. It's actually part of the reservoir right front
under the engine. And so I'm like, Oh, this is definitely coolant. Like once I saw that,
I'm feeling it. I'm like, yeah, that's what this is. This is anti-freeze inside of the body domain
controller inside the vehicle, right? This coolant reservoir and sensor is on the left front under
the hood. I don't know if I said left or right before, but it's the left front under the hood,
the reservoir, the sensor, and the body domain controller is right front passenger foot behind
a panel over there. So this fluid had wick through made it all the way through the harness and then
into the body domain controller. Now, we've seen fluid wick into modules, various other things,
right? Oil is very common to go into control modules, various cars. And I've seen coolant
Chevy Cruises very common to see that engine control module. And that's usually where you see
the stuff like coolant and oil is going into the engine control module. This one's a little
different. It's body control module, which is under the dash. Very interesting. But I went to the
plug and as I'm like pulling the plug out, I can see coolant dripping out. I'm like, oh, okay,
I think, I think we found it. So the level sensor is going to have to be replaced again,
part of the reservoir. We're gonna have to clean out the connector and the harness as much as we
can, but you can only do so much when there's coolant like literally inside of the insulation.
So the proper fix there is to cut the wire close to the module, solder it, and then seal it up
properly so that no more fluid can wick into the new module. And that's the second part of this.
So talk to the customer, hey, you're gonna need a body domain controller, coolant level sensor,
the cleaning, the repair to the wire, all that stuff. But of course, a new body domain controller
is like $14, $1500 cost at the dealer. I don't know exactly what it was, but it was quite a bit.
It is a 2018 BMW. So I mean, like, that's not that crazy. But of course, you know,
we got to program the whole car with it stuff, we're gonna do that. And, you know,
they're looking at, you know, a 23 grand all said and done with everything
to get this thing out the door and running. And so they talked to their customer,
they come back to us, and I'm not totally surprised that they did this for like, can you do a used
one. And I said, I don't know. Now, I've cloned a few fems and BDCs in BMWs before,
doing used ones, but they were older models. And this is actually a BDC two,
to newer version of it. And I have not done one of these before. So I'm like, well,
let me look into it. And it's mostly curiosity for me, because I want to know, can I do a used one
on one of these, can I clone that over. And turns out, I had part of the tooling necessary to do
this. There might be other options out there. But ACDP actually has a module for this. Now,
I had the, I think it's module three, which is the BDC cloning kit. And it actually hooks up
like the connection points are similar. But it doesn't read this particular module,
because that is for the older versions. But they have module 38, which ACDP, if you're not
familiar, it's you get the actual tool that hooks up to a tablet or a phone. And you could do the
programming there and can do key stuff. But for the actual connection and the license of doing
particular modules, you buy them on a modular basis, meaning that, hey, I do a bunch of old
BMW cast modules, I'll buy the cast module, or hey, I do these MSV 90s and these BMWs,
I'll buy this module, but I don't do body control modules in Jaguars ever. Okay, well,
don't buy the Jaguar body control module for that. And I kind of like that, you know, where you don't,
you can buy the whole kit and caboodle and get everything all at once, but you don't have to,
you can buy stuff as you need it. And as you see it, because odds are, I mean, just like a lot
of tools, you're gonna buy something that sits around and never it's used. This allows you to
order what you know you're going to use, or, you know, at least expect to use. And in this case,
hey, I got a car, I haven't bought this new module to do it. I'll give it a try. So we talked to him,
okay, let's source one, let's match the part numbers exactly. Okay, and then I'll get this
module for a CDP, no guarantees. But you know, if it works, it's X price, and that's a little
bit cheaper than buying a new one and doing that whole thing. We'll give it a try. So I ordered it.
And turns out it did actually work. It did the trick. And it took a little screwing around. It
wasn't exactly straightforward. They do have some stuff on their website. And the tooling does have
some instructions with it. But I had to play around a little bit to make this work. The one
hurdle that I kind of had to play around with until I got past it was the fact that the used body
domain controller that I got, did not have a key with it. Now, luckily, I had the key for the car
for the original. And I think that allowed me to do this. Had I not had that, I think additional
tooling or licensing would have been necessary. So just putting that out there, if it's like an
all keys loss situation, that makes things a little bit trickier. And not that it can't be
done, but I think it would require there's like an extra $200 license to do the all keys lost
version of this. But I was able to straight up one for one clone this thing with a CDP,
plug it back in the car, came to life, started up, zero codes, ready to go. Actually, it's
still at the shop, we're waiting for the reservoir to show up. But we did the rest of the repair,
getting it ready to go. My guys helped me get it all back together and stuff, but pretty interesting
one. And I wanted to share that on the show, both, you know, with the problem that I found,
interesting enough. But then also that, hey, there's solutions out there for, you know,
making used control modules work. Now, do we have to in that case, could you make the argument that,
hey, this car should probably get a new part, it's new enough? Sure. But you know, who am I to say
what people want to do with their money? And I like to know that I am actually able to do something.
So maybe those BDCs become discontinued someday, or the price just keeps going up for whatever
reason. I don't know. I'd like to know that I can offer an option to my customer, or I can provide
a solution where maybe there isn't one otherwise. And so now I know I have a tool that's capable
of doing this. And I know approximately what it takes to do it as far as time and effort,
so I can charge appropriately. And yeah, I thought it was pretty interesting. So that's where I'm
the feedback. Apologize if I don't get back to everybody right away. I do try to eventually.
But yeah, I'm a busy guy. I got a lot of stuff going on. So anyways, thank you so much.
With that all out of the way, let's get out there. Start fixing the world one car at a time.
About this episode
A 2018 BMW 640i comes in with a no-crank condition, and the host checks the push-button start logic and finds the BDC isn’t switching the starter command (terminal 50). Odd ~28–29V readings show up on terminal 30, but the real clue comes from a visual inspection: coolant has wicked into the BDC connector, traced to the coolant level sensor circuit (pin 10). The repair involves replacing the BDC and sensor, repairing/sealing the wire, and then cloning the module with ACDP—after which the car starts with zero codes.
This week on the show I share a case study on a 2018 BMW 640i that does not crank and has no relevant codes. We ended up getting the vehicle towed to our shop and during our diagnostics, we fond coolant in a module you might not expect. Listen to find out the solution.
