352: 2018 BMW 640i No Crank, Coolant In A Place You Wouldn't Expect
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.
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2018 BMW 640i
"This was on a 2018 BMW 640i real low mileage, fancy sports car BMW, and it came to one of our customers, a repair shop, as a no crank situation."
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 640i is a 2018-era BMW 6 Series Gran Turismo that uses a turbocharged inline-six engine and is known for being a fairly complex, electronics-heavy modern BMW. In this episode it’s the subject of a “no crank” case study, where the starter won’t engage and the car won’t start.
BMW 640I
"...at we dealt with recently. This was on a 2018 BMW 640i real low mileage, fancy sports car BMW, and it ca..."
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.
The BMW 6 Series is BMW’s luxury grand-touring lineup, typically combining a sporty driving feel with a more upscale interior and comfort-focused ride. In the podcast context, a 2018 640i is a specific 6 Series model that’s often discussed because its modern electronics and performance systems can create interesting diagnostic challenges even with low mileage. That’s why it may come up in a diagnostic podcast—there’s plenty of technology to troubleshoot when something doesn’t behave as expected.
no crank situation
"This was on a 2018 BMW 640i real low mileage, fancy sports car BMW, and it came to one of our customers, a repair shop, as a no crank situation. They did what they could, couldn't really get a whole lot of direction, ended up replacing the starter, and that didn't change anything."
“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.
A “no crank” condition means the engine won’t turn over when you press the start button or turn the key. It points to a problem in the starting system (like the starter, starter control circuit, battery/grounds, or ignition/start authorization signals), rather than a fuel or spark issue.
starter
"They did what they could, couldn't really get a whole lot of direction, ended up replacing the starter, and that didn't change anything. It was still a no crank situation."
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 starter is the electric motor that physically cranks the engine by engaging the flywheel/ring gear. If the starter is replaced but the car still has a no-crank condition, the fault is often in the starter control/trigger circuit, power/ground connections, or the vehicle’s start authorization logic.
start button
"And of course, hitting the button in the car couldn't get it to crank. It did key up."
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.
A start button is part of a keyless push-to-start system that triggers the vehicle’s starting sequence. In diagnostics, if the car “keys up” but the starter still won’t engage, it suggests the issue may be in the start authorization or starter control path rather than the dashboard electronics alone.
key up
"It did key up. They had a key that seemed to work, but nothing would happen when you would hit the brake and press the start button."
“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.
“Key up” refers to the vehicle entering an “awake” state when the key is recognized—typically powering up the electronics and allowing the start sequence to proceed. If the car keys up but still won’t crank, it indicates the wake-up/authorization step works, but the starter engagement step does not.
engine control module
"You're looking at engine control module. However, the engine control module is not in direct control of operating the starter, but it's going to play a role, of course, and especially with the engine starting."
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
"Transmission and the actual transmission range as read by the vehicle, that's also going to be an important input to make sure that it's in park or neutral. If not, of course, it's not going to let it crank."
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.
anti theft module
"And our body domain controller, which is the anti theft module... it also has a mobilizer data, it has key data, and it is going to directly send power to the starter."
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.
mobilizer data
"...it also has a mobilizer data, it has key data, and it is going to directly send power to the starter."
“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
"But let's look at some data pins and see maybe what it's trying or not trying to do."
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 diagnosis
"And so anyways, my text out there takes a look at it. We did not get a whole lot of great direction with this thing. And we realized that we were probably going to be getting into quite a bit of testing"
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.
A no-crank diagnosis is the troubleshooting process for when the engine won’t turn over when you press the start button. The approach often starts by checking whether the car is commanding the starter (relay command), then tracing whether the output circuit can actually deliver power, and finally verifying the inputs that drive the decision.
output circuit
"And then if it is, I'm going straight for that output circuit, and seeing, okay, is there an open, is there a connector issue, something like that."
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.
An output circuit is the wiring path and electrical control components that carry a control module’s command to a device (here, the starter-related components). If the module is commanding the starter but the circuit has an open circuit or connector issue, the starter won’t crank even though the request was made.
inputs
"Or if it says no, I want to look back at the inputs and see what's going on there."
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.
Inputs are the signals the control module receives from sensors or other modules that determine what it should do. In a no-crank diagnosis, if the starter relay command is not being issued, the technician checks the inputs to see what information is missing or incorrect.
BDC
"And kind of the same mindset [562.5s] of like, okay, what's the BDC, you know, trying to do or not trying to do. And I get into it and [569.0s] from what I can tell, all the inputs are there, right, it's happy with the key."
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.
BDC stands for body domain controller, a computer that coordinates many body-related functions (like key/start authorization and input monitoring) and communicates with other modules. In a no-crank diagnosis, checking what the BDC “thinks” is happening helps determine whether the car is receiving the right start/permission signals.
immobilizer
"There's no immobilizer information that seems out of place. Obviously, it's keying up. [581.9s] Break pedal position, that's working. Park neutral position, that's working."
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.
An immobilizer is an anti-theft system that prevents engine starting unless the correct key authorization is present. During no-crank diagnostics, technicians check immobilizer-related status/communication to confirm the car recognizes the key and is not blocking the start request.
terminal 50
"And this is where, [600.6s] when I was saying earlier, like starter relay command, that's not BMW. It's not, you're going [606.6s] to see something called terminal 50 is that's the starter command for most European cars, [614.3s] but definitely BMW. And so you want to see terminal 50 switched on, or yes, when you [620.2s] hit the brake and hit that start button."
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.
Terminal 50 is an electrical “starter command” signal used in many European vehicle architectures. When you press the start button (and conditions like brake pedal position are met), the body controller should switch terminal 50 on to request starter engagement; if it doesn’t, you can get a no-crank even when other inputs look correct.
datapids
"But I did see something else in the datapids that seemed odd. And I'll tell you where else I've seen things like this before."
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.
“Datapids” are the specific data parameters (data IDs) that a scan tool reads from a vehicle’s control modules. If a datapid shows an impossible value (like 28–29 volts on a 12-volt circuit), it can indicate either a real electrical issue or a scan/tool interpretation problem.
terminal 30
"But for the terminal 30 numbers, they were all showing 28 volts on them, or is 29 volts, something that it obviously wasn't at. And this is not a 48 volt vehicle."
“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.
“Terminal 30” is a common automotive electrical designation for a constant battery feed (typically unswitched power). In a normal 12-volt system, terminal 30-related readings should align with battery voltage; seeing 28–29 volts on a 12-volt vehicle is a red flag that something is off (either the reading or the circuit behavior).
scan tool thing
"When you see a datapid that's off, you should be thinking, especially if you're using aftermarket scan tools, is this a scan tool thing? Or is this an actual problem?"
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.
The host is distinguishing between a real electrical fault and an artifact caused by the diagnostic equipment. Some aftermarket scan tools can report odd or misleading datapid values depending on how they interpret module data, scaling, or communication—so the diagnostic process should verify whether the reading is physically plausible.
modules
"It's tucked up under there. Now, I know modules in that location on BMWs and many other cars, of course, this has a sunroof. Water damage is very common."
A “module” is a computer in the car that controls a specific system. If it doesn’t get power or a good ground, that system may act up.
In modern cars, “modules” are electronic control units that manage specific systems (like body electronics, lighting, or engine-related functions). They communicate over the car’s wiring network, so a problem in one module—or its power/ground—can cause symptoms elsewhere.
powers and grounds
"I'm going to check powers and grounds to it, because it's fairly straightforward to do that. Make sure I'm not missing anything. I was thinking maybe, oh, maybe there's a missing ground or a poor ground or something."
“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.
voltage reading
"And it's causing this voltage reading. Again, maybe that's this show is brought to you by automotive seminars."
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 diagnostics
"They've got Keith Perkins, John Thornton putting on classes like network diagnostics, mechanical engine testing, advanced electrical testing and GDI diagnostics, and a ton more."
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.
body domain controller
"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?"
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
"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."
“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
"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."
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.
Corrosion is the deterioration of metal due to chemical reactions, often accelerated by moisture and contaminants. In electronics modules, corrosion can form on pins, traces, or internal connections, increasing resistance or causing intermittent or permanent failures.
silicone
"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."
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.
In automotive electronics, silicone sealant is used to help keep moisture out by sealing gaps around components and wiring. The host contrasts a “sealed with silicone” module (less likely to corrode internally) with one where fluid can reach the control board.
coolant level sensor
"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."
This sensor checks how much coolant is in the tank. If the tank is leaking, the coolant can travel through the wiring and end up inside electronics.
A coolant level sensor measures the coolant level in the reservoir and sends that information to the car’s control systems. If coolant leaks from the reservoir and then wicks through connectors and wiring, it can contaminate modules like the body domain controller.
wick
"So this fluid had wick through made it all the way through the harness and then into the body domain controller."
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.
“Wicking” is when a liquid travels through materials by capillary action—like how a paper towel draws water. Here, coolant wicks through the harness and into modules, which is why the contamination shows up far from the original leak point.
program the whole car
"But of course, you know, we got to program the whole car with it stuff, we're gonna do that."
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.
Programming a replacement control module means updating the car’s software configuration so the new module is authorized and correctly integrated. On modern BMWs, this often includes aligning module coding/firmware and ensuring the vehicle’s systems recognize the new BDC.
module 38
"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..."
“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.
“Module 38” here refers to a specific ACDP tool/attachment/license intended to support programming and connections for newer BMW module variants. The key point is that tool support is modular—different BDC versions may require different ACDP modules to read/program them.
ACDP
"But ACDP actually has a module for this. Now, I had the, I think it's module three, which is the BDC cloning kit..."
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.
ACDP is an aftermarket diagnostic/programming tool ecosystem used to interface with specific BMW modules for tasks like coding and cloning. In this segment, the speaker notes that ACDP has different “modules” (tool licenses/attachments) for different BDC versions.
body control module
"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."
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.
A body control module (BCM) is an electronic control unit that manages “body” functions like lighting, wipers, locks, and other convenience features. Because it also communicates with the rest of the car, a BCM mismatch or missing authorization can contribute to starting/immobilizer-related problems.
part numbers exactly
"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."
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.
Matching part numbers exactly is critical when sourcing used control modules because the hardware/software variant must align with the car’s configuration. Even if two modules look similar, a part-number mismatch can prevent proper communication or require additional programming.
all keys loss
"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."
“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.
“All keys loss” means the car has no remaining authenticated key fobs in its memory, so it can’t verify the driver’s authorization. That typically forces a more involved reprogramming/licensing process for the immobilizer and related modules, making used-module swaps harder.
one for one clone
"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."
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.
A “one for one clone” is the process of copying the relevant configuration/authorization data from the original module to a replacement module so they behave as if they were matched. For immobilizer/key-related systems, cloning can be the difference between a module that powers up but won’t authorize the car versus one that starts normally.
zero codes
"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."
“Zero codes” means the car isn’t showing any error messages in its diagnostic system. It usually indicates the problem is fixed.
“Zero codes” means the car’s onboard diagnostics (OBD) are reporting no active diagnostic trouble codes after the repair. That’s a strong sign the fault condition has been resolved and the replaced module is communicating correctly.
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