Low Compression or Network Issue?
Customer complaint: No Start/No Crank/Just Clicks
When I go out to get the car, the battery is completely dead. I hook up my jump box and the car cranks fine, but sounds like there is very low compression. It is fairly cold in our area right now. About 20°F.
So, I go into the shop to get the guys to help push this thing in, and on the way back out to the car they ask, "What's wrong with it?" I tell them what I found so far, and one guy says, "I bet the timing chain jumped." Another guy throws out a guess, "Either that or the cylinders are washed down."
Now in the shop, I hook up my jump pack and the flash programming charger so I can check for faults and write them down. There are a lot of faults that could be caused by a low battery, so I clear them all and attempt to start the engine. It has that signature sound of low compression, and this makes the tech in the bay next to me ask, "So, did it jump time or is it flooded?"
These are the fault codes...
The faults I circled are the ones that came back after clearing all faults and cranking the engine. For those not familiar with BMW/MINI modules...
DME is the Engine module MRS is the Airbag module KOM is the Instrument cluster LEW is the Steering angle sensor EWS is the Immobilzer module
As you can see, there is no communication with the MRS or the EWS, and the DME shows a fault with the immobilizer code from the EWS, and there is a K-bus fault. At my shop I have Alldata, Identifix, and of course online there is newtis.info for BMW/MINI vehicles. From these resources, the best I can tell is that the schematic of the K-bus looks like this...
(As a side note, you should be aware that not all the modules in the schematic are on this car. It depends on trim level and options.) I've seen K-bus faults on these early Mini's before, so I know exactly where I want to go. The splice connector on the passenger side under the door sill trim...
In the picture, I have the plastic cover already taken off the splice connector for testing (and yes, I made sure the connector was not grounding out on the body while performing my tests). Also seen in the picture, one of the K-bus wires has been disconnected in the past by someone and taped back. I've seen this done before for non-critical systems like the Sunroof or CD changer. If something like that is taking the K-bus down, sometimes the customer chooses to just leave it disconnected instead of paying for something they don't use.
So, lets scope this circuit to see what we have!
Huh, It's got some activity, but it sure isn't 0 to 12 volts like it's supposed to be. Its about 0 to 1.60 volts right now. So, here's my plan: I'll start disconnecting wires at the splice until my bus signal returns to normal.
Well, I don't know if you have ever had to do that on these Mini's, but it isn't just a simple pull them off the splice job. The metal connectors have a portion that is crimped and locked onto the splice, and further more, they blocked off the only place to put a pick in there to bend the metal back. Anyway, it's a real pain, I get a few of them off, but this is taking some time and frustration, and my signal isn't improving yet.
I start thinking about what else I can do that isn't so painful. I look over at the radio and it's an aftermarket radio. Hmm...sometimes that can be troublesome, especially if someone has been hacking away at wires. I pull the radio and in the mess of wires I see a white/red/yellow wire (that's the K-bus, remember from the schematic).
I disconnect it and the K-bus is back to normal...
Great! I'm making progress, but wait a minute, the radio was already disconnected and taken out. What is this connector? It only has three wires. I didn't see anything in the wiring diagram like this. Is it aftermarket? Nope, there is a BMW mark on the connector. I guess I'll try feeling around in the dash to try to tell where this harness goes to. I can feel something way down there. I pull it up and out and this is what I find...
Is this an aftermarket device? Nope...
Well, it must be what is causing our problem since that is where that harness leads to. I unplug it and reconnect the 3-wire connector that was previously unplugged, but the K-bus still isn't happy with that 3-wire connector plugged in. At this point, the radio is disconnected, the iPod interface is disconnected, and this harness doesn't go anywhere else. It must be in this little "add-on" harness itself, right? After unwrapping all the tape and inspecting this harness, I find no problems. Wiggling it does not work to track it down. If the wires are good, it must be inside the funky connector for the iPod interface...
Finally, I have tracked it down to the source! So, now that the K-bus has a nice, clean, square 0 to 12 volt waveform, I should be able to communicate to the EWS and MRS. Nope, still no communication to the EWS and MRS, and the DME is still complaining about the immobilizer code from the EWS, and I still cannot start this car. And, what about the comments from the peanut gallery about the low compression being caused by jumped timing or washed down cylinders. Are these guys going to be right about that stuff too?
Find out tomorrow, when I update this case study with the fix. In the meantime, if anyone wants to comment, you are welcome to.
Okay, time for an update. Nelson is on the right track thinking about how the scan tool communicates with the EWS. Think about this, the "no comm" issue is between the scan tool and the EWS and MRS. Does this mean that the DME can't communicate with the EWS or is it just the scan tool that can't? There are more networks in this car.
Also, think about the wording of the DME fault, "incorrect code". Is that because of a communication error, or did it receive the code and it was incorrect? Nelson is correct when he says the car should start without the K-bus. So, if the DME is receiving an incorrect code, how can we fix that?
An additional piece of this puzzle is that one K-bus wire at the splice that was disconnected and taped back by someone in the past. I mentioned that I've seen that done before to eliminate non-critical systems that are taking the K-bus down. Was my assumption about that right? Did you take my assumption and just accept it?😁
I'm running out of time on my lunch break. I'll update with the fix tonight, and we'll finally find out if the low compression issue comes into play.
Boy, this is a long one! Lets wrap this up quickly if we can.
At this point in my diagnosis, I got to thinking, and I remembered that the DME and EWS have a dedicated network for communication. Now, even if you did not know this, it is in service info if you looked it up 😉
The wiring diagrams for all the possible communication wires on this car is a lot to take in, so I made a very simplified schematic because I want to make it easy and clear about what the heck is going on.
You can see in my schematic that the green wire is a dedicated network for the EWS to send the immobilizer code to the DME. The problem we have is that the DME says the code is incorrect. This can happen with a low battey. The modules get out of synch. with each other. I asked earlier how do we fix that. Well, good ol' service info has the answer, and so do most BMW techs. It's a scan tool function. In Autologic it's called DME/EWS alignment. There's one problem though. When I try to perform that function, I get an error message that says there is no communication with the EWS, and here's why...
We already know we have this problem, and we haven't fixed it yet. The K-bus is up and running, but we haven't gained communication to the EWS. What could possibly be the problem. Wait! What about that one wire that was disconnected from the splice and taped back? I assumed that was an unimportant wire that was taped off in the past. Could it actually be for the EWS? You betcha! I reconnect it and the scan tool can talk to the EWS and the MRS.
At this point you may be thinking, "Well, that wire must have been disconnected for some reason." That's true, but for what exactly, I don't know. The K-bus signal was still all good with that one wire reconnected.
You may also be wondering, "How does one wire at the splice give you communication to two different modules?" Good question. I don't have an answer to that, other than the wiring diagrams I have access to must be wrong. It would have been nice to check it with ista, because I believe that scan tool shows the topology of all the networks in the car and it's more likely to be correct. (We have ista, but only use it on rare occasions. Autologic covers 99% of our needs)
Okay, moment of truth time. I perform the DME/EWS alignment. Success! I turn the key to start it. Yes! Fires right up, no problem. What about that low compression sound from before? Don't know, don't care. The engine started right up and is running just fine for several days now.
I'm with you on your diagnostic path Steven. You need to get diagnostics up before diagnosing the root cause (this vehicle should start even if the k-bus is down.) Looks like you can talk to the Kombi correct? The Kombi is the gateway to k-bus. Which leads me to this question...what kind of K-Bus signal do you have right at the Kombi? 16 Pin -> Kombi (Gateway) -> EWS (K-BUS) 16 Pin
Good comments Nelson. You are correct, the vehicle should start even if the K-bus is down. That's a good clue for others reading this. And yes, the OBDII DLC connects to the KOM. I have updated the schematic in my original post to reflect that. I did not check directly at the KOM, but it should have been the same as at the splice. In other words, there was no issue there. Another helpful
Love the approach! I'm interested in the ABS codes: - The first one suggests there is some type of CAN network, how does that one look like? - Does the second one mean that someone tried to jumpstart the vehicle with 24V? Could there be blown fuses from that?
Hey Dmitriy, Glad to hear you like the approach 👍 Yes, there is a CAN network in this car. I was wondering if anyone was going to bring that up. Although, I will say that I did not scope it, as that CAN fault code did not return after clearing codes and there weren't any other CAN fault codes stored, so I decided that was not a concern at this time. The fault for system voltage over 18 volts
Nice, once we know the KOMBI had a good K-BUS signal and the K-BUS splice had a good signal, next step would of been going right to the EWS which is what you did. Great post Steven. This K-BUS diagnosis post can be used for E46, E39, E53, E38, E85, and some other MINI Coopers where the Gateway is the Kombi. (Non MOST bus vehicles) Only difference would be if the vehicle has a 20 pin diagnostic
Intermediate update has been added to the original post.
Hi Steven: I want to make sure that I understand the implications of your post. Are you saying that the disconnected wire is only required if a DME-EWS Alignment is required; once aligned, it's not required unless the alignment is corrupted? TIA, Guido
The EWS may communicate on the K-bus for other reasons, but as far as what you stated, yes that is correct.
Hi Steven: I can imagine a couple of reasons why they may do this. The simplest explanation would be that by keeping the cypher generator (or whatever term they use) separated from the normal use handshake circuit, they can prevent code generation spoofing. The regular handshake would use the K-Line but the generator would use the CAN lines. (Granted, speculation on my part.) Thanks for the
Great post Steven. I always love a to read a good BMW/mini case study. Havent been able to try this yet however I just got back yesterday from worldpac class for BMW. Couple of us were talking with the instructor, who works for BMW, and K bus issues where brought up. A tip I was not aware of till he told me was a quick way to see if you K bus is down is to turn on the four way flashers, if the 4
That is correct. The light module in k-bus vehicles are hard wired to the turn signal stalk, outside lights, and hazard switch (a few other things also). The Kombi recieves the signal over the k-bus indicating that the driver requests turn signal operation, then the the kombi lights up the turn signal indicator. The kombi can't get that signal from the light module if the k-bus is down. and
Steven, Thank you for a clear and concise write-up on a topic that I find fascinating. Given a choice, I would do this kind of work all day, every day. May I ask how you billed-out your time for this repair? Many thanks.
Hello Greg, I'm glad you liked the case study. I'm not sure what the boss decided to charge for this one. I'm on hourly pay, so I don't pay much attention to hours billed. I let other people worry about that. 🙂
Also wanted to thank you for turning me on to the newtis.info site - very helpful!