CANBUS Problem or Not?

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crunch81
Junior Member

Posts: 75

CANBUS Problem or Not? Aug 23, 2020 9:36:27 GMT

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Post by crunch81 on Aug 23, 2020 9:36:27 GMT

Hi People,

I have a very difficult question (for me), Here is some info:

Problem: I have a DTC in the scan results all the time. I always let it be because it was not causing me any problems.

The DTC which would trigger is "Invalid Data Received From ECM/PCM A" U040100 in module Brake booster

Now Last days I got multiple faults (alsways the same) with info in Display "Recuperation problem, see manual" (E-Golf) (recuperation is handled by the brake booster control module)
The DTC which triggered is "Brake Booster Control Module Implausible Signal" U103F00 in the engine module.

So, I first thought for sure to know the Control module on the Brake booster was bad, so got a second hand one and want to replace it (Very very %^&^((* Job)

I also am measuring all the wires for loose connections, bad ground and most important CANBUS problems.

Through the CANBUS you can measure voltage but the first thing to do is measure Resistance with battery disconnected.

Underneath this post I have uploaded the schematic.

This is my problem:

-T26f/25 is Can High from Brake Booster NX6 to Canbuswiring in main wiring harness.
-T26f/16 is Can Low from Brake Booster NX6 to Canbuswiring in main wiring harness.
Measurements between Can High and Can Low is 60 Ohms.

-T26f/18 is Can High From VX70 Brake Sytem Pressure Accumulator to Brakebooster
-T26f/10 is Can Low From VX70 Brake Sytem Pressure Accumulator to Brakebooster
Measurement between Can High and Can Low is 120 ohms.

Now, you see the resistance in T26f/10 and T26f/18 is not OK. It needs to be 60 Ohms.
I'd assumed that it could be that the CANBUS wires from the Brake System Pressure Accumulator were handled by the T26f connector of the Brakebooster, so I connected it back to the Brakebooster and hoped to see 60 Ohms
but nothing changed, it stayed on 120 Ohms.
I aslo checked T26f/18 to T4et/2 with multimeter (bleeping to check wire interuptions (don't know how it is called)
I also checked T26f/10 to T4et/3 with Multimeter for interuption
I aslo checked T4et/1 to positive
I aslo checled T4et/4 to ground

If I read the schematic correct, it seems like the brake booster connects the CANBUS wires from the Accumulator NX70 to the cars CANBUS system?
But if that is correct, then I need to measure 60 Ohms right?

My questions are:

1. Is 120 Ohms correct in this situation and why?
2. If it is not what do I have to do? What CAN I do?

Thanks a very much!!

Last Edit: Aug 23, 2020 10:11:40 GMT by crunch81

dv52 (Australia)
Moderator

Posts: 6,070

CANBUS Problem or Not? Aug 23, 2020 21:54:18 GMT

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Post by dv52 (Australia) on Aug 23, 2020 21:54:18 GMT

crunch: I really don't understand your measurements. Your measurements identify only one pin. The multi-meter has 2 x leads. Tell us where you placed each lead for each measurement (i.e. identify both pins on your wiring diagram for each measurement).

Second: as a general question - why do you suspect a CAN problem? Based purely on the DTCs that you describe, neither "invalid Data", or "implausible signal" suggest (to me) a CAN bus failure? Both these DTCs suggest (to me) that CAN messages are being sent and received as normal but that the information in the messages is not what the ECM is expecting. I don't know why the data is not as expected - but these errors don't suggest (to me) that there is a problem with the CAN bus!!

As I said my my reply to your other post, if the CAN bus, itself was not operating, you would see lots of errors from lots of modules and if the CAN part of the Brake module itself was faulty, the hex03 module would not appear on your scan at all and there would be DTCs in your scan report from other modules complaining about no communications from the Brake module!! Does your scan report contain these types of DTCs?

Don

Last Edit: Aug 23, 2020 22:01:28 GMT by dv52 (Australia)

Please don't ask questions about car coding/repair via the forum "Message" function. The better place for such dialogue is in the forum proper. That way, the wider expertise of the entire forum community can be applied to solve your problem - thank you

crunch81
Junior Member

Posts: 75

CANBUS Problem or Not? Aug 23, 2020 22:08:58 GMT via Tapatalk

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Post by crunch81 on Aug 23, 2020 22:08:58 GMT

Hi Don,

You are right, it’s not very clear in this way. I edited my post, this is what I did:

-T26f/25 is Can High from Brake Booster NX6 to Canbuswiring in main wiring harness.
-T26f/16 is Can Low from Brake Booster NX6 to Canbuswiring in main wiring harness.
Measurements between Can High and Can Low is 60 Ohms. Lead one on t26f/25 and Lead two on t26f/16

-T26f/18 is Can High From VX70 Brake Sytem Pressure Accumulator to Brakebooster
-T26f/10 is Can Low From VX70 Brake Sytem Pressure Accumulator to Brakebooster
Measurement between Can High and Can Low is 120 ohms. Lead one on t26f/18 and lead two on t26f/10

As for your second question: because I was checking all the wiring and have a wrong reading in two canbus wires I assumed my dtc’s must have had something to do with the wrong ohms reading on the canbus wires, at least I thought so till I saw your answer.

As for your third question: No, There are no DTC’s were your are referring to in my scan results, Only the ones in my first post above (I do have more but those have nothing to do with this situation)

Greetings Crunch

dv52 (Australia)
Moderator

Posts: 6,070

CANBUS Problem or Not? Aug 24, 2020 5:23:57 GMT

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Post by dv52 (Australia) on Aug 24, 2020 5:23:57 GMT

^^^^^ crunch: hmm.......... OK, now I understand your measurements - thanks. The answer to your dilemma is that the two sets of CAN pin-pairs on NX6 are NOT the same!!

Here's why :

Look at your wiring diagrams and notice that T26f/ 16 and T26f/ 25 are connected to the Powertrain CAN bus (see the circled numbers). This is what's called a high-speed CAN bus because it can operate at message speeds of up to 1 Megabits per second. High speed CAN buses use the circuit arrangement that I posted previously whereby there are 2 x 120 ohm resistors wired accross CAN_high and CAN_low at either end of the twisted pair. As I have shown on your other thread, this means that you should measure about 60 ohms resistance with a good quality multi-meter -but ONLY on a high speed CAN bus.

Now, look again at the other diagram and notice that T26f /10 and T26f/18 are connected to "Connection 1 (CAN bus high/low), in engine compartment wiring harness". Notice that this description does NOT identify any of the known high-speed CAN buses that are networked across the car - the words on the circled numbers simply identify a "CAN" bus. This means that these 2 x pins on NX6 are for local communications only - not for communications on the general CAN networks. In this case, these 2 x pins set-up a very confined CAN circuit with Vx70 - and only with Vx70!!

So, T26f /10 and T26f/18 form what's called a low-speed CAN bus and it differs from high-speed CAN networks in that the speed is only 125 K-bits/sec max. However, low-speed CAN has a special advantage in that it has fault tolerant capabilities. This enables the CAN bus to keep operating even if one of the wires is cut or short-circuited.

The exact same set-up could have worked if communications between Nx6 and Vx70 was via LIN protocol (which only needs one wire) - but LIN doesn't have the fault tolerant feature. This feature is why low-speed CAN is used in the Brake function - for safety reasons and because high volume data transfer is not needed (i.e. the Brake function continues to work even if one of the comms lines between the servo and the pump fail)

Anyhow, to explain your measurement results - for low-speed/fault-tolerant CAN, each device on the network needs a termination resistor for each data line: R(RTH) for CAN_H and R(RTL) for CAN_L. like this:

This is very different to high-speed CAN termination; low-speed CAN requires the termination to be on the transceiver rather than on the twisted pair.

So- I wouldn't worry about the 120 ohm reading

Don

Last Edit: Aug 24, 2020 6:03:41 GMT by dv52 (Australia)

crunch81
Junior Member

Posts: 75

CANBUS Problem or Not? Aug 24, 2020 7:01:15 GMT

Quote

Post by crunch81 on Aug 24, 2020 7:01:15 GMT

Don,

First off all I want to make clear that my answer is not because I doubt your answer, It is because I want to know and learn what I have missed in the Electric Diagrams.

So, the only way to know if you are dealing with a canbus on the car’s network or a local canbus network is the words:

-“In MAIN Wiring Harness” means a canbus in the cars network

-“In ENGINE COMPARTMENT Wiring Harness” means a local canbus/ low speed canbus just for 1 node

There are so many details on the electrical schematic, I wish they would put some more details in the

description on the right side. If they only used the words Local Canbus or Low Speed canbus it would make it clear right away but the phrases “engine compartment” and “main” is only to understand if you are a highly qualified technican.

There are also no ways to get to this information, I searched the whole internet.

If you were not here with your knowledge then I would have seen this as a faulty Canbus and had have hours of extra search work which was not leading me to nowhere.

Greetings Crunch

dv52 (Australia)
Moderator

Posts: 6,070

CANBUS Problem or Not? Aug 24, 2020 9:54:28 GMT lhenriquess likes this

Quote

Post by dv52 (Australia) on Aug 24, 2020 9:54:28 GMT

Hope this isn't breaking the forum rules!!

Aug 24, 2020 7:01:15 GMT crunch81 said:

First off all I want to make clear that my answer is not because I doubt your answer, It is because I want to know and learn what I have missed in the Electric Diagrams.

crunch: Not a problem. All's good- I entirely get your method. Understanding VW's wiring diagrams is not easy; they are very cryptic even for folk that I know who have been reading them for many years.

Perhaps you might read VW's document How to read Wiring Diagrams -it might remove some of the mystery from the material that you are looking at

Aug 24, 2020 7:01:15 GMT crunch81 said:

So, the only way to know if you are dealing with a canbus on the car’s network or a local canbus network is the words:

-“In MAIN Wiring Harness” means a canbus in the cars network

-“In ENGINE COMPARTMENT Wiring Harness” means a local canbus/ low speed canbus just for 1 node

No - those words in your response do not identify the nature of the CAN bus - they indicate the location of the connections in the car's loom. This is a diagram showing an overview of the data bus systems on a MQB platform car:

These are the high speed CAN buses (they operate @ 500 kbytes/sec) - except for the LIN data bus. In addition, there are a number of sub-mask CAN buses which are subordinate to the main CAN networks

Aug 24, 2020 7:01:15 GMT crunch81 said:

There are so many details on the electrical schematic, I wish they would put some more details in the

description on the right side. If they only used the words Local Canbus or Low Speed canbus it would make it clear right away but the phrases “engine compartment” and “main” is only to understand if you are a highly qualified technican.

VW uses it's own nomenclature and in many cases - yes, it assumes that those reading their wiring diagrams are professional repairers.

Also, it has to be said that for your exercise, using just a multi-meter to test the integrity of a CAN bus is very limiting and it only gives you a very rough indication. If you want to do the task correctly, you really need an oscilloscope - something like a pico-scope. But these types of diagnostic tools require a base level of knowledge

Don

crunch81 Junior Member Posts: 75	CANBUS Problem or Not? Aug 24, 2020 12:59:47 GMT Quote Select Post Deselect Post Link to Post Back to Top Post by crunch81 on Aug 24, 2020 12:59:47 GMT Don, Thank you! Totally understand now! Back to my challenge with measuring the other 22 wires Greetings Crunch

lhenriquess New Member Posts: 3	CANBUS Problem or Not? Oct 12, 2021 1:45:50 GMT Quote Select Post Deselect Post Link to Post Back to Top Post by lhenriquess on Oct 12, 2021 1:45:50 GMT Hello everyone, where can I get the complete diagram of the Canbus Network for MQB, VW golf GTI 2015. I'm about to install the Blind Spot System, and I'm in doubt which CAN network to use. thanks

dv52 (Australia)
Moderator

Posts: 6,070

CANBUS Problem or Not? Oct 12, 2021 20:19:22 GMT

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Post by dv52 (Australia) on Oct 12, 2021 20:19:22 GMT

lhenriquess: Hi.

On MQB platform vehicles, most of the control units allocated to the driver assistance systems are combined on the Extended CAN bus. This is the case for the 2 x Blind Spot Monitor modules. Look in the Wiring Diagram - the modules are called J1086 and J1087 (I don't have a WD for your model)

Don

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