Technical CANBUS very low voltage on CAN-L - steps to find the cause?

[bazmonaut]

Hi all

I am trying to fit an aftermarket cruise control, however this device refuses to initialise, saying it isn't recieving a CANBUS signal. I also have some other issues with the vehicle that suggest CANBUS problems.

It's a 2012 Ducato Maxi, 3.0lt with the comfortmatic gearbox. I have owned it for a year and a half.

I bought a cheap PC oscilloscope (Hantek 6022BE) and put it on CAN-H and CAN-L. I got a signal, but found that CAN-L was very low voltage at rest, like below 200mV. It should be around 2V I believe. So, I suspect this is what is causing my problems.

I suspect that I need to pull plugs off all the CANBUS trancievers (ECUs) in the van and keep checking to see if one of them is faulty, or if there is an issue with a specific wriing run.

Does anyone know how many of these devices there are, and where they are located? Any tips on troubleshooting this issue gratefully recieved...

 

[Anthony489]

Hi bazmonaut

The information I have on Ducato CAN buses is pieced together from a variety of sources such as the Fiat Training Manual, some have which have suffered in translation, so I don't have a clear and complete picture of operation. The following is only my best interpretation !

I don't know about 2012 models, but earlier ones had a B-CAN bus which was low-speed (50 kbit/sec) and used Fault tolerant signal levels of about 5.0 / 1.4 volts and 0.0 / 3.6 volts for the two lines. The two lines are referred to as B-CAN-A and B-CAN-B and the wires are White/Pink and Black/Pink. It is sometimes referred to as a "Comfort" or "Convenience" Bus, i.e. it controls less important features like door locking which are not especially time critical.

There is also a faster C-CAN bus which is high speed (500 kbit/sec) and used for more time critical functions like brakes, steering, engine and gearbox management. The two lines are referred to as C-CAN-H and C-CAN-L. This is not a fault-tolerant bus, I don't know the signal levels but they are likely to be 2.5 / 3.5 and 2.5 / 1.5 volts.

The pins 6 and 14 on the OBD - II socket connect to a local bus (probably also low speed) which is driven by a "Gateway" in the Body Control Module. The description of this is ambiguous but I suspect it allows you to see messages on both the B-CAN bus and a "censored" version of the C-CAN bus. What you can't do though is any meaningful hardware troubleshooting from the OBD socket because there is no direct electrical connection to either the B-CAN or C-CAN bus, it's all "one step removed".

If you suspect a hardware or wiring fault on the C-CAN bus you will have to probe direct on the relevant lines - see attached marked up schematics. Although these pre-date comfortmatic transmission (which is probably daisy-chained from the ECU) they give you the idea.

Bus transceivers are all differential, i.e. they respond to the difference in voltage of the two bus lines. This enables rejection of any interference voltage which is common to both lines. The ideal way to analyse this is to set up a 2 channel scope to display the voltage difference. With a single channel scope it may be more difficult to see the same signal that the transceiver does, unless the scope is battery powered and "floating" from ground so you can connect its input across the two bus lines.

 

[bazmonaut]

Ah thats great thanks so much Anthony, that's a great help.

I do think it also has a low/medium speed CAN as the relevant wires are in place in the OBD2 port. I have a 2 channel oscilloscope so thats all good.

I realised that I had one of the probes on "10x" which is why I was getting 'low' voltage... oops... so looks like there are no issue with the high speed CAN (that I can detect). Will work more after work today. Thanks again!