Technical Canbus terminators on Fiat Ducato X250

[Mobilicorpus]

Hello everyone! I have some issues on a Fiat Ducato X250 from 2010. The Airbag light is on and running a Multiecuscan scan reveals some quite a few errors. The light came on while driving in a particularly heavy rain.

B1001 - Control unit faulty
U1726 - Airbag Node (NAB)
U1706 - Brake system (NFR)
U1602 - CAN line error

Before replacing modules left and right, and because of the "CAN line error", I wanted to make sure the can bus is alright, but I have difficulty finding where it is and good places to measure.

I have checked the resistance between CAN-H and CAN-L in the OBD port and it is infinite. Is the CAN lines in this port actually connected directly to the bus or is it just proxied by the BCM?

I have also measured using a oscilloscope with the key on (engine off) and I think it looks fine. CAN-L idles at 5V and goes down to ~1V, CAN-H idles at 0V and goes to ~4V.

Where is the bus supposed to be terminated with 120Ohm?

I'd like to add that the last year or so I've sometimes had some weird glitches. The indicators sometimes only blink 4 times even though they are in the left or right positions. And at one time the windshield wiper got stuck in clean mode for 10 minutes or so. I am grateful for any advise!

 

[Mobilicorpus]

For anyone interested, I managed to clear the EEPROM of the airbag control module and it worked.

I found a can bus behind the radio unit and the levels looked just fine I think. Interestingly I still can't measure any resistance between CAN-H and CAN-L. I also don't know if that is connected to the airbag control module.

I think what might have happened is that voltage dropped during the heavy rain for some reason. Perhaps too much water got into the alternator for a short time. Or there're gremlins in the cabling. I did notice a solitary ground cable reached out from the harness by the airbag computer to a point on the chassis. It is a tiny bit too short for comfort.

I also found some excellent information of the busses here:

CANBUS very low voltage on CAN-L - steps to find the cause? | FIAT Ducato

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...

www.fiatforum.com

 

[Mobilicorpus]

Time for a (hopefully) final update.

The error returned after letting the car stand over night. After resetting again I was able to drive for an hour with no issues, but on the morning after it was back once more.

Since resetting worked for a few hours I took a chance and ordered a new second hand module and transfered a working EEPROM dump to that. And I am happy to say that the error has not returned for several days.

So apparently these modules can start failing completely out of the blue and they they can work for hours until the engine has been turned off for a while.

 

[Keith Spencer]

You need to check the termination resistors 2 x 120 Ohms in parallel. So you should read 60 Ohms reading with a dvom between pin 6 and pin 24 on the obd port. But before you do disconnect the earth lead on the battery, then check pin 16 and you should get 0 volts.
Infinite resistance would indicate a failure of the canbus network, or you are checking with the battery connected and energised. Which is most likely as you have checked with an oscilloscope and you are aware enough to assess the can hi low display. If 1 resistor is out then you will read 120 Ohms. and you will need to find out which module has failed.

 

[LPAE]

Hi Mobilicorpus / Keith,

I'm retrieving similar faults on the family 2010 X250 moho, except, dash is dim with no display screen (unless I earth out the obd pin 4 [chassis ground line], which otherwise has battery voltage - 3 sitting on it...)

OL on can lines at obd.
If I power my bluetooth VCI using a special cable from the battery (Launch X431), I get dash lights power up and BCM clicking into life, then can run diagnosis scan but only have comms with Engine Control (Visteon).
Checked all the interior ground connections and cleaned the 3 spurious points front left bumper.
Tried a replacement BCM and mating fuse box to no avail.
Have found no visual wiring issues having seperated all plugs on the C-CAN.
Brand new battery.
Cranks but no start - immobiliser light stays illuminated in process.

Any ideas where the 2 x terminating resistors are actually located? Sometimes these are 'digitally' energised and may not read at 60ohm parallel in obd if so. Can't imagine Fiat being that clever with them in a 13 year old van though...

Next steps I see are load testing all powers and earths to every control unit...
20 hours into diagnostics so far.

Liam

 

[Keith Spencer]

Hi Mobilicorpus / Keith,

I'm retrieving similar faults on the family 2010 X250 moho, except, dash is dim with no display screen (unless I earth out the obd pin 4 [chassis ground line], which otherwise has battery voltage - 3 sitting on it...)

OL on can lines at obd.
If I power my bluetooth VCI using a special cable from the battery (Launch X431), I get dash lights power up and BCM clicking into life, then can run diagnosis scan but only have comms with Engine Control (Visteon).
Checked all the interior ground connections and cleaned the 3 spurious points front left bumper.
Tried a replacement BCM and mating fuse box to no avail.
Have found no visual wiring issues having seperated all plugs on the C-CAN.
Brand new battery.
Cranks but no start - immobiliser light stays illuminated in process.

Any ideas where the 2 x terminating resistors are actually located? Sometimes these are 'digitally' energised and may not read at 60ohm parallel in obd if so. Can't imagine Fiat being that clever with them in a 13 year old van though...

Next steps I see are load testing all powers and earths to every control unit...
20 hours into diagnostics so far.

Liam

Hi Liam,
I assume you are saying that when you earth out pin 4 you get brighter dash lights?
If so, I would do a continuity from pin 4 to battery negative and you should get 0 ohms or very close to it, I would then check to a known good chassis earth point and ensure 0 ohms. Also check the continuity of the obd system earth pin in the same way.
If you are getting OL then you have totally lost the earth on your system and you need to sort out before anything else.

If this doesn't sort out the problem then next is below.

Crank no start with immobiliser light suggests the immobilizer is not allowing the engine to run.
You are at the point of needing a complete wiring diagram and full module layout diagram, so you can property think it through and decide how and where to test. The module layout will show you where the end resistors are. Good luck Sorry.