Can Bus

[Dlr]

Hi I have a skoda fabia 2017 that has a problem on the Can Bus, my car had a problem on the way home 

A light came on dash follow by 2 others and then it went into limp mode. The first light was glowplug fault followed by 

The DSC and EML light. When I returned home i pluged my reader into the obd2 port to see what was faulty but it was u

Unable to connect to car. I plugged in my obd break out box to check the Can Bus lines for resistance but insr3ad of seeing 60ohms it read dead short. The next day i checked the car and the lights had gone out but the short on the obd is still there so i can't  find the fault. Everything seems to work as it should be but the fault keeps happening

I can't find a diagram of what or where all the modules are so really in need of some assistance 

[Ootohere]

Welcome.

I will flag your post so a mod can move it to a section where hopefully you get an answer.

[J.R.]

I am sceptical about your diagnosis, what is a Canbus break out box and have you tried using a multimeter to verify the EOL resistance.

 

If there were a dead short none of the control modules would be able to communicate with each other and I do not think the engine would start, ECU needs to communicate with the instrument cluster for immobiliser, if it did run most of the other systems would either not work or go to default limited operating strategy.

[Dlr]

A break out box is a remote DLC that plugs in to the one under your dash. It saves you trying to climb under the dash. 

If you check the resistance on high and low can it should read 60ohm if all is working fine. It will read 120 ohms if one of the mcu are faulty

It will show a dead short if the wires are touching or a modules faulty as for not starting I not sure as I have. Never worked on the Can Bus before 

[J.R.]

21 minutes ago, Dlr said:

If you check the resistance on high and low can it should read 60ohm if all is working fine. It will read 120 ohms if one of the mcu are faulty

 

The first part is plausible but only if the manufacturer uses a 60 ohm EOL (end of line) termination resistor, this does what the name suggests and will confirm that the network is intact with no breaks along its length, I have measured it manually on the last car but am not sure the resistance is supposed to be 60 ohm, the EOL was located somewhere around the LH rear light cluster.

 

The second part, the 120 ohms in fault condition is a flight of fantasy, there are a multitude of faults that could occur in a controller none of which would affect the EOL resistance, disconnecting the controller being one of them, lack of power another, any internal circuitry fault etc.

[PetrolDave]

15 hours ago, J.R. said:

If there were a dead short none of the control modules would be able to communicate with each other

A dead short on the CAN bus at the diagnostic connector indicates a short on only the Diagnostic CAN, the CAN Gateway acts like a CAN equivalent of an EWthernet switch so a short on the Diagnostic CAN does not necessarliy mean there is a short on any of the other CAN buses in the vehicle (Powertrain CAN, Comfort CAN, Infotainment CAN, etc.)

[J.R.]

28 minutes ago, Dlr said:

It will show a dead short if the wires are touching or a modules faulty

 

 Agreed regarding the shorted wires but not the dead short if a module is faulty, that contradicts the 120 ohm if a controller is faulty, neither of them are correct according to my knowledge and experience, the best thing would be to interrogate the controllers using VCDS to find any fault codes.

[Dlr]

From have read if youu  meter the Can high and low it will be 60 ohms if you have not working it will be 120ohms as the 2

Have 120ohms terminators as I understanding it  forgive me as I have only just started work it all-out 

All I kno2 for sure is that i cannot connect to the car via the obd2 to see what faults on on it. 

I don't think VCDS will connect either as it wants to talk to the ecu 

 

 

[Dlr]

 sorry about the tyros in am in a car and the wife's driving is a little hard lol

From what I have 

If one is not working 

As both 

All i know

[Ootohere]

@Dlr  Is this Mk3 Fabia a 1.4 TDI?

[Dlr]

Hi. Yes it is 2017 

[PetrolDave]

23 hours ago, Dlr said:

i cannot connect to the car via the obd2 to see what faults on on it. 

Sounds like either there is a problem with the connections to the OBDII connector(could be power, ground or Diagnostic CAN), or the CAN Gateway has a problem.

 

I think you need to get a wiring diagram and do some old fashioned wiring testing.