Monday, 23 March 2015

CAN Error Testing And Repair

A Quick Look At CAN Network Testing



A vehicle arrived at our workshop today on the back of the wrecker. The fault reported was non-start and also the instrument cluster was showing a warning that the vehicle was not recognising that the gearbox was in neutral.

Although the tests and fault finding steps in this post are the same for most vehicle CAN networks, the vehicle we were presented with was an 2008 Iveco with a ZF AS-Tronic EcoLite gearbox.

The first course of action was to carry out a diagnostic code read on the transmission system, here are the results:

  • Fault Code - 806 - Clutch Actuator
  • Fault Code - 830 - Clutch Position
  • Fault Code - 833 - Clutch Position Open
  • Fault Code - 834 - Clutch Position Closed
  • Fault Code - 900 - Terminal 30
  • Fault Code - 907 - Relay Circuit 1
  • Fault Code - 909 - Relay Circuit 2
  • Fault Code - 865 - Gear Select
  • Fault Code - 896 - CAN Error Message
  • Fault Code - 918 - Clutch Valve 2 Closed
This, coupled with the fact that all power supplies and earths were present and correct, also that other ECUs reported communication errors with the transmission ECU, meant that testing of the CAN network at the gearbox was the next step.

The CAN wires are normally easy to identify at the ECU harness as they are usually a twisted pair, however if you have a wiring diagram for the vehicle available it will help tremendously.

I monitored the CAN network using a PicoScope however any 2 channel oscilloscope will do the job and should be setup as follows.

  • Channel 1 or A - CAN High
  • Channel 2 or B - CAN Low
  • 10ms rate
  • Channel 1 and 2 or A and B - DC 5v


A quick and easy way to tell if the CAN signals and wiring to the ECU are in working order is to disconnect the plug from the ECU and measure the CAN wires with the oscilloscope. In this case the results are in the image below.


Good CAN Network Waveform
Good CAN Waveform
As you can tell the CAN waveform is as you would expect. Each wave on the blue CAN High trace has an equal and opposite wave on the red CAN Low trace. This tells me that the CAN network is normal and working correctly with the transmission ECU disconnected. 

The next test is to reconnect the plug to the transmission ECU and monitor the CAN. Here is the result from the oscilloscope.


Bad CAN Network Waveform
Bad CAN Waveform
In the above image it is clear that once the ECU has been connected it introduces a definite fault and this is most notable on the red CAN Low wire. 

This led me to suspect the transmission ECU was fault and the next step would be to remove the ECU and send it for testing and repair.

Once we received the ECU from the repairers, we reconnected it to the vehicle and was able to confirm that the fault had been repaired by clearing the codes and checking that the vehicle started, which it now does!! 

I hope you found this post informative and now have a basic understanding of CAN fault finding. 

If you like these types of articles and would like to see more please leave a comment below.

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