Monday, 30 March 2015

AdBlue Warning Light On (Fixed)

Volvo FL AdBlue Fault

A Volvo truck was presented to our workshop today, the driver had reported a fault that the instrument cluster was showing a warning for AdBlue system failure. 

Naturally the first action to take was to carry out a diagnostic code read. However when we attempted to code read the AdBlue control module (ECU) it was found to be not communicating.

Instead carrying out a fault code read of the engine control unit was found to be useful, here are the results:
  • MID(128):Injection ECU, PSID:229, FMI (9):Data update frequency
  • Connection problem with the UDS (AdBlue pump module) ECU on the SAE J1939 CAN bus data line.
It became apparent that the AdBlue system was not accessible due to a CAN bus fault. 

The next step was to monitor the CAN network at the AdBlue ECU. As we explained in our previous CAN fault finding article, we first test the CAN network with the wiring block disconnected from the AdBlue ECU. The oscilloscope was setup as follows:

  • Channel A / 1 - 5vDC
  • Channel B / 2 - 5vDC
  • Timing - 50 nano seconds

Here is the waveform as viewed on the oscilloscope:

Good AdBlue CAN Signal
Good CAN Signal AdBlue System

As you can tell with the ECU disconnected the CAN network behaves as expected (equal and opposite traces between the CAN High and CAN Low). Next we reconnect the wire to the ECU and continue to monitor the CAN signal with the oscilloscope, again here are the results:

Bad AdBlue CAN Signal
Bad CAN Signal AdBlue System

It is clear that with the AdBlue ECU connected there is a definite fault with the CAN signal. Most notably on the red CAN Low line. This suggests that the fault lies within the ECU and not the wiring.

Next we removed the ECU from the AdBlue module and proceeded to open the ECU to see if there was any visible corrosion or burnt components. 

We found corrosion and a burnt component on the circuit board, here is an image of the burnt component: 

Blown ECU Component
Blown Component Between Pins

We proceeded to order a new AdBlue pump and ECU module, once received it was fitted the fault codes cleared and the the vehicle road tested. All was good the fault was fixed and the vehicle returned to the customer.

Hopefully this article helps you to diagnose CAN network faults and determine if the fault is in the wiring or ECU.

Why not leave a comment if you have had a similar experience.

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.