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.


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