24 Feb 2024

Shining a light on the root cause failure

I was recently asked by a trade customer of mine if we could “simply” program a new headlight module for a VW Golf MK7. They had replaced a control unit due to the dipped beam headlamp not operating. They advised me that after the unit was installed the light started working. with the other lights all operating correctly. However, there was a bulb warning in the dash display Please refer to Fig. 1.
At this point the request would seem normal and a straightforward job so we continued with the task in hand. Correctly programming new headlamp control modules on VAG vehicles will require the dealer tool for this manufacturer, known as ODIS (Offboard Diagnostic Information System). I connected this tool to the vehicle and carried out a full system scan. This is a common practice when attempting to program any control modules as programming errors can occur after the process is done and having a report before and after the programming is very beneficial. It can also aid in any warranty claims.  As you can see the only fault codes being set are “Headlamps No Basic Setting” and another fault code relating to “Right Headlamp Power Output Stage.” Please refer to Fig. 2. These codes were both permanent and would not clear. I expected to see the basic settings code, as the control module had not yet been programmed. However, the other code seemed odd to me initially.

Why programming is rarely straightforward
I pressed on and attempted to carry out the basic settings of the right-hand headlamp dipped beam control module, which our customer had replaced, but I kept receiving a communication error, as seen in Fig. 3. This seemed very odd as there were no communication fault codes present and all the front lights were operational.

After a few failed attempts I believed there could possibly be another issue with this headlamp which was not analysed initially by the original garage. Using the dealer tool, I accessed the information section and reviewed the wiring diagram for the headlight.
As you can see from Fig.4, there is a main voltage supply, ground, and CAN communication wiring to the headlight. A quick check of each revealed what one should expect when operating properly. Using an oscilloscope, I then checked the CAN bus wiring at this headlight.

To my surprise the CAN bus data signal was corrupted. This seemed very odd. Although the headlight was operational and no-fault codes were present at all regarding CAN communication, it was clear that there was an obvious issue with the data network. There could not have been any communication taking place between the control units on this network. These include the headlight regulation control module and the right/left headlight control modules. After disconnecting each control module in turn, I found the CAN Bus signal had recovered only when disconnecting the right-hand headlamp assembly.

The oversight
It was now obvious that there was more than meets the eye with this job. After contacting the customer and advising them of my findings they agreed for us to carry out further diagnostic work and identify the cause of the shorted CAN bus signal. The CAN bus wiring connects directly into the headlamp, from the connecter, and then into a control unit. According to the diagram, the connector is part of the headlamp assembly. I removed the right-hand headlamp assembly and located the control module which the CAN bus wiring goes to. I removed the headlight control unit from the bottom of the headlight assembly and found water intrusion present, causing the CAN bus signal to be shorted. See Fig.7. Unfortunately for the customer, this vehicle required a new headlight assembly and control module.
A replacement headlight control module and headlamp assembly were both installed. The basic settings function was run once again, using the dealer equipment. This time the function was successful. The headlights needed to be put into a setting position and then physically adjusted on a beam setter before confirming the adjustments were complete. The module then saved this adjustment as its basic setting. What initially seemed a straightforward job turned into a bit of a nightmare due to incomplete diagnosis. Many aftermarket garages will often not allow the diagnosis to take place elsewhere. I carry out a lot of programming for various independent garages across the South West and find this can be a regular occurrence. Correct reporting is essential. Before-and-after reports are a necessity and will often protect you against any shortfalls in the initial diagnosis. These reports include full system scans before and after the programming has taken place, and any other reports generated by the diagnostic tool in use for both failures and successes.