Process Makes Perfect
Process Makes Perfect

Process Makes Perfect


Technician Ben Martins emphasises the importance of a clear diagnostic process, which is particularly useful when diagnosing those more unusual faults.


Vehicle details: Mazda CX-3

Symptom: DAB amplifier issues

We hear a lot about having a diagnostic process, but never is this more important than when trying to diagnose those unknown faults that we don’t see every day. Not only can this keep you on the right course, but having a firm action plan can save you time. The process doesn’t have to be my process nor anyone else’s. The important thing is that you have one.

Vehicle ID

Are the details you’ve got on the job card correct for the vehicle you are being presented with? Sounds obvious I know, but with so many variants of the same model, this can often get overlooked. For instance, with the car in the following example, there can be options for Bose sound system, Lane
Departure Assist, Adaptive Cruise Control, Halogen or LED lights, with or without satnav, with or without centre screen – the list goes on. For this model, the key things we identified were that the vehicle had a centre screen, Bose sound system and satnav, which also came with voice recognition.

Customer interview

The customers have all the answers! Talking to them could give you that golden nugget of information. It may be something simple like, “the lights only flicker when it’s raining”, but what if you didn’t ask, and all your testing is done in the workshop which is hopefully nice and dry? The next time it rains, the customer will be back on the phone to you. When does it happen? How often? Has it always been there? If not, when did it start? These are the questions you should be asking.

The symptoms as described by the customer in this case were:

■ When attempting to switch the radio on by using the touch screen interface, the radio button was highlighted, but the menu couldn’t be accessed. The same condition was met when trying to use the command wheel in the centre console.

■ No audio or output to any of the speakers, including satnav, Bluetooth or voice recognition.

■ Volume also didn’t work and when adjusting settings for treble, bass, left and right or front and back, the selection could be made, but it reverted to the original position after a couple of seconds.

Verify the fault

The classic saying of ‘if it isn’t broke, don’t fix it’ springs to mind. But in all seriousness, how can we begin to fix something if it’s not behaving in the way the customer described it? As frustrating as intermittent faults are, sometimes it’s better to leave it alone until it manifests more frequently or if we can get authorisation from the customer to carry out extended road testing. We could stand back and throw parts at it in the hope that when the customer picks the car up, they won’t come back but the likelihood is they will be in the next day and now you’ve got to do it all for free. This is where a customer interview can help. If they can accurately describe how the fault occurs then replication should be straight forward.

With this case it was pretty straight forward. The fault was always there no matter what the condition of the vehicle. Ignition on, engine off, engine running, stationary or moving, it was a permanent fault. Great – should be easy, right?

Scan tool, DTC and data

There is always a place for serial diagnostics and it should never be overlooked. It can offer vital pieces of information to add to the puzzle, and if there is freeze frame data, this can give you the vehicle condition just before and after the fault occurred. This is especially useful when dealing with intermittent faults.

The Mazda had no warning lights on the dashboard, yet it definitely had a fault. Carrying out a global scan of the vehicle revealed no fault codes, but there was communication to the Connectivity Master Unit (CMU). It is the CMU that gathers the inputs and outputs to control the infotainment system. Another useful tip is to take note of the ECUs detected. You may not have any fault codes if the ECU (potentially) at fault hasn’t been picked up during the global scan.

Check for technical bulletins/known fixes or software updates

This can only really be done properly by going to the vehicle manufacturer’s (VM’s) site. It is definitely worth doing, as the fault you could be chasing could be rectified by a software update that you are not going to find using any diagnostic methods. Doing this could save you a lot of time. The Mazda had neither software updates available, nor any technical bulletins for a similar fault.

Description of the system

This can be done with product knowledge if you work specifically with one brand, but for those of us that can’t store all this information from every manufacturer, the best place is to find it is on the manufacturer’s site. You can use alternatives, but I find that if you want to know what you’re dealing with, the VM site is the place to go.

The new car features section is normally where you would find everything you need to know about how something works. So from here, I wanted to understand how the system operates and what controls the audio in the vehicle. From the VM site, I found that the CMU controls the entire infotainment system, such as communication between mobile devices and Bluetooth, and also the sending/receiving of both video and audio signals from units related to the entertainment system.

The Tuner and Amp Unit (TAU) is actually responsible for the audio output to the speakers, based on a control signal sent via a local CAN network from the CMU. Getting a better idea of the network itself is important at this point, which is where the CAN topology can help to see how it’s all connected.

Our earlier investigation into vehicle ID now comes into play as some ECUs, such as Blind Spot Monitoring, may not be relevant, which could lead us to thinking the problem lies within a system that isn’t actually connected to the vehicle. Our area of interest from a system description is the Connectivity Master Unit. This also highlights a local CAN network of three or four ECUs with terminating resistors in both the TAU and the CMU. What was strange in this case was that I couldn’t see these ECUs on a global scan. Reading through some further technical information, I found that there is an on-board diagnostic function within the CMU. The description was as follows:

On-board diagnostic function

The on-board diagnostic function consists of the following functions:

■ A malfunction detection function, which detects overall malfunctions in the entertainment system-related parts (with centre display)
■ A memory function, which stores detected DTCs
■ A display function, which indicates system malfunctions via a DTC display

This function requires access to a hidden menu which is where using the VM’s technical site is so important. You could spend hours scrolling through webpages but these days the information is there and usually at a fairly reasonable cost. In my experience, technical sites range between 5 and £12 per hour, so be specific with what you are looking for.

In this instance, you can access the menu by holding the command switch buttons ‘Volume’, ‘Audio’ and ‘Favourite’ for more than two seconds. From here, you can scroll through the menus until you find Read CMU DTC. It is here that I found the missing link, as a fault code had been generated U0184:00 – Communication error with TAU.

List of possible causes

This is where having gathered as much information as possible becomes helpful, as we can now list the likely issues:

1. Power supply to the TAU
2. Ground to the TAU
3. Wiring between the CMU and the TAU – open or shorted
4. TAU malfunction
5. CD player malfunctions 6. CMU malfunction

Make sure you pick up next month’s issue for part two.


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