M is the word

James Dillon on why you shouldn't always blame technology

Published:  12 May, 2016

We often talk about the relentless advance of vehicle technology, and about the challenges technicians face in trying to resolve vehicle problems, especially where the new technology ‘gets in the way’ of the problem.

That which gets in the way may be reduced through training and education.

Learning about new systems (and indeed refreshing knowledge on older ones) can lead to increased efficiency and improved customer satisfaction by getting it right first time. Without relevant training, the technician can be swayed by hear-say and misinformation, and often, the new or unfamiliar technology may be blamed unnecessarily.

Unfair blame

Each time we enter a phase of new technology, it is convenient to blame the unknown for problems that have arisen.

Readers of a more mature standing will have undergone several technology phase shifts. I seem to recall when engine management first became widespread in 1991, due to emission legislation changes, the black box at the heart of those early systems was roundly but unfairly persecuted. During this era misbehaving vehicles often had their ECMs or PCMs (or ECUs) changed unnecessarily. In many cases the vehicle still exhibited the deviant behaviour, despite having had several hundred pounds worth of a shiny new module fitted. This perhaps was the advent of the age of the M factor - M for Module.

Sometimes, when a technician runs out of steam with his testing, or runs short of time on the job, there can be a reflex reaction where the ‘M’ gets the blame, after all there is a chance it is at fault, and the dark forces that make the unit work are untestable, so it’s worth a punt isn’t it….isn’t it? Over the years, some technicians have come to understand that other root causes of vehicle problems are equally likely, such as wiring problems, sensor and actuator faults, etc.

Testing this stuff takes time, skill and equipment. Judging by the vehicles we see, those that have made multiple workshop visits elsewhere and appear with us plagued by ‘unfixable faults’, it appears that there is a sizable gap in the measurement and diagnostic techniques of many general repair garages.

What we generally see is parts escalation. The cheapest part is replaced, then the next cheapest and so on until the suspicion falls to the ‘M’, whether it’s a PCM, an ECM, an ICM or a BCM. The ‘M’ may have been replaced, but the fault still prevails. The testing skills gap is partially filled by businesses bench testing these units. Now, rather than replacing it, even after the parts escalation game has played out, the less costly (but more time consuming) step of sending it away to get it tested is considered by many to reduce the odds of getting it wrong. I still wonder if a bench test can ever be as good as an on-car test; I don’t think it’s supposed to be.

Rules to watch

One of the golden rules for diagnostic testing is to test the system under the same conditions as the fault occurs. This rule is broken with a bench test. Whilst bench testing can offer a bank of tests that some repairers are unable to perform themselves, it could be considered by its very nature as being narrow in scope. For instance, it ignores everything else which is attached to, or associated with; the control unit and the bench test may not be able to replicate the physical forces which are applied to it when it is in situ. Perhaps the bench test could be likened to a resistance test; if it fails, it’s definitely faulty, however if it passes, it might still be faulty. I’m sure that many have received the dreaded no fault found report.

The likely reason for this is that the cause of the problem is something that resides on the vehicle, that the local environment is causing the problem or that the bench test didn’t stress test the module in the right way. Bench testing modules can definitely help, but it may be a case of knowing the limitations of, and when to select this method. So, what practical steps can technicians take regarding workshop based on-car testing?

Communications: If it is not possible to communicate with one or several control units, it can be tempting to jump the gun and condemn a control unit. Multiple network U codes can be caused by power and ground issues, water ingress into connectors (short circuit), the poor fitment of aftermarket devices (entertainment systems), interference from the starter motor, simple wiring problems or a shorted component (such as the Citroen EGR valve problem). A good CAN test plan will help flush out the root cause.

Fault Cluster Analysis: Where codes and perhaps several codes are present, the examination of a common factor or link should be the guiding principle. Analysis of a decent wiring diagram should be made and a test plan defined based on this research. Signals, powers and grounds (including relays, fuses and the battery condition) should be examined (see our dillondiagnostics YouTube ‘TFSi video’ for an example).

Don’t simply rely on resistance checks; remember, a single strand in a multi strand cable will pass a resistance test, but won’t pass the required current.

Looking beyond

In common with other fault codes, Internal ECU Error codes don’t always mean what they say. They can be caused by shorted sensor signal supply or other sensor or actuator failures. In some cases, a failed actuator can physically damage a control unit, or the operation of that circuit could be suspended due to self-protection current limiting. One tip is to use a low current test light (500ma or less) in place of the faulty component and use the scan tool actuator test to attempt to drive the substitute load. This will (dis)prove the function of that driver circuit.

Consider that the sensor supply circuit can be affected by a short to ground. A single sensor can pull down multiple sensor supplies, and give the impression that the ‘M’ is in trouble. In this case, the OEM wiring diagram is your friend. It may be necessary to disconnect suspected components whilst measuring the reference voltage to check if it returns. In summing up, the key to diagnostic success is being able to measure the right thing, in the right way, at the right time, with the right tool. This is likely to involve multiple tools and a reasonable amount of time and skill.

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