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Fault finding (troubleshooting)



'A customer brings his car into the workshop'. The beginning of so many tales of endless searching for the cause of a fault. whereby, any number of faults are discovered during routine inspections, e.g., during the required services. Of course, all this and a complete picture is determined with the help of the owner/driver.

The first, and correct thing you're probably going to do, is ask about the error-memory. Indeed, let's give it a moments thought. What could it be? Basically the questions are always the same. If it's something to do with the electrical system, we want to be sure about the battery voltage, which, particularly under strain, shouldn't show a weakness.

This of course, is not enough, because it must be available there where it's needed. Due to the fact that, as everyone knows, current needs a return circuit, it may make sense to replace the cables individually. This should be possible if the cables are long enough. Although it may look a little odd, one can be sure that one is testing the correct cable.

Don't underestimate this. How often, when searching for a fault, does one have to retrace one's steps or even start from scratch again, because one has taken the wrong turning somewhere or only thinks that one has. If this could be avoided by being a little more systematic …

Should one be dealing with older control devices, don't forget that there may be several in- and output cables. When testing various mass terminals, of course also smaller voltage differences must be taken into consideration and it must be determined if sufficient current is flowing. In this case, the power relay is very important. At this point, a wiring diagram and perhaps also a test jack connector (pinbox) would be a good idea. This is why, searching for a fault outside of the workshop, is always only an interim solution. At the most, one probably allows oneself a maximum of 10 to 15 minutes, to find the fault right away.

So, off we go to the workshop. Under certain circumstances, a hoist is probably worth it's weight in gold. Sometimes it'a good idea to check the first thing that comes into your mind. It's annoying, if one thinks that something is needed and it's not available. I don't like to say this, but the rules are actually, quite clear. One tests the periphery and the wiring including the jacks, then in the end one replaces the control device. In the end one is wiser.

There's not enough space on this page, to curse about wild replacing. There would have to be a whole crew of 'experts', who maintain, that it could be this or it could also be that. They haven't even measured anything. In this case, all it is, is a remote diagnosis. The poor guy in the workshop changes everything in sight, hoping that in in end, all will be well.

As we know, there's always the spark of hope. Only when the spark dies, or perhaps when no more money is available, he starts replacing used parts with other used parts, a particularly insidious possibility of introducing even more faults into the system. In the end, the number of faults can be compared to the infamous can of worms, one has no idea of what's going on. The insanity of it all is, that even now the possibility exists, by using a systematic approach, of picking up one fault after the other.

You've noticed, that the replacement method is not what I'd recommend. It fits in well with a system of wasting, and also endangers used- and new parts. One cannot make that many mistakes when measuring. No, in my opinion, even dismantling to find faults, is a bit questionable.

Perhaps this can be demonstrated by an engine that won't start. What does one need, to get the engine to run?, we need fuel and ignition, with a Diesel engine, only fuel. Ok, maybe it's easier to divert the fuel-line than to install a pressure manometer. Apart from that, it tells us something about the amount, because pressure is also a result of having an obstacle in the fuel-line.

Now, does that mean that e.g., the entire rail in an intake manifold injection must be removed, allowing the valves to spray all over the place? Probably only in exceptional cases, since the injection signal itself speaks volumes. So, this together with sufficient pressure and a volume control on the tank flow-back, (almost) everything should be covered, or maybe not?

Oh yes, the ignition. Must you really take all the spark plugs out and earth them with an additional cable? Wouldn't the spark plug cable or the respective single-spark coil and a spare spark plug be enough? Indeed, whether the signal comes at the correct time or not, can't be seen anyhow. Most experts have built up their own pin-box, with the most important adapters for the (almost) damage-free penetration of the connecting cables.

Whatever the case may be, the manufacturers have proved, that they don't trust the workshops, not even their own. One speaks of guided fault searching, whose enormous costs, including training courses, are only worthwhile, because in the past, even more money has been wasted on unsuccessful measures. A workshop should be ashamed of itself if they allow a customer to leave the workshop without the fault having been found, or at least being prepared to try out further measures.

The guided fault searching is the duty of a workshop which is bound to a certain manufacturer. If these measures are not carried out, the cost of the guarantee work won't be reimbursed. In addition they would also have to carry the costs of any consequent damages. The guided fault searching is like a very tight corset, there is hardly anything that is as rigid as the computer, which tells you exactly what steps are to be taken. If it carries on like this, one day we won't even need mechanics anymore. Even the test-drive is prescribed and controlled.

Let's finally consult the device, that we should have looked at right at the start, the error memory. By the way this addiction to replacing is more fueled than curtailed. For some, it's sufficient if the text mentions a certain functional element. Thereby, these are nothing but vague indications, as to where one should check next. It is of course more difficult when dealing with an actuator. Is it working properly? Is it in fact, even working at all?

Is it perhaps not working, or not working properly, because it is not being correctly controlled? OK, test it again, to see which signal it is getting. Perhaps we won't even have to find it, because we've tapped the control device in such a way, that we can read out the Bus-signal and the feed back from the sensor. I was once told, that intelligent bank robbers once robbed a bank by using the internet and, that the bank denied any losses because they didn't want to admit to their customers, that they were vulnerable.

Faults which only appear sporadically, are of course particularly tricky, I don't have a patent recipe against this occurence either. Perhaps one should in fact, spend a bit more for the testing device for the OBD-jack. Whoever learns how to analyse and can set certain tasks for the driver, can in future, manage with a lot less dismantling. If I imagine repairing a motherboard, I also don't imagine that every soldering point to be checked, must be melted.

Unfortunately, in this text, we've arrived very late at the subject of measurement-value-selection when using a scanner. If one has drawn conclusions, which point in the right direction, this testing method is mostly more reliable than the error-readout. Consider however, the delay with which the data comes over. Although data-transfer for diagnostic purposes, does have priority in a number of Bus-systems, it's control device of course, does not neglect it's normal tasks.

This is a blessing for the system, perhaps however, not quite that much for the conversion of the values into, for other Bus-participants, directly usable information, e.g., the depicting of strain in Nm instead of volts. What if something has gone wrong? Is this value due to the conversion or is a replacement value being given because of a faulty measurement? Where ever the behaviour of an actuator is being tested, setting- element- and/or measuring errors are possible.

If I see a car on the road, with a cable stuck with adhesive tape, leading from the bonnet into the passenger side window, I tend to think, this could be someone who is searching for the fault rather than simply closing the bonnet with the comment: 'everything's all right'. Admittedly, there are those who measure and measure and have no idea of what they're actually measuring.

By the way, one can also 'drive' into the workshop. If, e.g., a dynamometer- or roller-braking-testbench is available. Here you can also test whether both sides of an ABS- or an ESD-system are functioning correctly, doing this on the road, would of course, be far too dangerous. Apart from anything else, an error can be erased and retested much more quickly.

At this point, we don't, by any means, mean to condemn the fault searching. It is something that really makes sense, but sometimes is wrongly applied. Perhaps the green-horn laymen will be somewhat deterred by the improved devices. In this case, the accompanying circumstances of the respective fault are explained more comprehensively, e.g., the RPM or the driving speed can be shown. 09/13





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