Automotive Technical Articles - Oscilloscope Diagnostics

System Diagnostics

Resistance is Futile.

Take yourself back to the college classroom for a moment and think about Ohms law. Recall the volts amps and resistance triangle. Did it really mean anything to you? Or was it just theory gone mad?  In this era of mass electronic system control in the motor vehicle, an understanding of Ohm’s law and the like is more important than ever, especially where efficient and accurate system fault diagnosis is required.

A foundation point to understand is that all electrical devices work on the principle that they consume power (watts) in return for work. This work may be moving an injector, actuating an idle control valve, rotating a fuel pump or heating the hot wire of an air mass meter. Watts can be represented by the equation volts multiplied by amps.

In many cases testing for voltage or resistance only gives us half the information needed for accurate diagnosis. Using these tests alone can lead to problems being mis-diagnosed or missed completely. Vehicle technology has progressed so far that the days of testing single dimensions in circuits as the first step in the diagnostic routine are gone.

Most practicing diagnosticians will tell you that before you get down to the nitty-gritty of probing and measuring, diagnosing faults in complex control systems requires a combination of assessments performed through a structured, logical approach.

Start at the beginning.

Start at the beginning - stringent owner/driver interrogation, then utilise your god-given senses (look, listen, smell, feel) and road test, basic engine condition checks follow (vacuum/compression and emissions), fuel pressure/flow tests, then diagnostic trouble code assessment - erase, re-test and review. When these assessments are complete, satisfactory dynamic system testing can commence. 

Dynamic system testing is where the technician assesses the control system under test in its ‘real world’ operating environment and if possible, in the same environment that the fault is evident. Amongst the ‘must have’ tools for dynamic system testing are an oscilloscope (either a digital storage oscilloscope (DSO) or an analogue oscilloscope (CRT)), allied to relevant test probes.

There has been a recent development in the oscilloscope arena. It is the Digital Phosphor Oscilloscope (DPO). This scope overcomes the individual shortcomings of the other two. It has the ‘fast, real-time’ display of an analogue scope, combined with the DSO’s ability to store/record waveforms for comparison and analysis. The majority of scope users will have either a digital LCD or an analogue CRT scope, due to the high cost of a DPO.  Currently DPO’s are confined to expert and ‘high end’ installations, but as the technology matures and the cost decreases, I’m sure they will become a more popular tool.

Combining the data acquisition capabilities of a scope with the versatility of a low range current probe, gives you a brace of powerful diagnostic tools that are hard to beat. The following examples show and describe the output from this combination of diagnostic equipment.

Current Limiting Injector

Fig. 1 shows the current waveform for a current-limiting injector circuit on the top, and the voltage waveform on the bottom. The first part of the waveform (1) is the peak current flow; the second part (2) is the hold part (lower current). Hence the term peak-and-hold for this type of circuit.

The current in this example builds to around 4 amps before the injector driver switches rapidly in a resisted circuit, thus limiting current flow to the injector to under 1 amp. This switch happens faster than the mechanical pintle can close, and therefore, the injector is held open continuously for the determined time period.

Figure 1

It takes a lot of power to open an injector fast, but very little to hold it open. The injector circuit has gone from around 16 ohms resistance (therefore under 1 amp total current flow) to around 3 ohms (and nearly 4 amps current).

Ignition Circuit

Fig. 2 shows a voltage and current waveform of current limiting in an ignition circuit. It's slightly different from an injector. Rather than cutting its current (as in the previous example), it limits the flow to around 7 amps (3). In this instance, the ignition coil is charging the secondary circuit ready for discharge, whereas the injector circuit is holding the pintle open

Figure 2

Essentially, by measuring amps dynamically and not static volts or resistance, we are looking at a function of the circuit. By measuring the current, we are also assessing many other elements i.e. coil supply voltage, voltage drop, circuit resistance, distributor sensor (hall or perm magnet) and ignition amplifier/ECM ignition signal etc.

In simplified terms, it’s a bit like the difference when assessing an engine’s emission output, between placing a probe into the exhaust pipe and looking through each influencing factor (engine vacuum, ignition system, fuel pressure and flow, injection duration, etc) to get the same result.

By using this method, we are starting at a key performance factor and if a problem is present, working back towards cause. It makes the diagnostic process much quicker.

An oscilloscope and a skilled user will be able to observe any information relevant to the system. Unlike code readers, the oscilloscope doesn’t rely on bespoke application software or connection harnesses to allow measurements to be taken. This has obvious benefits.

The question is “Why doesn’t everyone involved in diagnosis have and use an oscilloscope?” Well, the tricky bit is making sense of the image on the oscilloscope screen. This knowledge comes through training and experience.  Unlike the more popular measuring device, the multi-meter, it is not simply the measurement values that are of prime concern. It is also what the scope trace does during the measurement that guides the skilled operator. The subtleties of the trace, the 'cleanness' of the line, interference and oscillations are all vital factors for consideration during diagnosis. It is often the recognition of these subtleties that separate the experts from the rest.

Hopefully, this introduction to dynamic system testing and the oscilloscope have illustrated why they are useful tools in the diagnostician’s toolkit. I would hazard a guess that your local repair business that solves all the trade’s ‘unsolvable’ problems, will have a resident scope expert.  Does that not say it all?

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© James Dillon.  Date of article MMII.