The right tool for the job in hand
This month, James looks at the right way to go about testing and the value of resistance
THERE IS a well-used saying amongst those in the business community which goes along the lines of “if you can’t measure it, you can’t manage it”. This is also very true when it comes to diagnosing problem vehicles. It’s my belief that if the vehicle is exhibiting a problem at the time of test there is a symptom and if you are measuring the vehicle in the right way, with the right tool, you’ll be able to find what’s at the root of the problem.
This sounds very simple but what does this mean in practical terms? Well, this depends on the vehicle, the system and the symptom. An essential skill for the automotive diagnostician could be termed ‘appropriate tool selection’. It could be that for a rough running petrol engine, the gas analyser is the right tool to measure and manage the fault initially. For an auto transmission, which is struggling to select a gear, a scan tool and pressure gauges may be the right tools. For a parasitic current drain, a multimeter would most likely be the right choice. Each of the measurements that are made (and the accompanying data analysis) will help to eliminate suspects. The task is usually to measure and analyse until the root caused is discovered and proven beyond reasonable doubt.
Look busy
What about if you measure the system in the wrong way with the wrong tool? This could be considered as pointless as not measuring at all and just guessing what’s wrong. How about if we used a gas analyser to check for parasitic current drain? How ridiculous! This is an obvious example; obviously wrong to prove a point. However, less obvious but still inappropriate, what about removing a component from a circuit and measuring its static resistance versus measuring it on the vehicle, dynamically, with an oscilloscope? In general terms, measuring resistance alone to diagnose a fault falls into the same category as current drain with a gas analyser. You’ll look busy, you are using workshop tools, you can charge labour time for diagnostic checks and, to the untrained eye, you are putting effort in. However, in reality your efforts will do little to help discover the root cause of the problem.
Why then, I hear you ask, do most of the available data sources contain resistance values? The answer is not because it’s the right thing to do but more likely because it’s the easy thing to do. Information providers don’t do this on purpose to make your job difficult. They are simply re-presenting data which they buy from the vehicle manufacturer under licence. Their expertise is in packaging and selling data, not in testing and measuring problem vehicles. Their view could be that if that’s what the vehicle manufacturer says, then that’s good enough.
In reality though, is there a vehicle manufacturer who actually manufactures their own components, or should we consider them as vehicle assemblers? Isn’t it simply the case that they get their electronic ‘bits and pieces’ from the component suppliers’ market (ATE, Autoliv, Bosch, Delphi, Denso, Siemens etc.)? The component supplier provides specifications for their systems and components which the vehicle assembler may need to make sure that parts are compatible to their design. Is this the most likely source of where the dreaded specified values of resistance come from? Also, consider this; some of the vehicle manufacturers don’t consider the oscilloscope to be a necessary workshop tool. I have had this from the ‘horse’s mouth’ on many occasions from representatives of several vehicle manufacturers. So, why would these vehicle manufacturers ever consider providing comparative dynamic (oscilloscope) data which would help technicians diagnose system faults? I think we have our answer.
Resistance is futile
So, is measuring resistance for diagnostics is futile? The answer is, on its own, mostly. It is an indisputable fact that resistance has an effect on a circuit (consider Ohm’s Law) but it’s not a static value and shouldn’t be our only, or primary concern. There is much else in a circuit which has to be taken into account and which can be seen using an oscilloscope. PLEASE DO TRY THIS AT HOME. Remove and measure the resistance of a front sidelight bulb. Use Ohm’s Law to predict the current flow in that circuit. Refit the sidelight bulb and then measure the actual current flow. If the predicted and the actual values are the same, I’ll eat my hat.
There have been several jobs in the Technical Topics workshop recently, which highlight the need for dynamic circuit analysis. One of these was a 2005 basic model Astra. The fault was that the ABS light was on and the ABS wasn’t working. The previous workshop had interrogated the ABS ECU with a scan tool and pulled DTC C0040. This code related to an offside wheel speed signal fault. They had performed a resistance test on the unplugged wheel speed sensor, which was the same as the near side, so they tried to measure the voltage output. On spinning the wheel, the output didn’t fluctuate and the scan tool live data showed the same – zero speed, despite the wheel rotating. A conclusion was drawn that the reluctor must be in trouble. This vehicle uses a magnetic wheel bearing to excite the sensor which in turn, produces a waveform. A decision was taken to change the bearing/hub assembly – not a cheap job. Upon reassembly, the vehicle symptom was distinctly similar.
The ECU was next in line to have doubt cast over its ability to function but as the spend on the job was getting rather large, and with no discernible improvement, the vehicle was bought to the TT workshop for more detailed analysis.
Figure 2- The offside front sensor output
Current clamp
To accurately analyse the system we used the Micro Amps Clamp, see Main Image, in conjunction with an oscilloscope to read the small (several thousandths of an amp) output from the sensor. The system uses active wheel speed sensors which prod
use a very low current square waveform, the frequency of which relates to wheel speed. The current clamp was placed around the sensor wires, close to the ABS ECU to enable us to see exactly what the ECU was seeing. First, we checked the offside front sensor output, see Figure 2, then the nearside sensor output, see Figure 3. To our surprise, they looked similar.
We have only one of these Micro Amps clamps in the workshop, so it was not possible to review both wheel speed sensors live at the same time. This was not a problem though as we used an advanced feature in the scope’s software to allow us to overlay multiple waveforms on a single screen (we cover this technique in our Advanced Scope Training Course). With the scope setup and both waveforms on the same screen, the difference between the two sensor outputs became more obvious, check out Figure 4.
Figure 3- The nearside sensor output
So, although both sensors were measuring wheel rotation, the output from the offside wheel wasn’t registering in the ABS ECU. Notice how the amplitude from both sensors is close (7.1 uA, or micro amps, vs 7.2 uA), there is something causing a 3 uA offset which is affecting the offside sensor output. 3 uA doesn’t sound like much but it represents a 40% difference between sensors.
Figure 4- Both wavefroms on screen she clearer difference
This test shows that there was not a problem with the reluctor/hub and it shows that the sensor is producing a decent output. If this test had been done early on in the process, the first garage would have spotted the issue and the customer wouldn’t have paid for a hub and labour which wasn’t required. So, what was the problem? It was quite simple. There was a voltage drop on the wire between the ECU and the offside sensor which I found using the TESLite Pro. The fix was to graft in a new section of wiring and with the voltage drop removed, both sensor outputs were identical and the problem was solved.
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