Fig. 1

Current measurement, past, present and future

Oscilloscopes at the ready everyone, as in this issue Frank is looking at current measurement

Fig 2.

By Frank Massey |

Published:  01 December, 2021

I think it is very easy, with the fast pace of technology, to focus on the present while forgetting the lessons of the past. With that in mind, I’m going to take a review of current measurement and its application, past, present and future.
    
My interest in current measurement goes back to the 1980s, when control systems were very simple when compared to the present day. They did however have similar responsibilities.
    
I was already using oscilloscopes for our diagnostic tests as serial diagnostics were in their infancy. Our focus was on wave form profile and event synchronisation, for example cam, crank sensors, injector and ignition events. This proved to be a successful process. I will explain why a little later. As our reputation increased, we attracted more trade work. As is the case today, blame was often directed towards the PCM.
    
As our trade customers were often reluctant to bring the complete vehicle to us, we became involved in off-car PCM testing. This was an automated function test of the PCM’s ability to control actuators. We quickly became aware of false pass and fail results which, when tested on the problematic vehicle, proved to be current flow-related.
    
This raised our interest and concern, and we realised that a suitable in-situ test process needed to be found. In those days, all our electronic test tools came from the electronics sector as no satisfactory test tools were available within the motor industry, unlike today. We were directed to the hall effect current clamps so common within the PICO and other range of test kits.

Importance
To qualify the importance of the process, I will explain a very interesting fault from way back in the early 1990s.The vehicle; A BMW 850 V12 which employed two synchronised PCMs, each controlling one bank. The car came to us running on one bank with one ignition coil completely burned out due to excessive current flow.
    
We carefully checked the wiring for shorts, ordered a genuine new coil, swapped the PCM over to the faulty bank and successfully ran the vehicle on that bank. We then sent the PCM off for repair. When refitting to the vehicle, we observed it run on all cylinders for 5-10 seconds before burning out the other coil. Why was this? To condense quite a lengthy story, it transpired that the original faulty PCM allowed a current runaway within the coil primary circuit which did not show up under off-car bench simulation. This was the exact problem encountered with the network 500. By this time, all our off-car testing had ceased in favour of event and current path analysis using a scope with the vehicle intact.

Intervention
I have recently covered current flow in the ignition primary circuit (please refer to my two-part series in the  September and October issues of Aftermarket),  so we will begin with a simple saturated 15-ohm injector circuit. (See Fig.1).
    
You will observe that the current increase takes a slightly curved profile with a distinctive kink, this is the point when sufficient current flow allows the pintle to lift against the spring and fuel pressure. So, it was possible to predict sticking or late opening due to high fuel pressure without removal or injector bench testing, which was in its infancy then.
    
Previously the back EMF, normally around 80v, would indicate sufficient current and induction properties. Focusing on current analysis also confirms good voltage and ground paths. Switching will be found on power or ground and in some cases current flow may toggle within the PCM. Power switching and pulse width modulation (PWM) provides an initial high current peak with a reduced current period, allowing a much more accurate feedback sequential fuel trim.

Intention
Next, please refer to Fig.2, a PICO image as before, this time showing VAG power switched injector control. Given the range and variety in current clamps the world is our oyster when it comes to future application. When developing diagnostic process, we must first understand the limitations with test options and the various types of component control.
    
Having evolved from a simple saturated (on off) to PWM (on off variable period and frequency) to micro current and SENT (single edge nibble transmission) and data and diagnostics transmit only, our ability to diagnose components is being driven entirely to the serial platform. This is of course exactly what the VM intends.
    
How then might we use current measurement for control and response purposes? The control or command may be analogue, digital, duty, PWM, or binary format. However, all components that require movement or heat as a function require current flow and as such can be measured anywhere in the power or ground circuit.

Idle  
With this in mind, let’s look at a very simple idle air bypass valve. My intention here is to emphasise the importance of having the correct device. I will also show a similar example from a VAG swirl flap control. Please refer to Fig.3; Ford idle air valve current flow. Note the inaccuracy in current value despite using similar devices.
    
Now, look at Fig.4. Despite an obvious simple digital control signal, the current flow appears linear at 100mv = 1amp. Not possible, now compare the event with a more sensitive current clamp. Please refer to Fig.5.

Increase
To conclude my topic, I am sure many of your techs have replaced throttle body control modules, turbo vane control modules and EGR and swirl flap control motors. If so, I suggest checking the current path and feedback movement sensors. There should be an obvious synergy between current flow and movement, where a mechanical restriction will cause the PCM to increase the current supply. If this becomes excessive there will be a hard DTC, however this often leads to premature drive motor failure.





Fig 3.

Fig 4.

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