Scope school with Frank Massey
Frank Massey is one of the UK’s leading experts in vehicle Diagnostics with over 40 years of industry experience. He is renowned in the motor industry for his unrivalled expertise and knowledge as well as running a successful commercial workshop.
I have written many times about adapting skills and choice of tools long before vehicle systems became ever more integrated, I can reflect back over several years where many respected diagnostic organisations scorned the use of theoscilloscope. I can also remember that when the realisation of their vital role in systems testing finally became acknowledged the technical competence of most, so called, automotive scopes was nothing short of atechnical betrayal.
As part of my evolution I advocated in favour of specialist manufacturers from the electronics industry. Choosing an oscilloscope for the first time is like choosing a suit, it requires the help of a good tailor. Performance is much like development of a successful sports car, it starts from the chassis up. Scope performance can be focused in 3 key areas.
Acquisition,storage, and display. The problem, much like a top end sports car is that true performance comes at a price, until that is Pico’s automotive scope range came along. In keeping with the incredible evolution in automotive electronics the latest pico rainbow 4 channel automotive scope, represents the cutting edge of signals analysis.
My focus for this topic is based on a particular interesting diagnostic challenge together with the assets of Rainbow. Firstlet’s take a visit to the tailor.
Acquisition is how a scope captures incoming data, sample rates and triggering are key aspects. Vehicles, relatively speaking don’t use very high frequencies in control and communication networks. High speed can and flex ray are in the order of 1 MHz, that’s 1,000,000 cycles per sec. most control events are much slower in the kHz range, 1,000 events per sec. The problem however is that the scope is much like a policeman, to catch the villain he has to run much faster! X10 speed of the signal under test. Bandwidth is a scopes expression of capture speed rainbowoffers an impressive 400 MS/s sample rate with a 20 MHz bandwidth.
Rise time is another key performance indicator, events or fast voltage transitions like RF interference in can networks create error messages, in order to capture and display them a 250,000,000 always on memory is available.
Triggering is probably the most challenging aspect of event capture, this is the focal point from which the image is displayed, basic, advanced, pre&post options allow the capture of the most complex of events.
I have used some incredible scopes over the years the downfall is often display resolution most offer 8bit, rainbow offers 12 bit resolution with the added advantages of the latest PC screens.
So now you have a Tom Ford suit on your back let’s chase the villains. A vw polo 1.2 9N came to us with an intermittent problem with steering assist & abs The central electrics module reported 1598\566\ errors , steering assist module reported 566\ 1288 , all of which indicate driving battery voltage , terminal 30 signal low , charging performance problems .All of which would direct you towards a voltage drop problem ! What about excessive load or a short?
The next task is to monitor voltage at the battery terminals, voltage & current at the steering assist actuator and alternator smart charging control at the slip ring circuit. If yourmaths are good that makes 4 inputs, that’s why a 4 channel scope is a must!
I generally choose a sample rate of 4MS/s the higher it is the more detail you get, the down side is an increase in background noise, this can be filtered according to experience or confidence in evaluation, the up side is much greater detail when you zoom in. I choose no trigger (free run) with a long time base, in my case more is better.
The point of the test set up is to compare voltage drop across the fuse panel, at the same time to monitor correct alternator control when load is applied to the steering assist actuator and measure the all-important current draw.
CHANNEL 1 voltage dc post control fuse, note minimal load drop, normal
CHANNEL 2 alternator control duty cycle, note increase duty onload, normal
CHANNEL 4 steering assist, current on load 20 amps (1v=10amps), note current drop when steering wheel input is reduced, normal
CHANNEL 3 N/A
The pico image shows a peak current of 20 amps (1volt = 10amps) at the same time a minimal voltage drop occurs with no potential difference across the control fuse , at the same time you can clearly see the negative control duty to the alternator increase = increased load output , result, all is good .
This is how it stayed throughout extended driving, we did however remove clean and lubricate several critical contacts in the loom! The main power & ground leads to the steering assist were carefully checked, all grounds cleaned with battery terminal spray to finish. The main pcm sockets examined and lubricated. Is it fixed? I doubt it, I suspect an intermittent short in steering assist module motor. the point I expressed to the customer is that what the scope conclusively proved, is what was working correctly at the time of testing, and all we need now is opportunity!
