Steering repairs

Scoping skills come into play with a steering assist fault

Published:  19 March, 2015

By Frank Massey

I have written many times about adapting skills and choice of tools long before vehicle systems became so integrated. I can reflect back over several years where many respected diagnostic organisations scorned the use of the oscilloscope. I can also remember 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 a technical 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. The problem was, much like a top-end sports car, true performance came at a price, until Pico's automotive scope range came along.

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 FlexRay are in the order of 1 MHz, that's 1,000,000 cycles per second. Most control events are much slower in the kHz range with 1,000 events per second. The problem however is that the scope is much like a policeman, to catch the villain he has to run much faster - under test, ten times faster than the speed of the signal. Bandwidth is a scope's expression of capture speed and Rainbow offers 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 and in order to capture and display them a 250,000,000 'always on' memory is available.

A VW polo 1.2 9N came to us with an intermittent problem with steering assist and ABS. The central electrics module reported 1598/566 errors, the steering assist module reported 566/1288, 'Drive Battery Voltage: terminal 30 signal low' and charging performance problems. All of which would direct you towards a voltage drop problem but what about excessive load or a short?

I generally choose a sample rate of 4MS/s, the higher it is the more detail you get but the downside is an increase in background noise, this can be filtered according to experience or confidence in evaluation. The upside is much greater detail when you zoom in. I choose no trigger (free run) with a long time base, in my opinion, more is better.

Figure 2.





This is how it stayed throughout extended driving but we did remove, clean and lubricate several critical contacts in the loom. The main power and ground leads to the steering assist were carefully checked and all grounds cleaned with battery terminal spray, see Figure 3. The main PCM sockets were also examined and lubricated. Was it fixed? I doubt it, I suspect it's an intermittent short in the steering assist module motor. The point I expressed to the customer is that the scope conclusively proved what was working correctly at the time of testing and all we need now is opportunity!

FIGURE 2:? Current draw was measured from the steering assist actuator

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