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Audi S4 Case Study – Multiple ailments and a host of suspects

By autotech-nath on May 16, 2022

Troubleshooting faults in a workshop is rarely a dull moment whether you’re a newbie, beginning your deep-dive journey into modern vehicle fault assessments, or a seasoned pro who thinks they’ve seen it all. I am sure some of you readers can sympathise – my mind can sometimes wander, and I’ve been known to have the occasional brain fart, varying from slight to beyond moderate on the scale!

In preparation for the latest job to enter the workshop I read the job card the night before and made sure to have a brief conversation with the customer at drop off to get a better insight into the reported symptoms.

It was at the point of questioning the customer I felt one eye become ever so slightly lazy, picturing myself wearing a shaggy beige raincoat chewing a cigar and thinking about the ‘case’ in front of me – for those of you old enough to remember, and cherish, the memories of Columbo, detective extraordinaire. The irony here is that I floated the concept in my last piece

for AT that ‘we are analysts more than technicians.’ In some respects, we’re just as much detectives in the case of fault troubleshooting, which I think for a lot of us is the grit that keeps us doing what we do, to a good standard, day in day out. I digress….

Gareth found a useful collection of TPI’s all relevant to random alarm siren activity with no other DTCs stored.

The vehicle in question is a 2017 Audi S4 and had been in with us four weeks previously for a replacement battery. I had not paid much attention to the car in question at its previous visit. The customer’s concern was that the Stop Start system was not working and it never seemed to kick in, the A with a line through symbol would appear frequently when warm, but no matter what the circumstance or length of drive, it did not work. Furthermore, he asked us to investigate the heated rear windscreen not working on cold mornings, but the mirrors cleared quickly when the button was depressed. He also added an additional concern of reporting the alarm going off intermittently.

I began to consider the outcomes and fault patterns whilst ODIS was doing its thing conducting a full vehicle scan. Tool choice here was natural given I wanted to look at some key pieces of data and check a function had been conducted post battery replacement. Alarm triggers were going to be my go- to for the one customer concern, quiescent current analysis (battery replacement history, type, serial no, A/H) the next, and rich live data for the rear window concern. I had already created a 3-stage hit list, but I’d also reminisced of previous learnings on jobs and the importance of vehicle behavior patterns when it thinks one thing and is actually another.

In terms of stored DTC’s, there was not a lot to go on, 1- Alarm triggered, and that was it. Where to next? There was no guided fault finding or test plans created because there were no DTC’s. My next port of call before having my sacred brew and read was to put some more meat on the bone. It was unlikely in the time I had that I would get the alarm to misbehave going on the very intermittent frequency reported by the customer, but in terms of a quick road test (car was already warm from drop off ) I set off to observe Stop Start activity, and then a quick check of the rear window heater being switched on at the climate control panel, and a thermal camera to definitively confirm mirrors working, rear window not. The outcomes were as described, Stop Start did not work when I expected it should and the mirrors got toasty within a matter of seconds, with no sign of the window doing anything at all. With the faults confirmed, it was time to take a brief pause to do some research. I set about the following hitlist checks as a baseline start:

• Any TPI’s (Technical Product Index) relevant to any of my fault patterns?

• Current flow diagram of the rear window heater (note the setup is different for Avant and saloon so be careful, and in addition the Germans call it a rear window defogger, remember those search term definitions!)

Ensure data is the latest revision

The information search was useful. I fished a collection of TPI’s, see image above, all relevant to random alarm siren activity with no other DTCs stored, this bulletin has been updated several times. A TPI can have multiple revisions which is useful, but it can mean you come unstuck later if you are not reading the latest revision, as the guidance, or more importantly the SVM code, may have changed. The fix for the alarm was a Body Control Module software improvement, and had been through multiple changes, notably changing the SVM code for use within the ODIS SVM function, every time. I accessed the TPI through Alldata and the SVM code did not work. I rechecked the TPI’s within ODIS and then noted the latest change to the SVM and it applied correctly. Worth remembering when evaluating data and information, if in doubt, send a ticket as it may have changed in a short period of time and could leave you puzzled when you knew you were on the right track.

I was able to locate a current flow diagram, see below. Note the Asterisk marks that refer to a number code, advising on difference between model variants so you can follow the correct path for the control and wiring. I did not, however, yield a result on any relevant Stop Start issues.

Current flow diagram of the rear window heater.

Heading to the car it was time to begin some actual ‘doing.’ First port of call was the Stop Start issue. I headed straight for the battery management side of data. The reason being, I had pinned hopes on the battery replacement we had conducted potentially not being ‘registered’ in the car’s control unit, so it was still thinking it had a discharged poor state of health battery fitted. If this was the case it would validate the Stop Start issue without further troubleshooting, but in addition, may shed light on the rear window issue. ‘How so?’ I hear you cry. Well, little did I know some years ago, I had to investigate a rear window ‘defogger’ not working on a BMW 1 series 5-door hatch back. I ‘dithered’ away hours on drawing a blank on wiring checks, covers and trims off, checking this, checking that, to find that the car had the impression a poor state of health battery was fitted. I found this info eventually and checked the battery state of health data according to the IBS (Intelligent Battery Sensor) control unit log. The car had already had a new battery fitted a few months before being presented to me for troubleshooting. I registered the battery and low and behold, button depressed, windows getting warm! What?! Logical when explained and researched. Battery charge preservation becomes essential when the vehicle

(control supply) side of the circuit. One quick test and much of the circuit confirmed as good. Things then got a little more interesting. There appeared to be a ‘suppressor’ in the circuit, likely to reduce the effect of a high current consumer affecting something else in the rear screen when the high current consumer is activated. I checked the literature again, confirmed the rear screen was a busy affair, not only having the heater element, but also the antenna for the radio embedded within it. Further reading stated on my variant that there was a window suppression unit on the NSR D Pillar and a radio suppression unit on the OSR D pillar. With the NSR D Pillar off I confirmed the location and fitting of the suppression unit. Interestingly, at this stage, my sleuthing nature noted the presence of filler dust and body shop hallmarks, and rechecking in the NSR quarter area where the relay was located, there were also hallmarks of a previous repair. Now I’m not paid to judge, or spoil people’s days. The repair seemed adequate, but it did add an extra element we should always be mindful of when troubleshooting – human interference and disturbance.

The control relay was easy to access and test.

Human error

At this point I continued with my test plan. What’s happening at point B (the screen element connection) and what role is the suppressor playing? Above, you can see my test light showing a healthy voltage coming into the suppressor, confirmed by the meter, see below, right, and bright test light, but I had
no output at the screen just a stone’s throw away from the suppressor. I took the unit off given it appeared at the previous epicenter of damage/body repairs, checked resistance in preparation for a load test of the suppressor prior to ordering a new one (which at this point I had already found on the parts catalogue and was priced at a very reasonable £12 direct from TPS). My picture shows, I had failed to identify the fitment was incorrect, probably at rebuild stage by the body shop. The output wire for the rear screen was fitted to the centre bolt hole to hold the suppressor to the pillar, and not the output + terminal (ironically marked on the suppressor). No wonder it did not work. I moved the wire connection and cycled the window heater. Dew Dew (pun intended), on a thermal camera the window is now working! Stupid is as stupid does. The reality is this was as honest a mistake as the individual made when rebuilding the car in the first place. But it does show you have to have your mind in the zone and your wits about you when assessing faults. Test and check everything relevant, and always stop to reevaluate where you where, are, and plan to be within the task/process.

I smiled on reflection, and out of curiosity load tested the old and new suppressors. I had differing results so opted for the menial cost to advise and supply the replacement regardless of findings and despite proving the original could work. But nonetheless a useful learning, followed by the warm inner glow that I got the right culprit, I could leave on a Columbo quirky cliché and ‘limp’ off into the sunset. In reality it was pop to the loo with sheer excitement and collect the next job card!

I like documenting my tales and findings from the real- world workshop, as much as I like reading those from other like-minded technicians case studies. We need to keep up the fight, to research, understand, test and be confident in a job well done. Keep going, and never stop being the best you can be.



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