scope of diagnostics re ABS ECU

[hsq21]

Hi there,
I'm new here so I'm looking forward to a helpful experience !
Can anyone tell me, authoritatively and perhaps in simple layman-speak, how comprehensive is the diagnostic used by my Skoda dealer following (repeated!) ABS failures on my 2003 Fabia? It has taken 3 diagnostics and 2 new ECUs in the last few months to come up with an alleged solution (something about wiring modification being necessary on my particular model). Surely this should have been discovered first time round? Otherwise the expensive diagnostics are a bit of waste of time and money!

Thanks in advance for any offerings of info (and cash!)

[fordfan]

When you turn on the ignition the ABS will do a self check, and when moving forward for the first time you may well hear the ABS pump doing a self check. The ABS control unit stores fault codes if a fault with the ABS system is detected. To alert the driver of a fault with the system the ABS warning light is activated.

When a dealer does a diagnostic check, they will get any fault codes stored. They might be live codes (where the fault is still present) or they may be intermittent (sporadic) codes, where the fault has ocurred and then appeared to fix its self.

I say fault codes, what the machine does is display symptom codes, and not the cause of the problem. For example, we had an Octavia front wheel lift towed into us as it had been mis-fuelled. We sorted that out , and upon starting the car the ABS light was on. The codes told us that both front wheel sensors had failed and needed replacing. We knew better as the car had been towed with the ignition on, with the rear wheels moving and the front wheels not moving.

In the old days when the technician had the codes they would look it up in the workshop manual and undertake any testing/component replacement procedure specified. From what I understand these days the VAS machine that they use will take them through a guided fault finding procedure which they will not be able to deviate from. If doing a warranty job and they fix a problem without following this procedure, they may not get paid.

What the technician does to find the cause of these codes is based on his experience and knowledge of the product, the guided fault finding is designed to take these variables out of the equation so that a less senior technician can get a first time fix. So on the face of it, it is quite comprehensive, however with computers the output is only as good as the input.

Some fault codes will condemn a control unit outright, such as an EEPROM error code, which means that a chip in the ECU has had it, though what causes this can be a mystery, fault in the chip or an external factor such as an over voltage. In one case replacing the ECU fixes it and the other you are just treating the symptom. Do you spend more time looking for a wiring fault that may not be there once you have replaced the ECU and it checks out OK that it has been fixed?, if so do you think that the customer would be happy to pay? 'We've replaced the ECU and it appears to be OK, we spent a further 2 hours checking found nothing and that'll be £120'. - light the blue touch paper and stand well back.

What was your code, reading between the lines I suspect that it is a communications error code to the ABS ECU. It could be a transceiver fault in the ECU which a new ABS unit should sort. However with any manufacturer they don't like to acknowledge any short comings in their product. Where the main wiring loom passes through the plenum chamber, there is a grommet with a collar on it, on assembly they tape this collar into the loom, when the plenum drains get blocked the water builds up and pools around this grommet, the collar acts like a funnel directing water straight down the insde of the loom. Water in this loom can cause communications problems as the communications voltages are very small to start with, any loss is significant. I shouldn't think that this will appear in the guided fault finding.

So to answer your question, yes the diagnostics are comprehensive and the dealers invest 10s of thousands of £ in them, but they are constrained by procedure.

To have got the best you should have got the fault codes from the dealer and then posted them as there is nothing quite like this forum to gain knowlege about the real world causes and fixes.

[Andymod]

Take a very close look at the strip fuses on top of the battery, I think its fuse 4, they can crack and cause ABS faults. Had quite a few 9N polos with the same fault

[VRStu]

Could also have been caused by the infamous engine bay wiring loom. They suffer water ingress over the nearside wheel arch. There is a modification for it and if you have full Skoda history they offer some goodwill even out of wty.

[DGW]

Originally Posted by VRStu

Could also have been caused by the infamous engine bay wiring loom. They suffer water ingress over the nearside wheel arch. There is a modification for it and if you have full Skoda history they offer some goodwill even out of wty.

Is there a DIY fix to prevent this happening in the first place, eg Waxoyl or similar?

[VRStu]

There was a plastic part fitted I think Denis. Only affected the earlier cars so you should be ok.

[fordfan]

Saw one on a 53 plate car last week, a plastic tray that slopes towards the left, to channel the water towards that drain hole The V plate one I saw a while back didn't have this and had a drain (blocked) next to the loom.

*******YXY3009880 must be one of the earliest ones that I have seen, it had a November 1999 date on the battery fuse holder.

[DGW]

Originally Posted by VRStu

There was a plastic part fitted I think Denis. Only affected the earlier cars so you should be ok.

Thanks, Stu.