Hi everyone,

 

My son has a 2001 Octavia MkI 1.9 TDi. 10 miles into a run it conked out, could not be re-started and went home on a breakdown truck.

VCDS-lite  could not connect  to the engine module, although it would connect to others such as the abs system.

A quick check of fuses revealed fuse 29 blown, and it blew again when replaced as soon as the ignition was switched on.

 

No obvious signs of a short anywhere. When briefly replaced, fuse 29 operates relay 109 but even when relay 109 is taken out the short is still present.

 

So my questions are:

Has anyone come across this before?

Is there an obvious sensor that could be shorted out?

Can anyone point me to a wiring diagram so I can see what else is fed by fuse 29?

If it only feeds the ecu, then can anyone point me to a diagram of the ecu pinouts so I can trace the short?

 

If the ecu itself has failed, can I replace it with a secondhand one and how do I get around the immobiliser?

 

The only similar thread I can find is:   http://www.briskoda.net/forums/topic/122517-20-petrol-engine-wont-start/#entry1590242

which ends without a conclusion, but Teflon Tom seems to have access to a decent wiring diagram!

 

Hope someone can help,

 

Thanks!

 

Dave.

Please can anyone at least tell me whether the Haynes manual has a reasonably detailed wiring diagram that will help me find this fault?

Thanks,

Dave.

We had similar on a Felicia the other day.  You could try unplugging the ECU to determine if it's an internal ECU fault, though moving the wiring harness when unplugging it could move a chafed wire away from where it is shorting out.

I doubt that any wiring diagrams in Haynes will be detailed enough, but I may be wrong.

You can access the full wiring info for your car on a paid-for basis via erWin .   You have to register, and be logged in, and pay around £6 for an hour's access, but then you can download the so-called 'current flow' diagrams for your model.  You may be able to see, from the above current flow diagram, exactly which functions that fuse serves. Sometimes/often fuses serve more than one item, making it harder to track down the fault.

While you're in there, there's a load of other info available to download, to make the most of your money.

Thanks for that - I'll check out erWin and see what I can find 

Solved. Went on erwin and downloaded wiring diagrams. Not easy to read as all in german. Did I download from the wrong site?

 

Anyway, fuse 29 supplies the ecu as suspected, and on other models it must also have another function. There's another wire that comes to a dead end at a 14-way connector next to the inner wing behind the battery. This connector had corroded causing two other wires to overheat, melting their insulation and allowing the 'dead end' from fuse 29 to short to earth.

I cut off the connectors and soldered / shrink sleeved the wires and now ok.

 

Or almost. Maybe a coincidence, but now fuel and temp gauges both stay at zero, and there's about 4volts on the Brown/white 'earth?' wires. Not sure if these are genuine earths as the diagrams aren't clear where they would be grounded. If I do ground the brown/white then the temp gauge goes up to normal and stays there regardless of whether the engine is warm or cold. I've double checked all the wires I soldered!

 

At least it's running!

I think you might have picked the wrong site, all the ones I've downloaded are in English.  I've since got Mk1 Octavia ones myself, so can have a look at the Instruments section for you, what engine code is your car? (Not sure it matters but always useful to know when looking for parts or wiring info).

 

Good find on the corrosion related problem.

Thanks! Engine code is ALH.  I've found a few posts related to fuel & temp gauges not working pointing to poor soldering in the instrument cluster, but none says whether this also results in 4 volts on the brown/white.

 

I think I'll have to get the cluster out and have a look.

Confusing, as to me it surely looks like the two brown/white wires should be earth, connected somewhere behind the dash, however...

 

 

...The wire connecting the dash panel to the coolant temp gauge sender (G2) goes via (pin 4 of) a 14-way connector T14b, "14-pin connector, in front left of engine compartment (black)" which sounds suspiciously like the one you've been looking at for the fuse 29 issue. That must surely be related?

Edited 10 June, 201610 yr by Wino

I know it looks that way. I can check again tomorrow, but I'm sure I have connected the pin 4 wires (violet) to each other.

 

The brown/white was one of the wires that had melted and shorted out the dead end from fuse 29. If it is a true earth it should not be at 4 volts. It could be getting the 4 volts via the temperature gauge, as that gauge responds when I earth the brown/white. However, the gauge rises even when the engine is cold so I'm wondering if maybe the temperature gauge is designed to stay at 'cold' until the output from the sender rises above 4 volts?

The erwin diagrams that I have dont show where the brown/white wires are earthed (if they actually are)