Seat Leon: An obscure fault

A Seat Leon turns up at D&D Autos with an unusual problem, Matthew Pestridge comments on the matter

Published:  27 April, 2012

We have great working relationships with the local dealers in our area and whenever possible we are happy to lend a helping hand. This month we had a Seat Leon brought to us by a local franchise (though not a Seat dealer.)

The issue with the vehicle was that it would not switch off whilst the key was in the ignition, so the only means of switching the engine off was to stall it... which was hardly practical.

Intermittent

Unfortunately, as with a lot of faults I see, it was an intermittent fault. The dealer had attempted to fix the problem by fitting an ignition switch but this resulted in the car not being able to run at all. Unable to diagnose the fault they decided to call upon my services as we are specialists in various brands of vehicles, Seat being one of them.

As the situation seemed a little odd, I connected the Autologic tool and ran a scan to see what faults were stored in the whole vehicle. The information showed power supply issues to several ECUs. I traced this to a fuse being blown, by using wiring diagrams. Duly replaced, the vehicle was now a runner and so I was able to test the vehicle for the original fault.

All the power supply fault codes had cleared and the vehicle was fully functioning but the ABS traction light and the steering assistant light were both on permanently. The customer was happy that the vehicle now switched off and took the vehicle away with the ABS and steering lights on.  However, the customer returned a couple of days later saying that the vehicle would not switch off again. The vehicle was left with me again but the vehicle would not do the fault for me....the joys of intermittent faults!  I decided to look further into the ABS traction and steering assistant faults to see if they were linked to the issue.

Wiring nightmare

One fault code logged the rear left speed sensor so I decided to start there and have a look at what the cause of that was. Upon testing, I found that both wires to the Hall-effect wheel speed sensor were 12 volts. I suspected the wires may be shorted together causing the fault, so using the multimeter I did some testing and found there were indeed two wires shorting out together.  Further testing revealed there were a total of five wires shorted together, two were for the wheel speed sensor and three for the power supplies from the fuse box, two 'terminal 30' power supplies and one 'terminal 15' power supply.

Upon further testing of the wiring I found part of the loom chaffed on the battery tray. Checking the wires against the wiring diagram I found one of the wires was bridged to the terminal 30 and one was chaffing against 'terminal 15' -the ignition supply relay which I suspected was causing the vehicle not to switch off occasionally. The other two wires remained connected to the near side rear wheel speed sensor.  I then repaired the wiring, re-checked the ABS system and found that it had damaged the ABS ECU. I had to replace this along with the ABS pump as it had a common fault for the brake pressure sender. Once I had fitted this the vehicle ran with no issues and all the lights cleared.

Equipment

It pays to have the right equipment and the right diagrams to help diagnose obscure faults such as this one. It's all too easy to try to diagnose a customers' fault by throwing parts at the job, which can be costly for the customer. I find the best way to tackle tricky jobs is to be methodical.

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