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Raised from the dead
One day, an Audi Q7 with a 3.0 liter diesel engine stopped working and would no longer start.
Raised from the dead
The 13-year-old Audi Q7 TDI belongs to a Carinthian businessman and already has 170,000 kilometers on the clock when one day it plays dead and no longer wants to start. The premium SUV is towed to the nearest brand workshop and connected to a diagnostic device. An error on the solenoid valve N3 in the transmission control unit is displayed, specifically the error message is “plus-closed” and a missing message from the CAN bus is displayed in several control units. The decision is made in the workshop to replace the entire mechatronics of the 6-speed automatic transmission installed in the Q7. No sooner said than done, but after the replacement the error persists, the Audi doesn't want to start, the error message appears again. The disappointed owner consults the Klagenfurt technical trainer and diagnostic specialist Michael Pirovc, who takes on the case. A diagnostic crime thriller begins.
- Der Experte schließt den Audi an ein Mehrmarken-Diagnosegerät an und erhält ebenfalls die Fehlermeldung „Plus-Schluss am Magnetventil N3“.
- Aus Erfahrung weiß Pirovc, dass die Meldung möglicherweise auf eine Unterbrechung der Masse-Verbindung hindeutet und macht sich auf die Suche.
- Am Display des Diagnosegerätes lässt sich der Stromlaufplan des Audi Q7 aufrufen. Der Spezialist erkennt: Die Masseverbindung des Magnetventils geht zum Getriebegehäuse, das am Motor befestigt ist.
- Mit einem Multimeter misst er den Spannungsabfall und stellt fest: Der Masseschluss des Motors zur Karosserie ist offenbar unterbrochen.
- Auf der Suche nach dem Übeltäter überprüft Pirovc sämtliche in Frage kommende Stromleitungen und wird schließlich fündig.
- Das am Motorgehäuse via Kabelschuh angeschraubte Hauptmassekabel ist gebrochen und lässt sich einfach aus der Tülle herausziehen. Die Tülle hatte den Fehler gut versteckt. Der Experte konnte ihn erst entdecken, als er wusste, wo er nachschauen musste.
By bridging the break, the error is resolved and the Q7 can be started without any problems. The ground cable is replaced and the diagnostic mystery is solved.
Power cables in the engine compartment are exposed to both the weather and strong vibrations while driving. The heavily stressed wires are protected by thick insulation, but this makes it difficult to find a defect. However, an initially invisible cable break can be located if the current flow is checked using a multimeter. This practice requires a certain amount of experience and also the ability to find the circuit diagram that matches the error and interpret it correctly. An art that automotive technology apprentices train extensively at vocational school using practical examples and exercises. In hectic everyday workshop life, however, it can happen that replacing a seemingly defective part is the simplest solution to the problem. If this remains unsuccessful, it can definitely help to apply the basic knowledge you learned at school.