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I need some help or direction. I recently purchased a salvage a35 which had undercarriage damage. On inspection it appears the wiring cable underneath the vehicle had been destroyed, chafing the wires which I suspect caused a short circuit as they were exposed and were touching the undertray/gearbox pan.

I have replaced these wires and we’ve got power to the vehicle including ignition. Although the vehicle does not start or attempt to start. Having a look at the diagnostic machine it appears I’m getting no communication with any of the vehicles modules, nothing seems to work. Before I go ahead and look to get a new ECU, I would be grateful if anyone could assist and maybe point me in the right direction as to what to look for or test.

Also was wondering if the ECU has blown, would this cause me to get no communication with the rest of the vehicles modules such as the TCM/BCM/ECU/4 MATIC MODULE, SUSPENSION MODULE etc

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  • Welcome to Motor Vehicle Maintenance & Repair! Is your diagnostic machine communicating with the ECU? Have you checked fuses to ensure the modules are getting powered up? Commented Dec 31, 2023 at 18:54
  • Hi Paulster2, many thanks for getting back to me. I have checked the fuses in the engine bay, all fuses appear to be okay. Unfortunately, I’m not best with electrics so unsure how to test the relays. In terms of communication with ECU. I am able to get power to the vehicle along with ignition. However once plugged in I get no communication from most of the modules in the vehicle. It does read a few modules such as ambient lighting etc but majority of the modules come as as ‘not equipped’ on my diagnostic machine. Commented Dec 31, 2023 at 19:19
  • That sounds more like a shortcoming of your diagnostics machine, not your car (at least at this point). Commented Dec 31, 2023 at 20:03
  • I will try another diagnostic machine, however once the vehicle has power and ignition there are a number of systems which don’t appear to work at all for instance the front headlights, the steering wheel controls etc. Which leads me to believe there is definitely an issue with the systems communicating or either the modules are just blown. If the ECU was at fault would this cause the other modules in the car to not work? I don’t really want to plow money into the car without knowing the engine and gearbox are okay, so my main aim is to get the vehicle up and running. Commented Dec 31, 2023 at 20:26
  • You stated there are (was) issues with the wiring harness. You sure this isn't the cause of things not working? Commented Dec 31, 2023 at 21:02

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Nearly every circuit on this car has fuses, if they are in fact all good then check your voltages at data link connector(dlc). Pin 16 at DLC is your battery power, pin 4 & 5 your grounds, pin 6 and 14 your communication to gateway module which has access to all communication networks (CAN and Flex-ray). A shorted wire on a bus will inhibit communication to any module on the bus.

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