Dangler19

Hi, I have recently bought an A6 C5 avant from England only to find that the main 150a terminal fuse at the battery had blown and been replaced with an 80a audio fuse (who knows why). This has also blown and has now been fitted with an 80a MCB only for diagnostic purposes (easily reset). So the main reason for the thread is that I am trying to diagnose why the 150a fuse would have blown. The voltage dial in the car drops to around 13v when the Bi-Xenon or climate control module is turned on. Once the fan speed is turned up in the car the MCB trips. I do have a new fuse terminal unit but am reluctant to fit it until I find you what caused it to blow in the first place. Anyone have any ideas how to take this further? Regards, Daniel