@niggly. Both the Front and Rear sensors are used on the O/S suspension arms. They compare each others position through the MASTER ballast. When it has worked out the adjustment, it tells the SLAVE ballast, and BOTH adjust the Headlight aim height (certainly a lot quicker than me trying to explain it) Rear of the car is more likely to have more movement than the front , because of the way you load the car.
I don't know if you have read any other bi-xenon post, but I have had ballast issues. The bulb would go out after 2-3 minutes, changed the ballast, this happened to both ballasts. The ones I fitted WERE genuine, BUT hadn't been through the Volvo Pre-programming ,so Self-levelling couldn't be achieved.
Now, the really strange thing here is, since Refitting the original faulty units, (and Indy garage programming ) my lights haven't gone out once in 3 months !!, (kiss of death) . They do have a delayed adjustment though . The car (06 v50) hadn't had a update for 50k 5yr, but why work now?
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