i have a 2004 audi a6 2.7 twin turbo , i have replaced the battery ,cables and altenator , when the car is running with lights and a/c off the voltage is at 14 but when the car is at idle and i turn the a/c on the voltage drops to 12v any suggestions ?

 

Take it to an Autozone type store and ask them to test the alternator and battery -- make sure they are good. As long as it reads 14v when rpms are up, I would not be too concerned. There are a lot of electronics in this car and with lights, radio, a/c fan running it will draw a hefty charge - more than your alternator is putting out at idle - and will draw down your battery briefly. Once the rpms go up, the battery recharges and you should again read 14 v.

Other thoughts: That leaves the voltage regulator. Also, there was a recent post about an alternator change and the owner put in 2-3 alternators (120amp) and kept having charging issues until he replaced it with a 140amp unit for an All-road.