Okay didn’t want to make a new thread. OP you’re problem is very similar to mine.
2004 V8 fully loaded keyless entry air suspension towing etc. it was a one owner with 150k miles and very clean. Since I own a Phaeton thought it would be easy to track this voltage issue down. Owner was up front about it and I honestly figured one of the following would have fixed it..... it hasn’t and I’m at my whits end finding the source of the problem.
I have replaced the Alternator
Replaced the B+ battery cable.
Replaced the positive jump post
Replaced engine ground wire
Removed all ground wires (two in passenger plentium / two under passenger frame rail near SAP / and two grounds on each footwell inside.
Removed carpet on both sides and inspected wire looms for splices. (Re connected all splices with new connections just Incase )
Brand new battery
New air compressor
New air compressor relay
New serp belt
Rebuilt Kessy with new MOSFET chips and tested all resistors
Removed all major fuses cleaned contacts in fuse box in engine bay. Driver dash and passenger dash and all connection points under driver seat.
There are 0 faults present until about 1 hour into driving.
I can hear relays start to click and you start seeing the low voltage errors start popping up one by one effecting modules furthest away from the battery. Eg trailer , air suspension control module. Rear diff etc. headlights flicker too and if really bad fuel cap / large evap leak. Intermittent cat bank two. All codes are sporadic and never show if voltage stays at a normal range.
However resting voltage is 12.9
Idle voltage is 14.1
If rear windshield heater is on with stereo, headlights, fog lights , all 4 heated seats you get 14.5 volts at the cig plug inside and at battery terminals, positive post in engine bay etc.
But once the car has been driving for a bit you start to see the voltage hover around 13.9 then around 13.8 is when the faults start to appear.
Is there anything I missed ? I’ve been working on this for a month now.
I also replaced the following items before I knew that this electrical issue wouldnt go away.
Rebuilt transmission
Timing belt
Timing chain cam solenoids both banks
Valve cover gaskets
All vac tubing for egr to intake flaps
Cleaned intake manifold
Removed carbon build up on intake valves
New spark plugs
New coil packs
New 02 sensors x4
New cats
New brake vac hoses
New power steering rack
New upper and lower control arms front and rear
New brakes front and rear
Rebuilt both front and rear diffs with new fluid
Cardin shadt / carrier bearing
New rear drive line
New level sensors x4
New tpms sensors
New xenon ballasts
New xenon bulbs
So now about 15,000$ in parts and countless hours on my part I now have a perfect paper weight...