Hi all,
My 2011 TDV8 Vogue SE is still having low voltage issues (https://www.fullfatrr.com/forum/topic66148.html). A new battery was installed 8 weeks ago and the I did a reset using a neighbors Gap tool. The alternator seemed to be working fine and the hidden 753 screen showed charging at 14v+. A few weeks ago I got a EMS fault and the transmission started to shift at a noticeably higher rpm. at this point the hidden 753 screen showed charging at around 12v. The fault could not be cleared with the Gap tool so I charged the batter overnight and then was able to remove the error. At this point my garage suggested changing out the alternator under warranty as a first step.

Sometimes the car starts first time even though the battery voltage is less than 12v. Other times a get just a click, but the car starts after holding the start button down for 6-10s, which is what the OM says to do. The battery is generally in the range 11.7-12.3v and yet still starts the car.

Before taking the car to the garage I charged overnight on a trickle charge. In the morning the voltage read 14.3v (car not opened, bonnet switch cabled tied down) and dropped to 12.9v within a minute. why would the voltage drop so quickly? Is this the BMS at work. 24 hours later the battery had dropped to 12.7v.

Yesterday, while the car was in the garage I was able to try a test on a 59 plate TDV8 and the 753 screen showed 11.5v (checked with a voltmeter) and yet the car started with out a problem.

I was under the impression that the BMS should keep the battery at 12.7v and that the car shouldn't start if the battery voltage falls below 12v. But both my car and the one in the garage both start despite the battery voltage being around 11.5-11.7v. Is this normal?

Phil

see this

https://www.fullfatrr.com/forum/topic50836...p;start=15

and see my post number 9 on this thread

https://www.fullfatrr.com/forum/topic66378.html

the thicker wire replacement is a common upgrade for many of us, once that is done, then look to other causes, but generally it does the trick !

Do the starter relay mod as per FFRR14 in the first thread. I had almost identical symptoms. I fitted a new battery, a new starter motor and a new alternator chasing this but still had low voltage and starting issues. Thing is, the BMS is actually quite clever and, even though you’ll see low 12V as soon as you unlock the car as so many ECUs and lights etc, it will still start the car with lower voltage that you would reasonable expect.

However, the inability to start is almost certainly due to the single pathetically thin cable that goes from the fuse box in the engine bay, and works it’s way about 5m around the engine, dropping in gauge about half way along its length, to the starter motor. It’s the trigger wire that closes the relay in the starter motor solenoid that allows the high current feed direct from the battery to spin the motor. It’s simply not up to the job of holding the solenoid closed if conditions are not optimal and age/corrosion just create extra resistance down the wire exacerbating the problem.

If you want a ‘try before you buy’ solution, just run a suitable wire in parallel between the starter trigger wire in the fuse box and the starter solenoid. I’ll put money on that working and you having no problems starting as low as even 11.5V once done. If that does work, then I can’t recommend any more strongly to do the full relay mod as FFRR14 describes as an almost bomb-proof solution. I also did this mod about a year ago and, since I did, the car has never failed to start. Not once. In all weathers, and in all battery conditions. In the grand scheme of things I’m glad I did replace the starter motor and alternator as I’ll now hopefully have many tens of thousands more reliable miles out of those parts, as the originals had done 100k. However, for the sake of about £20 in parts and a few hours to do a meticulous job, the additional relay is probably the single best mod I’ve done to the car.

Good luck whatever you choose! 👍 Andy

2010 4.4TDV8 Vogue SE in Santorini Black with Ivory interior
2017 Audi SQ5 3.0 V6T Quattro in Volcano Red
2001 Audi Allroad 2.5 TDI manual with low-range in Highland Green. Currently SORN whilst undergoing some serious restoration!