Just bought a 2005 C6 A4 a couple of months ago and it had electrical issues so I replaced the alternator and battery with brand new parts. This fixed most of my issues except at a particular time. If the car is cold or has been left a 15 minutes or more it starts on the button with good battery voltage (normally 14-14.6V depending on how long it's been sitting). However, if I start it quickly again after stopping it (pretty much always when stopping for gas) it will still start fine but the voltage is extremely low (11-12V) and gives all the dash warnings.

Does anyone have any ideas on what this could be? I'm 99% sure it's not the alternator and battery since they're both brand new. When I was researching I found something about needing special nuts for the battery terminals (part 11612153) but these appear to be out of stock everywhere so I can't give that a go. Maybe a connection somewhere?

 

Quite likely the starter solenoid is shot. They run less than $20 on Amazon.

 

Problem sounds more like a connector problem.

So since you know that the battery and alternator connectors are tight since you just installed them, would check the connector at the starter solenoid were the alternator and battery cable come together, the fuse box terminal connector, as well as the ground connector of the battery cable to the block just above starter.

As for voltage readings, remember that its giving you the digital voltage reading at the ECM, and not the cluster itself. So could be a bad ground at the ECM, or the ECM/BCM/Lower fuse box connectors that are the problem.

Hence charging system feeds the engine fuse box terminal, which you have the upper and lower box connectors,


Which fuse box feeds the BCM through it connectors,


And the BCM feeds the power to the modules, including the ECM, Which the ECM sends the digital signal back to the cluster to display that voltage.


As for the charging system itself, start the motor cold, turn off the head lights, the radio and HVAC so the radiator fan is not running, and check voltages.
Alternator off it back rubber covered terminal to the body of the alternator should be 14.7 volts.
At the battery, should be 14.5 volts,
From the engine fuse box terminal, to the body of the alternator, 14.5 volts.
If you are getting readings lower at one of the points, then go back and check the main connectors mentioned above again.

If the charging system voltage are fine, but your getting reading lower than 14.1V at the cluster, then the problem is from the fuse box, through the BCM, to the ECM with either the postive side, or ground wire at the ecm connector.

Last one, stock OEM valeo alternator only puts out about 14 amps at idle, so if the engine is hot, and the raditor ran is running once the car is started (or you have the A/C on that will force the raditor fan on), then the radiator fan is pulling everything that the alternator is putting out at idle, and will be a min or two of some higher RPM driving until the battery catches back up with it charging from the charging system after it light drain from starting the car, to be at a higher voltage again.

 

Thanks a lot for your suggestions, I'm going to give all these a go starting with the easiest which will be turning the air con off before I get gas and update.

 

So I tried turning the AC off before getting gas and this actually worked. Started perfectly (as it normally does anyway) but no errors on the dash and voltage reading normal at the dash. I'll try it a few more times before considering the matter closed but very promising at least!