2008 Ls3 C6 A6, Headers, CAI, aftermarket coils plugs and wires
I am having an electrical issue, i replaced the battery and alternator, and the ‘service charging system’ still appears on the DIC, My volts on a cold start are 14.3-14.5v but after 5-10 min of driving they go to 10.8-12.8v, when driving the car handles okay, but when idling at a stoplight the car rough idles and the volts drop super low. recently the car starts to turn off after idling for too long. Any suggestions?? please help it’s been hell

 

constant service charging system with voltage and dash reading above 13.5 volts, you have a C5 alternator in the car, and not a C6 one. The are both pretty much the same, except the voltage regulators in them, where the C6 voltage regulator has an extra working terminal instead (voltage regulators connectors look the same for both).


As for voltage drop out with heat, best guess is the alternator cable that connects to the starter solenoid that is the connector for the battery cable to the starter solenoid as well, that nut is either loose, or the entire terminal is cracking away from the solenoid from the starter not given a heat shield to protect if from the starter heat.
That is short of another connector not being loose,and added heat causing the problem.

If its the starter solenoid the problem itself, then it can be replaced on the starter, with a new solenoid costing about $20.

Simply put, power from altenator goesto starter solenoid, then to battery cable up, to engine fuse box, through it to BCM, BCM to ECM, and the dash gauge, is telling the voltage of the ecm, not the dash gauge itself. So could be loose or connection problems in that positive line, or could be as simple as loose ECM ground that is causing it to see lower voltage isntead. If you have a tech 2, can start the car and start checking module voltages, to see if loosing voltage at BCM, or down line of it to the BCM instead.

And one last thing, stock Valeo OEM alternator may put out 140 amps at full tilt, but at idle, it only putting out about 14 amps. I bring this up, since if engine is at idle, and radiator fan kicks on, the fan pulls 18 amps, and it will cause the lower voltage output of the alternator to car.

 

If you have a DVOM you can do a quick voltage drop check on your charging system to rule out high resistance in the circuit…run the car at 1500 RPM with electrical loads (high beams, blower etc…) and check as in the video…0.1-0.3 is good…0.5 volts is max.