A4.B5.DIY.Engine.Electrical.ChargeSystemTroubleShooting
From QuattroWorld Wiki
How to troubleshoot your electrical charge system
The dash voltmeter should be a little over 14 when the car is running.
You alternator should be giving your battery around 13.8-14.4 volts. You can test it with the car started holding one end of your multimeter on the large terminal of the alternator and the other to a ground. If it reads 13.8-14.4, you alternator is fine. If not, it's dead.
You can also do a voltage drop test on the cable going from the battery to the alternator by putting the positive multimeter lead on the larger alternator terminal and the negative multimeter lead on the positive battery terminal. The voltage listed will be the amount of voltage you are losing from one end of the cable to the other. Ideally you want to see 0, but less then 0.4 is acceptable. More than that means your cable is bad.
You also want to check the voltage regulator which is the smaller cable on the alternator. With the car started, put a voltmeter on it and you should be seeing 14.4. If the voltage regulator isn't getting power, the alternator can't regulate the power.
You can also check the positive and negative wires for their resistance. You should see less then one ohm resistance on both, .5ohm is acceptable. This can be done by setting the multimeter to ohms and putting the positive lead on one end of the cable and the negative on the other.
To test the battery, you need a full charge. The easiest way to do that is take it to your local autoparts store. To check the rest of the charge system, use the steps above. The tests the autoparts stores do on your alternator are insufficient.
Dillon (nunya)
