Problem is likely with a burnt diode in your alternator. If the ignition is off the diode will continue to draw power and your alternator light on the dashboard will illuminate. It basically turns on a circuit in your car thar should be off, and in the process draws power from your battery, will usually eventually result in a dead battery overnight. Could be something else, but this is the likely culprit... this isn't only with m42's, pretty much all vehicles could have this same issue.
Here's some info I copied and pasted from another site:
ALTERNATOR DIODE FAILURES
One of the most common causes of charging problems is the failure of one or more diodes in the alternator. Alternators have six diodes (three negative and three positive) that convert the alternating current (AC) to direct current (DC). They are called a diode trio because each negative diode is paired with a positive diode.
When the engine is running, charging current from the alternator flows through the diode trio via the BAT (B+) connection on the back of the alternator. A little current also flows through the charging light indicator circuit. On GM alternators, the indicator light circuit is terminal 1. On European alternators, the indicator light circuit is usually called 61 or D+. On Asian alternators, it is usually labeled L. This terminal leads to the ground side of the alternator warning light. When the alternator is charging, the diode trio supplies voltage to the ground side of the indicator light. This offsets the battery voltage applied to the positive side of the light, causing the light to go out once the engine starts. If the alternator stops charging, current flows though the light circuit from the positive side causing the charging system warning light to come on.
If one of the diodes fails, it may cause the charging system indicator light to glow dimly. If two or more diodes fail, the light will get brighter. At the same time, the feedback current from the diode trio will reduce the alternators ability to produce current. So the more diodes that fail, the less power the alternator will generate.
A bad connection or open circuit between the alternator output terminal and the positive battery terminal will force the charging current to follow a parallel route through the diode trio and out of the alternator. This heavier than normal current flow through the diodes will cause them to overheat and fail. Consequently, if you have replaced an alternator before because of bad diodes, and the replacement fails for the same reason, there is likely a bad connection or open circuit between the alternator BAT (B+) terminal and the positive side of the battery circuit. Do a voltage drop test to check the entire circuit.
Topic: battery light stays on (Read 4967 times)