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ICQ>
Tom ---
Let me add a little to Jean's good advice. The inductive meters are good to have for just this diagnostic. However, you can also tell a lot with a voltmeter by measuring the voltage at the battery under different conditions. A fully charged battery should measure around 12.6 to 13.0 volts with no load, not charging. The temperature will influence battery voltage somewhat.
If your alternator is working, you will see about 13.5 to 14.5 at the battery while charging, again, depending on temperature and load, and also depending on the state of charge of the battery. If it is discharged, expect to see the voltage lower for awhile. For this diagnostic, you need to know you have a good battery -- a battery with a cracked conductor (high resistance) may show higher voltage because it imposes no load on the alternator.
A common problem with worn brushes, as Jean mentioned, is that they wear down to the point they can just barely provide current to the battery -- essentially a trickle charge -- but not enough to make up for cranking. In this condition, the idiot light does not come on, but you're still not charging.
To protect myself from this situation, I robbed a voltmeter from a turbo cluster and installed it in a (non-turbo) 240s I drive. It's very helpful to monitor the system voltage -- I've already found worn brushes this way, without getting stuck on the highway with a dead battery.
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