Put a volt meter on the battery, car running. Note the voltage.

Now, turn the on blower, again note the voltage

Now, turn on the headlights, again note the voltage.

You shouldn't see much of a voltage drop at the battery if the charging system and battery are ok. If you do, it's either the alternator, or the battery. I'd say that normal drop is probably less than a volt. You should never be below 12 volts when the car is running, and really you should be upwards of 13 volts. If you start out at 13 or above, and then drop down to below 12, then it's a systemic issue as described above.

Now repeat the test but this time put the negative test lead to chassis ground. The results should be identical. If you get more voltage drop testing this way, then the problem is a ground.

This is by no means an exhaustive or bullet proof test, but it will probably point you in the right direction.

If you see no significant voltage drop when you put on the lights and blower, then the problem is somewhere downstream of the battery.


-tp