Why does the voltage meter go down to about 10 when I turn the headlights on I Have put A New alternator and new Battery It Charges Fine untill I turn on the Headlights
More than likely, a bad ground connection somewhere makes the dash meter read incorrectly. First, check the BODY to battery ground lead. It should be separate from the battery cable that goes to the engine block. Then, more difficult, the ground lead for the instrument cluster in the dash…
Get a $10-20 multimeter and measure what the battery voltage really is.
If the battery cables are the side-mount style, remove both cables from the battery, and peel back the rubber cover for the positive cable terminal. If there’s a lot corrosion under this cover, it can cause a voltage drop to vehicles electrical systems. And this corrosion can leach down into the cable itself. So when corrosion is found, it’s best to replace the cable.