Yes, working on the car is 25F really sucks. There's a post below yours about working in -22C, that oughta warm you up. right?

The alt light popping on when you hit a bump indicated movement of a wire or something. When the light is full on, that means the light circuit has a good ground, not the one going through the alternator brushes, and THAT means the alternator is not charging.

The voltage at the thin red wire should not be battery voltage. It is what's left after coming through some of the warning lights.

Before you spend much more, try this. That red wire is one of the group that (1) terminates in the gray connector on the fiorewall, and (2) is subject to disintegrating insulation.

So put in a shoo-fly, (by-pass in non-railroad terms) and see if that helps. You need to run 16 gauge red wire from the alternator to the RF fender and thence to the firewall and the connector.

At the connector end you will need to open the removable side and clip the terminal off the old wire and solder/shrink tube it onto the new wire. This will leave the original wire isolated at both ends.

If this doesn't fix it up, let's do some more sleuthing. It's the pits to shell out for a new alternator (or anything, for that matter) and not get the problem fixed.

Good Luck,

Bopb

:>)