You might be surprised. Most alternators are grounded purely by the mounting bolts, and more than once I have found a high-inpedance or entirely open circuit due to corrosion, or simply the bolt being a little loose. I spent a very cold day with my sister over christmas when my car stopped charging the battery, and we kept having to get jumpstarts from people all the way home to get a little power into the system, then drive as fast as possible to get home before it ran out

Had to do this four times over a 20 mile journey, in the snow. Bit of a pain.

It turned out to be a salt-corroded bolt that had such a high surface resistance the alternator was effectively isolated from the engine. When I cleaned it, everything started working perfectly, so I ran a heavy copper strap from the alternator body to the engine to make sure it wouldn't happen again.

Of course, normally the positive output is fine, but without a good ground you get all sorts of odd results, as you know.

Patrick