[brucepick]

We drove Volvo 240's for about 15 years, I've removed and replaced those alternators plenty times.

If you disconnect that small red wire the alt MAY not put out any current. If so, the alt puts nearly no load on the engine. You'll save fuel if you do it that way. Just be sure to insulate the loose wire end. I like a bit of clear vinyl tubing stuck over a wire end, and duct tape that to the wire so it won't slip off.

To test whether the alternator is disabled or not - - -
Definitely rev the engine up to about 2000 rpm or so for a bit: sometimes with the exciter wire disconnected, the alt will not charge till it builds up some rpm speed. On my Civic, there's a connector with 4 small wires. With the connector removed, it doesn't charge at first - BUT once it gets up to about 2000 or 2500 rpm, it will start charging and keep charging till you shut it off, no matter what the rpms are!

An electrical multitester or voltmeter will tell you if the alt is charging or not. Plus probe on the battery positive, neg probe on battery neg or clip to ground. About 12.5V is battery normal resting voltage, it will be less if the battery is running the car with the alternator disabled. Normal charging voltage is about 13.5V to 14-ish.

If you want to read volts while driving you can get a plug for the cigarette lighter. Just buy some cheap doodad with a plug, snip the wire leads, and connect to your voltmeter. Just don't let the leads short together when it's plugged into the lighter socket.