Look for 13.5 - 14.5 volts (roughly) at anything above 2000 rpm.
The r/r has two failure modes (I've had them both!):
1) The voltage will drop below 12 V -- meaning the r/r isn't charging the system and you are running off battery power (but won't be for long because the battery will die).
2) The voltage will jump to like 18 - 19 V -- meaning the r/r is failing to regulate the voltage. This is the more common failure mode. It will 'cook' your battery. This happened to me on a multi-day trip. Stopping and then starting the engine sometimes made it work, but a couple of times I had to let it cool off before it would regulate again.
This has been a problem with older VFRs as well (had one fail on my '85 VF1000R). Some have failed at low mileage, but most seem to fail around 20 - 25k miles. The r/r on my '94 lasted until 55k miles.
Consensus is that the failure is heat-related; the replacement OEM r/r Honda now sells has a decent heat-sink mounted on it and is a much better unit. I think that unit was introduced in '01 or so. If you don't see cooling fins on top of your r/r, you have the old type. You can see them by removing the seat.
One key item that is often overlooked is to also make sure that you have good grounding points on the frame. Remove the bolt holding the green grounding wires, and use a brass or steel wire brush to clean the ring terminals and mating frame/bolt surfaces. Even if they look ok, cleaning can make a difference. The primary grounding point on a '94 - '97 is on the right frame above the brake pedal area, don't know about other models.
Also periodically check the white connector where the r/r plugs into the harness. If it's discolored (brown), that is due to heat. Keep a close eye on it. These will burn to the point of melting if the r/r gets too hot. (almost happened to me, but I caught it before the r/r failed). Honda recognized this problem and sells a connector repair kit.
I highly suggest that you install a voltmeter, and carry a spare r/r if you plan any multi-day trips. My voltmeter saved me from the aggravation of a tow and a day or two waiting for parts, far from home. This is the only weak point on the VFR, so best to prepare for it!
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