More from MikeC:
Having the voltage at 16+ is a sure sign of a fried regulator inside the alternator and/or solder blobs breaking off and rattling around inside it. Either way it needs to come out and be professionally rebuilt or replaced.
Replace the charging fusible link before installing the new one. If you want to double-check the wiring, use a small 12volt turn signal light bulb in place of the link. Install the alternator, hook the bulb in, connect the battery, and check. If the bulb is lit at all (even dimly) then there's a problem someplace. Now turn the key ON but don't start. You might see a faint glow. A bright glow is bad news. If your car fails this bulb test, isolate the alternator again (disconnect the battery first!) and re-test. If you still have glows, you've got wiring harness issues between the battery and alternator.
If the new alternator passes the bulb test, undo the battery & bulb, install the correct charging fusible link. Re-connect the battery. Before starting, use a voltmeter to test the smaller wires feeding the alt:
* +12volts on the white+green wire with the key ON or OFF.
* +12volts on the white+yellow wire with the key ON only. It might not be a full 12volts; you should see zero key OFF and +something key ON.
If you don't get these voltages, the harness going into the alternator might be bad. Unplug this connector and re-test it directly. Not having these voltages will confuse the regulator (part of the alternator assembly) causing +16volt type of outputs.
Bad Power Draw:
If you look at the wiring diagrams, you'll see that the alternator's field coil gets power from a LONG stretch of wiring... and that same wiring feeds the dash voltmeter. Basically, the path is:
* start from the battery + post
* fat white wire to the fusible link box IGN (ignition) link. Anything that is powered only when the ignition key is in ACC/ON/START is fed through this link...
* from the IGN link to the ignition key's "input" post.
* from the ignition key's ON post back to the engine bay. And also to the fusebox, to the dash guages, etc.
That wire feeds the front fans, alternator field coil, turn signals, etc. I.e. a lot of fairly high-current applications. ANY bad connections/resistance in this wire will cause the symptoms you describe. It's also the cause of the "dancing" voltmeter when the turn signals are working. If you put a real voltmeter on the battery posts you'll see MUCH less voltage variation than the dash guage indicates.
Start by disconnecting the battery (key OUT of the ignition) and taking out the fusible links one by one. Inspect them carefully; any signs of melting or burning or discoloration on the insulation means the link cooked at one time (i.e. tried to act like a fuse) and is bad - it must be replaced. Get a new link from the dealer; they're cheap. DO NOT MAKE YOUR OWN WITH REGULAR WIRE - IT MUST BE FUSIBLE LINK WIRE! If the link is okay, clean the contacts on it AND on the box completely. Smear in some dielectric grease (any parts store will have it; it comes in a tiny toothpaste style tube) to help reduce future corrosion. Also undo the connectors from the battery posts and clean them, follow the fat white wires and clean their harness connectors (again using the dielectric grease when you re-connect). With tiny needle-nose pliers you can re-crimp the female ends of the various connectors so they have a tight grip. Unplug the alternator wires and clean/grease them. Then go to the battery "-" post and clean it. Follow the beefy black wire to where it bolts to the bodywork under the battery. Undo that bolt, clean ALL wire ends you'll find there, and the bodywork part. This is the main ground input on the car. Good grounds are critical on any car. Follow the fat black battery wire to the side of the engine block and clean that end too. Under the dash, by the steering column, you'll find the connector for the ignition key harness. Clean up those connections as well.
See how much that helps. If your dash voltmeter still drops a ton under load, take the alternator out and have it tested - most auto parts stores will do it for free. Once it's known to be good, put everything back together and run the engine at idle, turn signals ON, a/c cranked to 65 deg to make all the fans run, etc. What is the battery voltage both measured directly at the battery and on the dash guage? If the actual battery voltage is below 12.5 volts; the alternator circuit isn't working well enough. You may have to run new beefy wires from the IGN link output pin to the ignition key to supplement the degraded factory wiring. That wire gets old and developes a fair bit of resistance.