Tach help

15V is too high. You need to start by making some tests.

1--First and foremost, unbolt the regulator and notice whether the bolts are stripped or can be made tight. Scrape the firewall around the bolts, as well as the regulator mount. Use star washers.

2--Check the ground. Run the car at a good fast idle, and run check 2 and 3 first with all accessories off, and again with lights, heater, etc running. Stab one meter probe directly into the top of the battery neg. post. Stab the other directly onto the regulator mounting flange. You are hoping for a very small reading, the less the better, and zero would be perfect. Over .2V means you have a ground problem between battery NEG and the regulator case

3--Check the hot side of the ignition harness. Turn the engine off, but switch the key to "run." Stab one probe into the top of the battery POS post. Hook the other probe to the ignition "run" buss. Either at the "key" side of the ballast, or at the blue field wire on the alternator. Again, you are hoping for a very low reading. More than .3V (three tenths of one volt) means you have a voltage drop problem in the path ---------

from the battery -- fuse link --- bulkhead connector -- ammeter -- in harness splice -- ignition switch connector -- through the switch -- back out the switch connector (on dark blue "ignition run") back out the bulkhead connector -- and to components under the hood, ignition system, alternator blue field wire, regulator, etc.

Your top most suspects are, the bulkhead connector, the ignition switch connector, the switch, ammeter connections or the ammeter itself, and in rare cases (but it does happen!!) a failed "in harness splice."