It's Electric. woogy woogy woogy.

Finally heard the 69 AMX startup the other day. 

If your tach is bouncing off of zero irregularly, it's probably just the vibration from the open headers. One of my cars had a working tach, now it's not, but it does flutter off zero at a very rough idle. Look at the tach wiring or maybe the tach itself. Unfortunately, I don't know how to test the tach. If anybody else has a suggestion, let's hear it.

If you have a very old battery charger, you can test a tachometer. The old chargers, or DIY, put out a 60 hz signal. Some older chargers put out a very "poor" signal and you sometimes have to use a diode to clean that up. + to all the tach wires but the black or ground, black or case ground to the charger -. Readings on tachometers:

Edited by Trader - Apr/11/2021 at 7:47am

The original OEM Faria made tach MAY not be compatible with your MSD tach adapter which could explain the low reading. The engine to body ground wire would not affect the tach reading as the tach is not dependent on the body ground. You may want to check with MSC and let them know you have a Faria taut band, series wired tach.
Someone with an audio signal generator, like an electronics repair shop, can test your tach using a 60Hz test tone. As indicated by Trader, a 60Hz signal will provide a reading of 1800 rpm. 30Hz 900 rpm, 120Hz 3600 rpm, etc. The signal generator should be set for about 6 to 8 volts output on square wave.

As for the high beam indicator, check to make sure the bulb, bulb socket, the copper lands and the pins on the printed circuit board a clean. The pins should also be tight to the PCB. They could be tack soldered to the PCB copper trace if needed. The high beam bulb is fed off the white wire going into the cluster plug. If your high beams are working then the white wire should have 12 volts on it and that should make it's way to the bulb in the cluster.

Do both the green turn signal indicator lights work when you turn on the 4 way flashers ? I'm sure you know that there's a separate flasher for the turn signals and that's fed off the switched side of the ignition switch through the fuse panel. The fuse is the 14 amp Accessory fuse.

I have read that other people have used the tach adapter on the 69 (just here on the forum) without issue. I'm thinking the issue lies further down in the tach, not so much with the tach adapter.