Gauges and turn signals don't work, GEN lights up.
 

Hey guys, I was driving around after finally getting her running for the first time in ages and she was doing great, but recently I had a really weird issue.
My fuel and temp gauge don't work, and my tachometer drops back to 2k if I rev past 3.5k or so. Also, my turn signals don't activate. Everything else works, including my hazard lights. It sporadically started working twice while driving, but only for a few minutes before not working again. The weird thing is, whenever it isn't working, I see a light that says GEN, which I'm presuming means generator. I'm presuming it's fine though because I haven't had any trouble with the battery. I replaced what I thought were the relevant fuses but that didn't change anything, and they all looked fine.
I'm terrible with electrical stuff so if anybody could help me out that would be much appreciated!

GEN is the alternator.

Ah whoops, makes sense since that's actually in the car. Either way though my battery is doing just fine, and if the light turns off when everything starts working I feel like the alternator itself should be fine. I feel like it's some kind of connection related to the alternator but not the alternator itself. That being said I'm still just shooting in the dark.

Could be a connection, most likely its your alternator or voltage regulator. If your light comes on, your system isn't properly re-charging the battery. Your battery is like a reservoir of water. Your lights, radio, heater fan, all consume power (water) from your battery. The alternator re fills the battery for the other systems consumption. If the alternator's output falls to low, your light comes on to alert you. A low output will drain the battery slowly because the draw(s) on the battery drain it faster than the alternator can re charge it. A bad connection on the alternator could prevent it from recharging, plus the light can come on if the alternator starts to overcharge the battery due to certain failures.

By far, the most common failure is something in the alternator or voltage regulator which drops the voltage
enough to trigger your light. It can be an intermittent problem where it's not putting out enough (the light comes on) then it is (you light goes off) making it come and go. You might need to recharge or replace the battery too. You did say that you just got it running "for the first time in ages". I imagine the battery is old too.

Thanks Banzai! I replaced the battery recently so I doubt that's the issue. I guess I'll just look for any loose wiring and check on that voltage regulator, hopefully that'll fix it!

Check yer grounds. One on the drivers shock tower, one up under the dash and the one from the end of the muffler to the hanger in the back. There may be others but those are the main ones. You have a grotty connection somewhere.

Another thing is does it only go away at certain revs? If so get the alternator tested or check it with a DVM tapes to the windshield to see the output when it is failing.

Banzai has described what the issue may be. One spring when i got my FB out of storage I had a similar electrical issue, with engine OFF or idling I would have lights on the dash, GEN and other random ones.

It turned out to be the voltage regulator in my alternator failing. At low revs it would not put out enough voltage to charge the battery.

Check the voltage regulator and output using a voltmeter. When my regulator went I could see an AC component to the alternator output.

Rx-7 alternators did not have built in voltage regulators until 81. Works the same way, but SA's have an external, separate regulator in the left rear corner of the engine bay. A loose or bad ground as Tim describes could be the culprit too.

Dash warning light wise,same symptoms for SA and Fb from not charging sufficiently. Need to have .5-.75 more volts than battery voltage coming from alternator to turn off battery/charge/warning lights.

Only 79s had an external regulator.

You are right, my mistake. Forgot for a moment they went electronic in 80. External voltage regulators went with the points ignition in 79.

That happened when my alternator belt broke and I was running on battery. You're probably not getting enough charge from your alternator for some reason.