charging system help

i have a 1990 mustang lx 5.0 and im having issues with voltage drop while the car is running. it has a new alternator and a new optima red top battery but when the car is running, the voltmeter jumps around and when the lights, fan or stereo is turned on it drops so low it almost makes the car stall. help please?

What kind of alternator is the new one you have? Just because it is new does not mean it is working properly. Are you able to swap in a different one temporarily? Also I hear Optima battery quality has gone down the drain in recent years (which sucks cause I really want one).

Check the battery cables for corrosion and clean as much off as you can with a wire brush. Corrosion is an insulator and can be inhibiting the voltage if significant.

If you have a multimeter, use it to check the voltage from the battery with engine off and when engine is running for accurate measurements as the dashboard display doesn't provide very good values for diagnosis.

You can also take your car to an Autozone or Advance Auto and have them test your battery and alternator for free. They can usually identify the problem if it is a bad battery or alternator.

i replaced the the ground and power cables already. i noticed that ground from battery to block was into aluminum on engine, is this right? theres only a drop when engine is running, it cant keep up with any accessory running. engine off its fine

and i swapped out 2 alternators with same results

Cut the two black/orange wires near the back of the alternator. Then run a jumper wire to the battery positive post. If everything goes good then your fusible link in the alternator circuit is going bad.