ok i have a 88 rx7 GXL. the problem is the voltage dips to about 10, then returns to over 12, whenever i turn on lights, roll down power windows, adjust suspension, etc. basically any extra load on the electrical and it drops then returns. BTW this is on command, i mean it does this no matter what. the previous owner gave me a extra engine bay harness and said that was the problem, just replace it. i was just wondering if there was a way to find where the drop is coming from.
Thanks

 

bad alternator or battery. Not a voltage drop.

Anytime the voltage drops under 13.5 volts while the engine is running is a bad bad thing and indicates that the alternator is bad or not wired correctly. You need at least 13.5 volts just to charge the battery.