I have a 98 ranger with the 4.0. When I started it the other day the voltage guage started to fluctuate rapidly a small amount up and down, and the lights and everything else electrical fluctuated accordingly. Then it stopped and seemed to be charging fine, then it started fluctuating again, then it showed it was overcharging and fluctuating, then dropped to no charging, then came back up, over and over. I changed out the alternator and the battery with no fix in the situation. Does anybody know what would cause this?

Thanks for your help.

 

Is your belt loose? Maybe tensioner not working properly.

 

Bad grounds or loose connections on the starter relay, or possible loose connections in the fuse box, or bad regulator.
The alternator will put out a pretty steady voltage, raising amps as the rpms increase. The meter shows VOLTS in most late models not amps. The volt meters wobble when they are being supplied wobbly voltage or they are being supplied intermittently by loose connections.
Your lights flickering in time to the wobble indicate that the supply is being varied from the alternator.
When the engine is off, the alternator is taken out of the circuit, and you have no flickering, I suspect.
If you have a regulator separate from the alternator, I'd replace that before spending time on the wires. A bum regulator will not regulate very well and the volts will be all over the map as it functions - fails - functions.
tom