Electrical charging problem

96 XJ Sport 4.0

Last night the GEN light came on and gauge started dropping. I immediately thought the alternator was going out (reman alt I put in about 3 years ago - battery is an interstate about 2 years old). This morning I tested at the battery and got 12 volts, started it and got 12 volts again. I drove to the parts store, GEN light came on and gauge started dropping again. Then, as I pulled into the parking lot the gauge shot up to about 15. Parked it and had the parts guy test it, read 13.2 volts at idle and 13.5 under load. Driving home, GEN light came on and gauge starts dropping. Turned on the heater and headlights, gauge dropped a little then shot up again. Parked and left it idling and gauge started dropping again a minute later. Just tested at battery and got 12 volts running and after shutting it off. Any insight is appreciated, thanks.

Begin with basic trouble shooting of the start and charge systems. Remove, clean, and firmly reconnect all the wires and cables to the battery, starter, and alternator. Look for corroded or damaged cables or connectors and replace as needed. Do the same for the grounding wires from the starter to engine block, and from the battery and engine to the Jeep's frame/body. You must remove, scrape, and clean until shiny, the cable/wire ends, and whatever they bolt to. Jeeps do not tolerate low voltage, bad connections, or poor grounds.

you might have blown your fusible link between the alternator and battery. use a multimeter to test continuity between them while you wiggle the wire

If you can stop and open the hood when it is dropped, take a hammer and rap the alternator and see if it chages. The brushes on the slip rings can get bad and stick. A jolt will normally loosen them for a while. Not a permanent fix, but a diagnstic tool. The same goes for wiggleing the wires and connections. Do only one at a time otherwise you won't know what effected it.

A good battery should read over 13 volts (13.75 nominal) it may be weak and alt. can't keep up with running a load like headlights and trying to move the battery voltage higher than it can still hold. 12 volts is really a "working number" for components, operational is more like 14 volts average. Do your wipers run slower with the engine not running? Motors draw more amps and make a good "load test".

Thanks for the suggestions, I put in a new alternator. Problem solved.