Download the 2002 Jeep KJ Service Manual below...gives you information as to how things work and section 8W has the wiring diagrams.
The Owners Manual.....check very carefully if you have it in your glovebox mounted horizontely at the top.....will tell you that the Battery Icon comes on if you have a charging issue.
This is true.....you may have a charging error caused by a bad alternator or a wiring problem so you may need to have your alternator tested.
BUT!! They do not tell you that this Battery Light can also come on if there is a problem with the Battery Temperature Sensor mounted under the Battery....I found this out the hard way on my 2002
Export CRD.
So first of all get a cheap Digital Multimeter...you do not need a fancy one!.
Measure the voltage across the battery terminals:
Should be about 12.4 volts with engine off.
Should not drop below about 10.5 volts while cranking.
Should be about 13.5 volts while engine is running.
Check battery terminals are clean and tight.
Check the engine grounding points coming from rear of the two engine heads going to chassis are clean and tight.
Get back to us as regards the voltages measured above.
Now as regards the Battery Temperature Sensor!
The early KJs such as yours has a sensor underneath the battery, facing upwards and spring-loaded.
This measures the temperature of the battery and accordingly allows the PCM to slightly adjust the charge voltage to the battery from the alternator.
One side of this battery sensor goes to ground...I had the case on my KJ that this Ground went missing somewhere inside a large cable harness...I just added another ground to chassis.
You can remove this sensor. Measure the resistance of the sensor at room temperature. Make sure when you measure resistance that you do not hold the meter leads with your bare hands....your internal body resistance affects the reading...wear gloves!
You can heat up the sensor with a hairdryer or place it on an incandescant lamp to heat it up. As it gets warmer the resistance should change if the sensor is working correctly.
Re-fit this sensor after measuring it...make sure one of the leads is connected to Ground!
The Index Page of Section 8W gives you a Component Index where you can find a component such as the Battery Temp Sensor and it directs you to the relevant Wiring page....make sure you are looking at the correct engine type!
Jeep KJ Manuals:
www.colorado4wheel.com/manuals/Jeep/KJ/
