2000 Jeep Cherokee

The alternator has to be tested while the problem is occurring. When the light turns on, use an inexpensive digital voltmeter to measure battery voltage while the engine is running. It must be between 13.75 and 14.75 volts. If it is low, suspect worn brushes in the alternator. They can be replaced but most people just replace the entire alternator.

Alternator was tested at autopart store while problem was occuring alternator tested good

HOW ABOUT THE OLE DISCONNECT THE BATTERY FOR 5 MINUTES, THEN RECONNECT TRICK?........JUST A THOUGHT..........WON'T HURT NUTHIN'!!!

THE MEDIC

ok I will give it a try thank you

Find the two small terminals on the back of the alternator. Measure the voltages on them when the light comes on. They must be measured while the engine is running. One will have full battery voltage. The other one is the important one. It must have less than battery voltage but not 0 volts. If you find the same voltage on both terminals, the wire from the alternator to the voltage regulator inside the Engine Computer is broken or the regulator circuit is defective. If you find 0 volts on the second terminal, the brushes are open.

NO!! Don't disconnect the battery. That works on cars with breaker points, but the ripple and potential higher voltage can damage computer modules.

Well I went back and had the auto store exchange the battery thinking it was just a bad battery so technically it was diconnected but didn't do anything to fix the problem I really believe it has to do with the ignition because when I had it replaced right after is when the problem started I have had the alternator tested in all possible ways

"Ignition" is a system made up of a lot of parts. Which part was replaced?

The thing that you actually put the key into and turn to start

Ahh. That would be the lock cylinder that turns the ignition switch. Those will not cause the problem you described. The alternator, ignition system, (spark plugs), and fuel pump all run on the automatic shutdown (ASD) relay. If there was a problem with the switch, the engine wouldn't run so we can rule that out.

The only way to know for sure what's happening is to measure the two voltages on the back of the alternator when the warning light is on.

Let me explain why I think that a little better I had the lock cylinder replaced by my neighbor who's a mechanic (not an actual shop) due to it sticking bad and finaly breaking well since the new ignition switch was put in whatever the mechanism is that releases the key works about hmm I'd say 1/8 of the time most of the time it doesn't release and the key is left in it. It wasn't until I had it changed that the battery light started randomly coming on. Is there something between the battery and the alternator that maybe I'm missing?

The voltage readings on the back of the alternator are the only way to know what's going on. If they are correct even when the warning light is on, it is possible the ignition switch is stuck between two positions and power is being lost or reduced to the instrument cluster. That is not common and it should cause other things on that circuit to also stop working. I think the radio and heater fan are switched with a different part of the ignition switch but I can't remember for sure.

Ok just fyi yesterday before it actually died the stereo did go beserk and then stop and if I stop turn my jeep off after the battery light comes and turn it back on the light doesn't come back on it comes on after driving a while again.....don't know if any of that helps any

Well, the argument could be made that the connector pins in the ignition switch connector could be heating up causing degraded connections, or a diode in the alternator is failing from heat buildup, or the voltage regulator is failing after it gets hot, . . . or it could just be a coincidence. The clue would be if the warning light always turns on after about the same amount of time after starting the engine.