99 Cherokee electrical problem

Hey folks. I am having a very strange problem with my Cherokee. It's a 99 with a 4.0 in it. Which evidentally makes it weird because it's newer than a 98 and according to any generic lit. that makes it strange.

I digress..

The problem happens when it's particularly cold and never when it's warm. Warm being above about 45 degrees.

The problem:

I start the truck and all is lovely and well. Drive for a bit and the voltage fairly quickly drops from 14v to about 12v. No apparent reason, but it can be aggravated if I use more juice (headlights, heater on speed 2). It have never happened with out the headlights. I assume theur power draw is the reason. So the voltage drops and will typically sit there for a short period of time and then the voltage guage will suddenly drop off altogether and the idiot light comes on. The trucks keeps running but it's clearly on the battery.

Generally if I turn it off and then back on it will reset and be OK. But the problem is getting worse and frequently won't reset the system.

The alternator is a 2 year old junkyard alt (didn't solve the problem then, but it wasn't a huge issue then) and the battery is about a year old.

I asked the parts store and they seemed to think the regulator was part of the alternator and not neccessarily independantly serviceable.

Any ideas?

Thanks in advance!
Morgan

The voltage regulator on newer XJs is controlled by the computer and is not a part of the alternator itself. The good news is that you can hook up a code reader to the OBD II port and see what kind of codes it is giving you. Once you figure out the codes you can go from there. Most large auto parts stores (NAPA, Schucks, etc.) will read the codes for you under the assumption that you will buy what ever is needed to fix the problem from them.

Wiley Coyote said:

The voltage regulator on newer XJs is controlled by the computer and is not a part of the alternator itself. The good news is that you can hook up a code reader to the OBD II port and see what kind of codes it is giving you. Once you figure out the codes you can go from there. Most large auto parts stores (NAPA, Schucks, etc.) will read the codes for you under the assumption that you will buy what ever is needed to fix the problem from them.

Click to expand...

The bad news is if it is the regulator then you'll have to replace the computer to fix it. I would check the wire from the alternater first and the ground from the block to the body for corrosion. Eather one of these could cause problems.

Thanks guys.

For anyone remotely curious.. I checked the grounds and harness.. the problem was that the "plug in piece" on the back of the alternator was falling off. Not broken, just half unplugged.

Pushed it back in and it works like a champ. I like the simple fixes.

I would have replaced the $ECU$, all the wiring, and the alternator.

But that's me.

And my luck.

Welcome to the wonderful world of the '99 XJ! I bought a '99 specifically because it reportedly had fewer mechanical problems than the other newer XJs. This year IS known for electrical problems, though, and many are initially attributable to poor connectors/connections.