low volts and check engine light

I have a 98 with the 4.0 and an auto trans. When i turn my defrost on every thing is ok untill i turn the fan to 3 or 4 then the head lights go dim and radio dies. When I turn it back to 1 or 2 every thing gets better. I had the alternator checked and they said everything was good. My friend said that it could be the voltage regulator????? Where is that?

Your post title says that you have a check engine light? If so, pull the code. Use the technology provided to you by your Jeep. Your OBD2 system is pretty sophisticated and the trouble codes are really invaluable in isolating the root cause of your problem. Post your code here for interpretation.

I had a 98 that did the very same thing. You need to replace the Alternator!! It is not your regulator as that is part of the ECM and the rarely go bad!

I would say it's prob. the alternator as well. I've had an ECM go bad though, so you never know, but you'd prob. have other symptoms too if it was the ECM. I'd definitely say it's the alternator.

So it turns out that i had a bad terminal conection. When I cleanet the battery post all was fixed...... duh

Great that you figured that out without replacing parts like it seems that everyone wants to do all the time. Easiest way to check out charging system is with voltmeter. Should be about 12 volts with engine off and about 14 with engine running. Whenever anything is suspect, clean the battery terminals and the grounds.

As I started reading this post the first thing that crossed my mind was clean the battery connections. The reason: my 97 XJ showed a weird High Voltage reading and "check guage" light came on when my terminals were not making good connection due to build-up.
I know it's easy to say you know the answer after it's been stated but let's remember guys before you go throwing parts at something do the free stuff first. We've all likely replaced parts that didn't need it at one time or another till we found the real problem. It's not much consolation for me when I lie to myself and say "well at least I don't have this or that to worry about now".
Glad you found your problem and it was a cheap fix.....Al