weird electical issue

ok I have an 87 xj 4.0 auto. When it's cold outside my voltmeter jumps the lights dimm and then brighten up, once she warms up it ok reads like 14 volts. I put a new alt. in last year the bearings seized up. I checked and cleaned all my grounds. Any Ideas? bad alt. or battery?

My vote is a bad battery. Old or worn out batteries do not like cold weather...

hook up a meter when its cold and see if you have voltage fluctuation causing the dimming i would suspect the voltage reg which means new alternator again

yeah do have fluctuation when cold

now will I be able to swap the alt. So it has an external regulator? I have one out of a 94 jeep.

I thought the regulator was in the PCM.

Nevermind; for Renix.....I'm not sure where the regulator is.

so you guys think its the alt?

For Renix, the regulator is in the alternator itself. Have you checked belt tension?

There is a wire coming from the ignition switch to the back of the alternator. Tells the alternator when the ignition is on and it's OK to charge. You could be getting low voltage through this wire, for various reasons. On yours, maybe once the motor is running and the voltage is up, then it turns the alternator on. A possibility, I've had trouble with this wire before (I mud a lot) in various Renix years, the connection at the back of the alternator gets corroded. A good spot for a dab of die electric grease.
Low battery voltage may also be a contributor, the starter really sucks the battery down.