tranny won't shift when headlights are on

:confused: recently my transmission has been acting up when ever i have my headlights on, which is necessary alot in the middle of winter. i rebuilt my tranny 18 months and 8,000 miles ago and has been fine until now. i notice the dashboard voltmeter shows only 12 volts when the headlights are on, 14 when they are off. connecting a voltmeter to the battery shows 14.2 volts with the engine running whether the lights are on or off. battery hold a good charge, starts immediately on the coldest mornings. yesterday cleaned all connections at the battery, ground post at the front under the hood, checked fusible links and connections at the firewall behind battery, all seem good. today i noticed
if i apply additional load to the battery such as power windows the voltage drops even lower at the dash and the headlights dim. voltmeter at the battery stays at 14.2 volts. when i rebuilt the tranny tested all shift solenoids and they were good, and again shifting is normal with the lights off. changed trans fluid and filter 2 days ago, shift is even smoother, when lights are off, but when lights are on i cant get past 2nd gear, and often have to shift manually to 2nd to get out of 1st, at times wants to slip into neutral, as when i apply power to windows. where should i look next? voltage reg? something seems to be drawing volts down beyond battery and alternator as charging seems unaffected. 94 safari awd 180,000 miles

Take the negative battery cable off of each end and clean the terminals with a wire brush or file. Get 'em as shiny as Rudolphs nose! Also check the body ground wire connections. Reinstall and tighten all connections. If voltage remains at 14.2 volts, the regulator is doing its job. If the problem continues, there could be a problem with the rectifier bridge or the brushes in the alternator. Judging from the tests that you have done, the alternator testers at Autozone and other parts stores most likely will not spot the problem. Best to take it to a shop that specializes in electrical systems and have them test alternator maximum output.

:)went under the hood to recheck the negative ground wire connections again this am, noticed the ground stud in the radiator support was a teeny bit loose, and then spotted several smaller ground wires connected under the stud which i hadn't even seen as they were under a lip in the support. took those off and cleaned them as well, tightened all up and now dash voltmeter reads the full 14+ volts with the lights on, power locks, windows, etc operating. tranny shifts normal with lights on! I can drive in the dark again!
thanks much old master! Merry Christmas to all. Now if I could only do something about the loose nut behind the steering wheel.

Glad I could help and glad you got her going! Thanks for posting back with the fix.

As for the loose nut, sorry can't help ya, got one myself and can't figure it out!