Gear Loss

Hi All

Just joined . I have a 2006 LR3 TDV6 2.7 Diesel Auto . All is good however each time I turn the lights on I lose all use of the gearbox process as in it revs up to 4,000 revs does not change up yet still travels around 10 miles and hour. I stop turn lights off and all is restored and gears work normally . Am due a mega flush this month but seems to be an electrical issue cant be the transmission as works fine without lights ! . I picked up a trailer 3 weeks ago with a standard connection which gave an HDC fault during the journey which I reset and no issues since but lights now control my gears ! Any help with an obvious diagnosis will be checking battery shortly but seems totally weird as a connected issue hope you can help

Check for corrosion on the battery terminals. Also check the TCM connection for corrosion as water can drip onto the harness and enter the TCM.

Measure voltage on the battery with engine running and see how much it drops when you turn on the lights cos if it drops much the alternator is gone and low voltage affects the gearbox functionality

Thanks will get these points suggested checked

Thanks for your suggestions will get this checked also