Re: Towing with a Territory
You should only tow in performance mode which locks 4th out. Your transmission is overheating in 4th and the ECU kicks it back to 3rd and will also turn engine fans on to cool things.
Also this may indicate your bands are worn and slipping under load...also oil in trans will be overheated and burnt which makes slippage worse.
I had a very similar problem whilst towing for first time recently and the same as you in "D" - then I read the handbook which says to drive in Performance Mode - ended up adding external cooler and flushing old fluid out - hoping this will fix things.