toyota emina overdrive in at 80???? why?

i've just bought the above in diesel turbo 2.2 and its automatic 1992, it had another engine and gearbox jan 2006. dont know much about its history but seems to run really well apart from the fact that the overdrive doesnt come in til i hit 80. is it possibly wired wrongly somewhere? and if so where???? gearbox oil is fine so i really think this could be wiring but where????
any help much appreciated