The two aux fans in the front are for the ac system and switch on and off somewhat independent of engine temp.
http://www.peachparts.com/shopforum/diesel-discussion/155355-87-300d-aux-cooling-fan-diagnosis.html#post1187059

If your engine overheats my 2 cents would be on a faulty fan clutch on the cooling fan behind the radiator. It shall engage around 90C and you should really hear it blow air when from 2000 rpm.