G/box: Auto AW70/71: ATF: To Change Or Not To Change?

[TheHungriestBadger]

Hi all,

First post here, but I've been reading avidly for a little while now. Last weekend I acquired a '90 240 GL Estate with the auto 'box (presumably an AW-70). I am currently debating whether to begin changing the ATF or not; it certainly needs it since I don't think it's been done in 135k miles. It does smell burnt and is dark in colour.

Upon picking it up, I noticed it was low and topped up about 1L. The car took positively to this, with a much-reduced delay when engaging Drive or Reverse. Shifts are smooth, and I don't believe it's slipping? Certainly, the speed picks up okay across the board, although it does have a tendency to just sit around about 2k with the speed coming up smoothly, above 2k all is normal... Is this slipping? Kickdown works fine, no clunks shifting up or down.

So, the question: do I leave in the probably burnt ATF because it just works, or do I begin the process of gradually changing the fluid over? My planned method would be to drain 2L out of the oil return into a large drinks bottle and then replace it with 2L of fresh ATF, drive for 100-200 miles and repeat over time until it starts running considerably cleaner.

Thing is, I will need this car pretty much daily over summer, expecting to cover about 1500 miles by September. I'm currently thinking it's pretty much a 50/50 on whether changing the fluid would improve or degrade performance, although the fact that it responded positively to topping up is perhaps a good sign?

Sorry for the long post, but better to be thorough, right?

Cheers in advance,
Adam