Not that Jiffy Lube types are the best ones to comment on transmission
issues, but they do see a lot of them. Likely they had some comment, to
which you've not referred, as to the color of the fluid. ATF fluids are
uniformly dyed with a temperature sensitive red colorant. If the fluid
exceeds acceptable temperature levels it turns to a "straw" (yellowish
beige) color and, when *really* heated, to a brown or almost-black
color. At the latter point, it may give off an unpleasant odor that
clearly associates with burnt material. I don't personally know that
the S90 has a transmission radiator or other heat exchanger, but it
certainly sounds as though that cooling device is out of the loop *or*
there's significant slippage in one of the clutch packs. If a clutch or
break is slipping, it can provide the majority of the "dark color" to
the fluid.

bob noble
Reno, NV, USA

"Bob Noble" <the.nabob (AT) worldnet (DOT) att.net> wrote

I recently changed my ATF fluid in my 740 M46 with overdrive because the
overdrive had gone. Fixed the mechanical problem in the overdrive with a £5
spring that had broken. When I first split the overdrive, which shares the
ATF with the gearbox, it was like brown water that came out and smelt of
burnt circuit boards. The gearbox had done 200,000 miles with me, and
probably the rest with the first owner without a change. I put new oil in
once I'd fixed the overdrive problem, and I found that the gearchange was a
bit more slicker and the car felt like it was travelling easier. I know it's
a different car but the oil is the same. It definately has a shelf life. I
was led to believe the gearbox was sealed for life, but I guess it was a
shorter life than they thought. BTW, my gearbox and O/D unit are fine after
clocking up another 10,000 miles with the new oil.