This picture shows new automatic transmission fluid. Clean, fresh automatic transmission fluid is usually red,
smells like fresh oil, and is absent of gritty matter. Dark fluid that has been left in the transmission too long
and thus has been burnt, is probably full of gritty wear material, and smells burnt. Leaving transmission fluid
like this in the transmission too long will result in internal damage to the unit in the form of increased wear,
scorched clutches, and bands, and worn bearings and bushings.
Color
Transmission fluid should be red. Dark brown or black indicates that the
transmission was overheated at one point. There are dos and don’ts regarding
changing the fluid on transmissions that have been overheated.
On vehicles with a neglected transmission and high mileage, changing the
fluid after it’s been burned is a crapshoot. If the transmission has overheated, a
couple of things happen internally. The seals harden and allow fluid to pass by
them. Less fluid in the system decreases hydraulic pressure. The drop in pressure
causes additional overheating and friction buildup. In addition, the glue that
affixes the clutches’ friction material to their steel backing crystallizes, so that
when new high-detergent fluid is introduced into the system, it scrubs away the
crystallized glue. The friction material then falls away from the steel backing,
rendering the transmission useless. At this point the transmission has to be