Auto Tranny Fluid is Brown

Last night I dumped the dark brown tranny fluid out of the torque converter from my 91 XJ. The XJ was slipping in high gear. I am sure the fluid had not been changed in 50K miles. My XJ has a rebuilt 4.0 going in this weekend. The XJ has been used as a daily driver by my FIL and he also used it to tow a 2o foot inboard to the lake several times a month. The Jeep has not been off road, but I intend to change that. First question- Should I fill the TC up with fluid before I reinstall it with the motor this weekend? Second question- Like I said this fluid was brown, should I try to clean the converter out with something before reinstalling? Third question- Any suggestions about the fluid I put in?

Chances are that the tranny is toast. Brown fluid means that the tranny overheated and burned the fluid. Adding new fluid now may make things worse. There was a thread about this not too long ago. The detergents in the old fluid have broken down and a varnish has built up on the drums and bands that helps them “stick”. The detergents in the new fluid will break down this varnish and the tranny will slip even worse. There is a saying about auto trannys; change often or not at all. If you already have the engine out now would be the time to swap out or rebuild the tranny.