tropics

For the gurus on the board, got an unusual trans leak. First the basics, 94 4Runner, 2wd, A340E, 180K miles, no codes, no funny shifting, just spewing fluid out. Heres the unusual part, I tried searching, but could'nt find any info. The trans is leaking on the drivers side, on the case above the trans pan, there is a donut shaped rubber gasket that looks to be only friction fitted in the case. In the center of the donut is a kind of round cap. the FSM doesnt mention the purpose of this item, not sure of the purpose. The gasket was loose so I pushed it back in to try and reset it. After 17years or so its probably hardened and worked its way loose. Anyone know what that item is? Sorry, no pics. I thought it was weird that the gasket/o ring is only friction fit since when it gets loose fluid will come out.