Lincoln/Mercury Repair: transmission fluid, worn clutch, many different reasons

Question
Hello,
I've been practicing general mechanics for almost 20 years and I've always wanted to know what are the exact cause of transmission failure after changing the fluid in a transmission with high mileage. I've come across many different reasons but I trust your expertise to set me straight. Thanks and best regards, John.  

Answer
Hi John,

Without any positive proof, there is a condition in transmission fluid that develops over time we call "varnishing". Once varnish is present in the transmission it must stay there. I believe that varnish is made up of old fluid with fine particles of clutch material. I feel that it creates a bit of extra friction needed to hold a worn clutch pack. New fluid is mostly detergent and will clean the varnish off the clutch discs and make it nice and slippery. Which is great for the valve body, but will cause the discs to slip on the plates. I would say that half of the transmissions we overhaul are after a couple of weeks to a couple of months of a fluid change. In fact, I won't even do fluid changes here because I know what happens! Incidentally, my 2000 Navigator, that is not babied, has 115,000 miles on the factory fluid...