For context: I have an older Toyota 05. 160k miles. Transmission fluid looks kinda brown and mechanic said I should do a flush and quoted me 300$. I’ve also heard that at some point you shouldn’t change the transmission fluid if it has t been changed in awhile. Why is this?
In: Engineering
That’s what we call a hoax. The reason this hoax keeps getting perpetrated is that someone will notice their transmission having issues so they’ll bring it in for a flush. Not long after, the transmission will fail and the flush gets blamed even though the reason for the flush was because the transmission was having problems. Sometimes forcefully flushing can move clutch material that has built up in passages to locations causing binding in valves and pressure manifold switches, etc. If you just do a drain and refill, that should not occur.
I’ve been rebuilding transmissions for 30 years. There is nothing in old transmission fluid that helps a high mileage transmission to keep operating normally. Transmission have filters that remove particles from being able to flow throughout the internals so there are no particles to help with grip. If anything, old fluid breaks down and loses its ability to assist with grip.
Transmissions are not as complex as people think. High mileage transmissions typically will have worn down clutch plates which will cause slipping. Seals and o-rings stop sealing and can crack causing hydraulic fluid loss which causes things to not engage properly. It’s rare that were see catastrophic failure where things actually break inside. Also, clutch material can build up in passages before they get to the pan to be filtered which can cause issues with activating pressure switch manifolds and can cause issues with movement in the valve body.
Transmission rebuilds are expensive because of the labor. I can spend 14 hours removing a transmission from a vehicle, disassembling said transmission to get to a $15 part deep inside to replace it, reassembling and reinstalling said transmission. That $15 part may cost $2500 in labor. However, a refurb transmission may cost $2500 installed so there’s no cost benefit to the vehicle owner either way.
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