What causes most transmissions you see to fail?

Summary:

Jaden splits the answer between older and newer units. On older transmissions the cause is usually straightforward wear and tear from age, with clutch failures and band failures as their characteristic problems. Newer transmissions, specifically six, eight, and ten speeds, are failing through torque converter shudder, and he traces the root cause to the valve body, which he calls the brain of the transmission. Because these are pulse width modulated systems, the valve is constantly moving on a percentage basis, and that constant motion wears out the bore in the valve body. The worn bore bleeds off pressure to the torque converter, which destroys the converter clutch and produces the shudder. He then describes the cascade: debris from the converter enters the shared fluid, clogs passages, sticks valves and solenoids, bleeds off pressure to the clutch packs, and burns up the transmission.

Answer:

So that's a complicated question. The older stuff that we still do, it's mostly just wear and tear and they're getting old and it's time to rebuild it. They had their own issues as far as clutch failures and band failures. The newer ones are having a lot of issues with torque converter shudder. Six speeds, eight speeds, ten speeds, they're all having those issues. And part of the issue that they're having is because it's a pulse width modulated system. The root cause is usually the valve body. The valve body is like the brain of the transmission, and with a pulse width modulated system, the valve is always moving. It's a percentage based system, and so it's constantly moving. And what happens is they wear out the bore in the valve body, and then it bleeds off pressure to the torque converter, and then it winds up taking out the clutch in the torque converter, and it winds up shuddering. And when it shudders, it ruins the clutch in the torque converter. The torque converter and the transmission share fluid. Then the torque converter junk gets into the transmission and starts clogging things and sticking valves and sticking solenoids. And then it bleeds off pressure to the clutch packs, and then it burns up the transmission.