The hardest transmission to rebuild is a CVT transmission, also known as continuously variable transmission.
CVT or continuously variable transmissions are the hardest transmissions to rebuild because of their belt and pulley design as well as their strict software calibrations and their tendency of internal wear to destroy the metal cones and most mechanics actually recommend just replacing the CVT transmissions, instead of attempting to rebuild them.
Rebuilding any transmissions can be very difficult, although manual transmissions are easier to rebuild than automatic transmissions.
Dual Clutch Transmissions are highly complex, requiring advanced diagnostic tools and strict adherence to specific factory-mandated torque sequences and procedures.
The combination of intricate mechanical components and sensitive electronic controls makes them very challenging to service.
Older, mechanically-operated automatic transmissions (like the TH400 or C4) are relatively straightforward, but modern units (such as modern BMW or SAAB transmissions) are built as hydraulic computers.
They require clinical levels of cleanliness; a single piece of lint from a shop rag can jam a spool valve and destroy the rebuild.
They also require specialized tools and strict software/sensor calibrations.
And while manual transmissions are simpler to rebuild, they still are more difficult to rebuild than an engine as they also still demand and require precise measurement of clearances as well as careful shimming and exact reassembly.
And just 1 improperly set synchronizer ring or worn slider, will mean that you need to take the whole assembly apart again.
Rebuilding transmissions is best left to a transmission shop and rebuilder who has the expertise as well as all the proper tools to do the job.
The number one killer of transmissions is excessive heat.
Excessive heat causes the transmission fluid to oxidize and break down, which robs the transmission of it's vital lubrication and also creates severe internal wear.
Although heat is the number one killer of transmissions, other things also increase the excessive heat, which accelerate the death of a transmission.
Having low or contaminated transmission fluid, doing lots of heavy towing and hauling, neglecting maintenance and aggressive driving habits all factor into and cause transmission failures.
Transmission fluid acts both as a lubricant and coolant for your transmission.
And having leaks or old, burnt fluid in the transmission also lead directly to increased friction as well as catastrophic overheating.
It's important to keep an eye on your transmission fluid and never allow it to get too low.
If the transmission fluid is low, you should add some more of the correct transmission fluid and change or have your transmission fluid and transmission fluid filter changed every 20,000 to 30,000 miles to extend the life of your transmission.
And exceeding your vehicle's factory towing limits or even placing any extreme strain on your drivetrain causes the torque converter to slip and generate intense heat, which kills transmissions.
If you don't have a transmission cooler and do any towing, it's recommended that you add a transmission cooler to help your transmission and transmission fluid stay cool.
As for not changing the transmission fluid, failing to change the transmission fluid and failing to replace any clogged transmission filters, allows the transmission to operate with degraded heat trapping fluid, which also kills transmissions.
And even slamming on your brakes, rapid acceleration and shifting gears while the vehicle is also still moving shock the system and spike the transmission fluid temperatures.