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The 4R100 was introduced in 1998 as an evolution of the E4OD and shares many of the same parts (and some of the same issues). These units proved to be fairly durable from the factory, but time and heavy use can expose a few weaknesses. The aftermarket has provided some clever solutions for these that turn a fairly durable unit into a very durable unit. As with any modern unit, it's susceptible to valve and valve bore wear and those are replaced with improved aftermarket components. Some common failures specific to this unit are:
Plastic front lube/drainback valve that is prone to melting and restricting lube is replaced by aluminum
Center support wear allows it to move in the case causing damage to other components and is addressed by machining the center support to allow a steel ring to be installed to stop wear and act as reinforcement.
Overdrive piston return spring snap ring tends to pop out of its groove potentially causing a catastrophic failure and a locking ring that essentially acts as a solid ring stops this failure
Apply pressure to the intermediate and direct clutches is fed to supports that ride in the case with only tight clearance holding the oil pressure. Wear and case flex open up the clearance causing poor apply pressure. Spring loaded tubes and seals keep a constant seal to the supports, especially under heavy loads that flex the case when apply pressure is needed most.
Original design used two narrow rear case bushings that were prone to premature wear in these heavy use transmissions. A one-piece bushing with increased surface area has proven far more durable.
Again, there are plenty of other components replaced in these units that are normal to any later automatic transmission; the list above is 4R100 specific. Our experience at PowerSource is unmatched, so feel free to contact us via the Chat option or call us on the toll-free line and we'll be happy to do more than read a script to you; we have actual building and repair experience.