The 4L60E was introduced in 1993 as the successor to the 700-R4 (4L60) and uses the same gear train design, and even some of the same parts, but changed to electronic control with an aluminum valve body, and all but the earliest units employed a PWM (pulse width modulated) lockup for the torque converter. These units lose some fluid integrity over time causing various shifting issues, 3-4 clutch failures, converter lockup issues (sometimes imperceptible, but manifesting with a P1870 code that will cause a hard 1-2 shift after the code is set) They do have some gear train wear and failure points; most commonly the reaction shell stripping or breaking causing loss of reverse, second, and fourth gear. A lot goes into these units, but here's a list addressing the most common problems:
Fluid to apply the 3-4 clutches takes a pretty long path before arriving at the clutches themselves and there are numerous loss points along the way that are addressed: this is why adding extra clutches to the drum isn't a long term solution. Pressure regulator valves get 100% replacement to help address this and also reduce line pressure instability.
The PWM lockup cycle of the TCC regulator valve wears the aluminum valve body resulting in fluid supply loss to the converter clutch; the control module picks up on this and maxes out line pressure to compensate which results in a very hard shift into second gear. The valve body is machined and a new valve and sleeve are installed to fix this (this process applies to other valves as well, but this one is almost always the first one to fail) TCC regulator valves are replaced 100%.
The original reaction shell was prone to breaking at the neck or the splines stripping. The replacement shells have a neck radiused at the failure point and the splines are hardened. These also get 100% replacement.
As with any transmission oil control is key, but this one has a lot of potential loss points. Machining is critical and there's a host loss points addressed: check balls, bushings, drums and shafts, as well as AFL, TCC, shift, and boost valves. These units get 100% replacement of the forward and reverse abuse bore plugs to prevent 3-2 tie-up, the common 3-4 clutch failure, and delayed reverse engagement.
The 4L60E is a generally reliable unit that benefits greatly from the aforementioned technology. 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.