How do you convert a factory 4L60E engine/transmission harness to control a 4L80E when swapping transmissions behind an LS?

Here is what the documentation says. Each point links to its source; details depend on your exact computer, harness and transmission.

• The Quick 4 kit includes both the early large and late small output-speed-sensor harnesses, with black and purple wires terminating in the controller's white TSS connector. (US Shift)
• The Quick 4 includes a separate input-shaft-speed-sensor connector that is used only for 4L80E applications; 4L60E/65E/70E transmissions do not use it. (US Shift)
• For 1999-2007 Gen III truck/LS-style harnesses, the 4L60E-to-4L80E conversion guide moves the transmission-connector brown wire from cavity U to cavity S, and repurposes the removed white wire (cavity S) and tan/black wire (cavity T) for the 4L80E input-speed sensor. (Uploads)
• 1999-2002 PCMs use blue/red connectors while 2003+ use blue/green connectors, but the transmission-swap pin locations are the same. (Uploads)

Confirm against the instructions for your exact parts before wiring. More: Transmission control in our tuning guide.