I provide some tips and pointers for inspecting and assessing used GM 4L80E transmissions for purchase. Some of this video's presentation applies to the TH400 family as well.

There are a few different reasons folks purchase used transmissions, including those looking for a rebuildable core or a good-used unit to install as a short-term (usually) budget-driven solution to get their vehicle back on the road while they save the funds and plan for a proper rebuild or replacement later.

Unfortunately (unless you have x-ray vision), it can be very difficult to know if a given transmission is actually serviceable, functional or a grenade job just by looking at it or superficially inspecting it. That said, there are a few things you can do to weed out poor candidates for either rebuild projects or "as-is" replacements and improve your odds of success in the used transmission market.

Tech Notes: ATSG states the wiring harness changed in 1993; torque compensator feed eliminated after 1993; however, if you have a 1991-1993 model year unit that has the earlier harness/VB, you can install a Rosta harness, part number 34446AK (works on all years 1991-2003), updated BW non-vented solenoid and adapt the VB and plate to work with the updated EPC (See Transgo link for instructions below). Valve bodies that use the initial vented electronic pressure control solenoid (EPC), produced by Bosch, will need to be adapted to work with the updated Borg Warner/Holly EPC. See page 5 of Transgo's SK-4L80E shift kit instructions on how to do this procedure: https://res.cloudinary.com/transgo/wb...