While not as sensitive as some shifting bands, the Reverse band adjustment on a TH400 or 4L80-E transmission is critical, and failure to get it right has tripped up even the best builders. There is nothing worse than getting the transmission installed, putting it in Reverse and then not going anywhere or having no engine braking in Manual Low. The main reason for this is that band adjustment/servo travel is not easily checked or adjusted. There are OE tools (J38737/J21370-10) for checking and selective length pins for adjusting travel, but the tools are expensive, hard to locate and difficult to use. Additionally, the OE selective pins have been discontinued. The solution: make your own tool to set pin travel.
Most likely you have what you need using spare parts lying around the shop. There are three styles of servo covers for these units, but just two are needed for making servo-checking tools: a TH400 and a late 4L80-E, which works for both late and early units (see Figure 1, above). You will need to cut a window in the cover to see the piston. Drill a hole down through the top of the cover so you can tack weld a nut to the top. A 7/16-20 fine thread is preferred, as this makes for a convenient .050-in. travel per turn. Then, thread the bolt down in until it just makes contact with the piston. You can make yourself a handle to help count your turns or just leave the head of the bolt.
Once the pin is assembled in the piston, install it in the case with the tool bolted down over it. Tighten the bolt until it is just barely contacting the piston. From here, you have several options for checking travel: Either measure from the top of the cover to the top of the piston and zero your calipers, or measure from the top of the bolt to the cover. Tighten bolt down until the band is seated and check your measurement. The piston should travel .125-in. You can also just count the turns of the bolt. With the 7/16-20 thread, the .125-in. travel equals 2.5 turns (see Figure 2).
There is more than one way to determine pin length/servo travel in these units. There are two main things that really matter. One is to make sure the band does not drag on the reaction carrier when released. The second is that the band is gripping the carrier well before the piston is bottomed out in the bore.
Read more stories from our TASC Force Tips series here.
Caleb Perham is a Sonnax product support representative. He is a member of the Sonnax TASC Force (Technical Automotive Specialties Committee), a group of recognized industry technical specialists, transmission rebuilders and Sonnax technicians.
