TH400 problems

[Craig M]

Started a trip with the 82 dually this weekend. Had a 2000# pull trailer with about 4000# on-board. Weather was warm and headed east. Pulled first couple of major hills fine. When we got to Baker grade transmission had issues. I have the Gear Vendors behind the 3 speed TH400. To keep rpm's up on grades with a trailer I typically manually downshift through the 6 available gears. When manually in 2nd gear it was shifting back and forth between gears on its own. It did not do this when the gear vendors was engaged. Anything easier than a rebuild? Transmisssion has only 87,000 miles on it but many of those may have been with a trailer (I am 2nd owner and did not get to talk to prior owner, but vehicle was set up as trailer hauler).

[More Power]

The TH400 uses vacuum modulation to interpret throttle position. So, I'd eliminate the vac pump, vac lines and VRV valve mounted to the injection pump before suspecting a trans issue.

Some later TH400's used an electrical signal from the VRV assy to force a drop into passing gear (2nd) at WFO.

MP

[Craig M]

The reason I am worried about internal problems is the "surging" is occuring when I have allready manually put the selector into 2nd. It should not "upshift" or surge into 3rd when it has been manually placed in 2nd. It is happening at high rpm so there may be some type of bypassing or?