[Captain J]

Originally Posted by Dead Parrot

From your description, sounds like you have a TH400.

Choke light on: Make sure the choke isn't stuck. Stuck choke can cause a way too rich condition which plays with vacuum and thus impact shift points.

Trans: Verify that the trans fluid level is correct. Low level can cause shifting issues.
One quick test - take a clean hose, pull off existing hose from modulator, attach test hose to modulator, suck on hose. If it keeps vacuum, modulator might be good. If it doesn't, modulator has leak and needs replacing.

I’m not sure if it’s TH400 or R700. When starting the truck it will automatically run as choke is on, then when I press the pedal rpm drops.
Oil level is good, oil clean, I’ll do this test.
However what’s this switch/solenoid that is located at the gas padel with 2 wires (red goes to fuse box 12v, the other goes to side of the trans)?