Erratic shifting/Hunting for Gear TH350

[BadBowtie]

Maybe buy a new TH350 fill tube and indicator?? Drain any excess fluid and add a full bottle of Lucas Transmission Conditioner in.

If it is the original TH350C the electric plug would still be on the driver's side,up above the oil pan flange.

When those lock-up TH350C transmissions "go" most owners upgrade to a 700R4 or place a rebuilt, regular, TH350 back in.

 

[Ovals_and_Bowties]

Life happened (wife was sick, water line broke, etc) and I almost forgot about this thread but small update:

*I did find a vacuum leak on the brake booster but fixing it did nothing for the trans. It just bounces between 2nd and third with slightly softer shifts now.

*It's worse at cruise than at light accel especially around 20MPH.

*Shifts normal on medium-light accel as long as I don't let off before 35MPH.

*Detent hooked up or unhooked; over or under adjusted, makes no difference on the erratic shift.

*It'll hold third in drive on deceleration all way down to like 18MPH. If I'm stopping for a red light but it turns green it'll try to take off in second. Not sure how slow it goes before shifting to first but I know at a full stop it will shift to first automatically.

*I'm suspecting valve body cross leak but this is my only running vehicle right now. So I need to get something else going before I try pulling the valve body.

*I think I'll go out and put the pressure gauge back on the tranny. I dug up Ron Sessions book and the spec seems to be about a 7PSI line pressure difference between idle and 3k RPM if the gov is working. My crank journal for 1&2 is a 0.001" out of round, think the old 305 will tolerate revving up to 3k long enough to run the test? It's got 10w-40 in it and holds about 35PSI at idle and quickly climbs to 60PSI with throttle.

 

[Ovals_and_Bowties]

Another Update:
A) I have devised a test to check for internal cross leak: check the 3rd gear pressure tap at 15MPH whereby the trans holds 2nd in drive.

B) I have a theory about what's happening. Either GM or somebody later on installed a governor for a CAR with a 2.73 rear. I.E. it wants to command an upshift too soon for a truck carrying cargo. Thereby lugging the engine, which the modulator "sees" then commands a downshift. But once downshifted engine vacuum goes back up, thence, dropping modulator pressure allowing the governor to, once again, command an upshift. Rinse and repeat.

OR it IS supposed to be in third at 20MPH and the torque converter clutch is engaging when it shouldn't or the stator clutch in the torque converter has failed.

I will disable the lockup and road test the vehicle.

I likely pick up a governor calibration kit change the weights and see if the problem goes away.

 

[Ovals_and_Bowties]

Final update should anyone else ever have the same problem.
I figured it out the filter in the trans got a small tear in it for whatever reason and the valve body sucked up enough crude that the 1-2 and 2-3 shift valves were a little sticky. Not enough to completely stop it from shifting. Juuuussssst enough to make it hunt.
After disassembling the valve body and giving it a good cleaning and a little polishing on the valves it no longer hunts for gear.