Technical TH350 acting weird

My transmission was acting strange today. At low speed in high gear (very light throttle, if any) the transmission can't decide what gear it wants to be in. It wanders back and forth between 3rd and 2nd.
Up, down, up, down,.. until I add some throttle or slow way down.
First check was fluid level and it looks clean and at the right level. The kickdown cable is hooked up and adjusted. Another clue might be the trans failing to downshift with heavy throttle, but that has been an ongoing problem, whereas the trans hunting for the right gear is recent.

Does anybody here have an idea what could be the cause? I'm thinking I need to throw a new modulator at it, but I'm only guessing.

 

at least check to see if there is ATF in the vacuum hose at the modulator

 

check the govenor, could be sticking

 

How many miles, or years on this trans?
Last filter, fluid change? What did it look like inside?
Vehicle weight?
Rear end ratio?

 

I just dealt with this issue just a couple of weeks ago. By chance, did you have the transmission out for any reason? I pulled mine to swap out a broken flex plate. Gave it a bath and a seal change while it was out. It never went back to normal. First I had no gears... then it would engage. Finally got it 'going' and it was doing exactly what yours is. I ended up scoring a rebuilt from Craigslist. I did get a lot of info on my thread if you want to search it... entitled "Th350 Mystery"

JC62

 

Check to make sure there is manifold vacuum at the modulator. Ported vacuum will make the trans fall out of high gear at low throttle openings.
Bill

 

Make sure the modulator rubber hose isn't cracked and leaking vacuum.

 

To answer some of the questions, it is unknown how old this trans is or how many miles are on it. The fluid was surprisingly clean, with no burnt smell to it at all.
The vehicle is 3000 lb, with probably a 3:30-ish gear. The rear end is an old 8.75" GM with drop-out housing...not sure what gearsets came in them.
The engine does have a cam, so I expect that vacuum is low when idling... maybe that is confusing the modulator?
I will crawl under there and check the modulator hoses and vacuum signal.
If I opt for a new modulator, should I get an adjustable one? Could the adjustment help with these symptoms?

 

Had this happen to me and turned out I had knocked the hose off the modulator while doing something underneath the day before.