Truck transmission wizards need some advice

[the old grind]

So , we have a 1200+ rpm-stall converter not stalling(?) and we think below 1000 it's something in the transmission causing the engine to stall instead?

I've driven hot rods with Vega torque converters that wouldn't budge much below 2500 rpm ... because the converter slips below stall speed. There's not much 'hookup' there.

If someone tried to get the newer model converter/trans to work they may have done an iffy job of connecting bits*. Was the truck just bought this way or was it driving fine for months or years and is this a new symptom? (*electrically triggered solenoid in/on the valve body)

Occam's razor isn't always your friend if sticking to a first conclusion denies testing-out other ideas. All we know about this F-150 is year(s), make, model and symptom. :2cents:

 

[super55]

It seemed like it always did since I bought it but it seems to be more pronounced now. Part it might be because the idle screw is turned up just a tad more to make idle smoother in park. Last winter I put in a rebuild 351 because the old motor threw a crank bearing due to low oil pressure. So I had the motor out and actually am kicking myself for not replacing the torque converter while I had it looking at me right in the face. Mating the two back together actually went really smooth it dropped right in with no issue. Timing and vac and are good and replaced the old motorcraft carb with a 500 CFM holley 2 barrel.

You do bring up an interesting idea. The C6 trans was used a lot in old ford diesels where the stall is considerably lower. It could very well be the wrong torque converter. It really feels like it is a locking torque converter and the clutch pack inside isn't releasing the turbine side from the flex plate side.

I believe the valve body could be an issue possibly as well but seems the valve body focuses more on fluid flow from 1-2,2-3, kick down 3-2 and reverse.

 

[the old grind]

This ^^ All of it.