Turbo 350 down shift problems

Updated: 09 Dec 2024

I have a turbo 350 behind a sbc 350 in an 87 cutlass, with a shift kit and stall torque converter and an automatic shifter. My question is when I down shift to 1st It will not shift until I slow down almost a dead stop. When it doesn’t down shift the whole car shakes and the trans makes a loud thunk noise like it wasn’t ready for the shift. What can I do to find out what’s wrong? I’ve checked my vaccum lines an theyre good, fluid is brand new, maybe a little overfull. Only thing I can think to check is my governor but I have no problems in any other gear, which makes me think it can’t be the Governor. The trans was sitting for a while before I put it into the for the record. Thanks

Comments

When it does down shift*

Check the torque converter?

It’s brand new, it’s a 3000 stall and it locks up perfectly everytime at that rpm

How does the transmission choose gears? You mentioned it has a governor, which is one input, but does it have a throttle cable or vacuum modulator? or is it electronically controlled?

You have an aftermarket shift kit. Sometimes those new valve bodies are designed to “lock out” 1st gear except for standing starts. This is because 1st in some muscle cars is too high and can spin the tires, so it is sometimes beneficial to start in 2nd to go faster.

I am not really familiar with GM transmissions, so I probably can’t help you that much. I recommend flushing the fluid and changing the filter if it has not been done within the last year. Otherwise I guess check some GM forums! Good luck homie.

TH350s have a vacuum modulator, governor, and kickdown cable. That’s… it. Pretty simple things, really. For an automatic.

You have some valve sticking in the valve body probably, especially if it has sat for a while.

So what’s the best way to fix that? Take it apart and free it all up? I thought since the shift kit is also only about 2 weeks old it wouldn’t be a sticking problem but maybe the mechanic over looked that

Hmm the fluid and filter is all new, (2 weeks old) I don’t have the kick down cable hooked up because I was told with my manual shift I have I didn’t need but maybe that’s wrong? Vaccum to my knowledge is good. I’m not sure if the Governor makes a difference but I haven’t checked that. Maybe that or my kickdown is what’s causing it

If your kickdown wasn’t set right, it should shift too high. Something you can test - unhook the kickdown from the carb/throttle plate and zap strap it to something so it’s pulled as tight as possible (the last little bit is extra hard so make sure you get it). Then drive around. It should shift up and down at about maximum rpm regardless of anything else. At least, my car would do that. Not sure with the shift kit. I didn’t have a kickdown for a while and it shifted normally, it just didn’t kick itself down a gear when I jammed the gas.