th350 problem please help

[coyotekid]

i have a th350 with unknown mileage in my 78 c25 .. I am going to try to keep it short as this is my first day and post on here so here it goes.. the trans does not ever downshift from third to second on the highway trying to pass people its the vacuum style kick down and it does something because if i disconnect the hose it shifts like absolute crap. it also gets hot fast.. cruising on the highway the transmission hump (no interior carpet) gets hot enough it hurts to touch for any long amount of time and i am running the radiator atf cooler plus an auxillary cooler.. all advice is appreciated because im out of ideas... thanks

 

[Jarhead79]

Mine wouldn't shift right when I'd try to pass someone, found that someone had put a rubber hose that was too big on the modulator valve. It's a rubber hose that goes to a metal tube then a rubber hose to the vacuum port on the intake manifold or rear of carb, not sure which exactly, I fixed that and adjusted my shift cable, mine shifts better now and firmer, but if it's getting hot I'd say it's about toast, or if your exhaust is too close, the heat from it might be heating up the trans. I'd pull your pan and filter and see if the fluid is burnt or smells burnt

 

[firebane]

2 things need to be hooked up for a th350 to function properly.

1) Modulator on the right side of the case
2) Kick down linkage on the carb

You have to ensure #2 is in place and adjusted properly or it will never kick down for you.

 

[coyotekid]

the truck never had a linkage going from the carb to the trans .. not sure where it would go i dont see any random linkages on the transmission just the vacuum connection that goes to the carb?

 

[yevgenievich]

There is a cable that goes from the carb to the transmission. It is needed to downshift. Other option ia to run the cable in to the cab and pull on it when downshift is desired

 

[77 K20]

I suppose you could always drive it like a manual trans. If you romp on it and want it to downshift pull the gear selector lever from Drive down to 2nd... then move it back when you are done.

 

[firebane]

There is a cable that goes from the carb to the transmission. It is needed to downshift. Other option ia to run the cable in to the cab and pull on it when downshift is desired

This. If you are missing this cable your hurting your poor transmission

 

[Georgeb]

Set it up right before you smoke that trans. You need the cable and the vacuum modulator.

 

[77 K20]

I had asked my mechanic about a year ago about the kickdown cable (I was having bracket issues with the throttle and kickdown) and he said it isn't really necessary and would not damage the transmission by not using it.
Now if it was a TV cable then the transmission would be toast.
The kickdown cable just downshifts for you when you need the passing gear.


As far as it getting hot- are you sure your aftermarket cooler isn't partially plugged or blocked? Things shouldn't be that hot.

 

[coyotekid]

Other than no downshift and running hot the trans shifts smoothly however it seems like the shift between 1st and second at wot is early causing it bog a little bit ..... How hard is it to swap one of these things to a 4 speed manual ??? I hate autos .. I'm not a novice machanic and have done quite a few engine and transmission swaps and have worked a few years as a fabricator/welder.. Im just new to the GM world ha-ha I got tired of fords letting me down so I made the switch !!

 

[robert8096]

You sure it's a turbo 350? Turbo 350 has the kick down cable to the carb. A turbo 400 uses a switch at the gas pedal to control kick down. Here is a chart to confirm which trans you have.