Thread: Shifting problem with a Turbo 400 Trans

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I have a shifting problem with a Turbo 400 Trans. The problem is when the truck is cold the gears will hold; 1st to 2nd will shift at 25mph, 2nd to 3rd will shift at 70 mph. After the truck warms up (10 – 15 minutes of driving) it will shift fine. 3rd gear will kick in at 45mph
The specs are Chevy ¾ Ton 4x4 with a 1984 style 350 engine with a 1989 T.B.I. unit. Edelbrock Cam & Intake Manifold. Hedmen Headers, the Trans is a Turbo 400 which was rebuilt 500 miles ago and the issue did not go away. I have brought it back to the shop where the trans was done but they say it is a vacuum issue so they will not touch it..
I have changed the MAP Sensor and the O2 Sensor and have gained some more horsepower but it is still shifting wrong when cold.
Thanks in advance for all your help.

Originally posted by grvdl
I have a shifting problem with a Turbo 400 Trans. The problem is when the truck is cold the gears will hold; 1st to 2nd will shift at 25mph, 2nd to 3rd will shift at 70 mph. After the truck warms up (10 – 15 minutes of driving) it will shift fine. 3rd gear will kick in at 45mph
The specs are Chevy ¾ Ton 4x4 with a 1984 style 350 engine with a 1989 T.B.I. unit. Edelbrock Cam & Intake Manifold. Hedmen Headers, the Trans is a Turbo 400 which was rebuilt 500 miles ago and the issue did not go away. I have brought it back to the shop where the trans was done but they say it is a vacuum issue so they will not touch it..
I have changed the MAP Sensor and the O2 Sensor and have gained some more horsepower but it is still shifting wrong when cold.
Thanks in advance for all your help.

None of that makes any sense. there is nothing on the motor that would give you more vac. hot than cold that I know of. you can check you vac. at the trans. if you wont to, then you'll know. get yourself a vac. gauge T it into the vac. hose going to the trans and put the vac gauge in the truck with you while you drive down the road. a stock motor will have about 15" of vac. setting or driving down the road at light throttle and will go down to 0" at WOT. once the motor rpm catches up the vac. will go back up. get the gauge in the truck with you and go drive it cold and watch the gauge and then watch the gauge after it gets hot and see what the dif is. it don't take much vac. to make the tran. shift maybe 10" i don't know what cam you have but it should be enough vac. to shift the trans anyway. vac. gauge don't cost much and you need one in your tool box anyway. if the trans wont shift into hi gear at 45 mph then the vac. would have to be at almost 0" if the trans wont shift into hi gear and you have 10" of vac, or more then its gonna be the gov. bad, bad modulator or need to change you fluid and filter, or for some reason the passing gear switch is bad when its cold. the wire going into the trans. on the left side is your passing gear. you might wont to unhook that wire anyway just to be sure. wont hurt nothing to drive with the wire loose just wont have a passing gear. do what I've told you here and let me know what you find.

Sure ya got the vacume hose to the modulator valve routed to direct manifold vacume ???? sounds to me like ya may have that vacume hose routed to a temperature/vacume switch......

Originally posted by HWORRELL
Sure ya got the vacume hose to the modulator valve routed to direct manifold vacume ???? sounds to me like ya may have that vacume hose routed to a temperature/vacume switch......

thats a good answer HWORRELL, but i cant imagine him going back to the trans. shop with a problem that simple and they not fix it for him just to protect their trans. if they wouldnt it would be the last one they would do for me.

Originally posted by lt1s10
thats a good answer HWORRELL, but i cant imagine him going back to the trans. shop with a problem that simple and they not fix it for him just to protect their trans. if they wouldnt it would be the last one they would do for me.

I know but 35 years of this and I still get amazed by some of the stuff that comes into my shop.......

Originally posted by HWORRELL
I know but 35 years of this and I still get amazed by some of the stuff that comes into my shop.......

i guess, its something i would have done in my shop, so he wouldnt break something else and blame me for it.