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T-400 upshifts too fast

6760 Views 10 Replies 5 Participants Last post by  mailbox1024
Need some help here guys. Have a 72 Corvette with a 1968 327 350HP engine and TH400 trans and 3:73 rear gear ratio. The trans upshifts way too fast. The trans already had a shift kit installed before i purchased the car at a proffesional trans racing shop in the mid west. When i accelerate, it shifts right away and goes to top gear in a matter of seconds. Is this a problem with the trans modulator or something else? Car bogs when it shifts so quickly, will do this at any RPM. I can cross the street and be in 3rd gear doing only 10 mph! Then it gives me difficulty in downshifting to a passing gear.
Any suggestions will be appreciated.
PS.. Just rebuilt the engine and have a good steady vacuum at the modulator.
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i had the same problem with my th350. most of ur problem is with the modulator. i had an adjustable one put in and that took away most of the problem. but also look at your vaccum line from the modulator to the intake. the line could be pinched shut, clogged, or its not getting full vaccum like it should. just be careful with the adjustable modulator... if you set it at to high of a shift you will have to drive with a very heavy foot for it to shift. in turn you will have to opposite problem you have now.

Thanks for the input. I already ordered a new modulator from Summit Racing , should be in this week. As far as the lines and vacuum goes, I ran in a new line, and vacuum is 20lbs and steady. I tried to adjust the old modulator and it made no difference at all. The one from Summit is adjustable too, but they said it is preset for factory settings and shift patterns.
thanks for reassuring me that i went in the right direction....
no prob glad i could help :thumbup:

if that doesnt fix the prob id take a look at the solenoid and spring that control the shifting but that means you gotta take off the valve body at that will just give you nightmares lol.

yeah, i had that all apart last year when i had it up on a lift. Everything was replaced in the valve body. I tried a new shift kit thinking the old one was beat up on before i got the car. I wouldnt have replaced the springs, but i got a free kit from a friend so i figured that i would just attempt it. Same problem and no change in shift pattern. Should have swapped out the modulator when i had it up on the lift.
thanks again........
Maybe I can help you a little more. :welcome:

The transmission governor shifts the trans when you have wide open throttle, zero vacuum to the modulator. The modulator counteracts/overrides the governor to make it shift sooner.

If you have NO vacuum to the modulator WOT, it will shift at 5000 rpm or whatever the governor is set by springs and weights.

Adjusting the modulator allows you to adjust your partial throttle shifts within a range the modulator is calibrated for. If that range is not acceptable, a different modulator is needed.
So what are you saying? I need to replace the modulator? I have excellent vacuum going to the modulator now. i just have a very quick upshift. The trans will shift to 3rd gear by the time i reach 20mph! Just a tad too quick for me. If i accelerate just to pick up my pace, it will shift to high gear and bog down on me. The shift range is 1000- 1500 rpm 1st, 2nd,3rd. then it will remain in high gear unless i manually downshift.
the vacuum reading of 20 inches seems a bit high .

Do you have a vacuum line hooked directly to the intake manifold and the modulator with no other hook ups in the mix?

The vacuum must drop off as the throttle is opened to delay the shifts.

I am also curious if the governor springs were changed when the shift kit was installed?
No, i have a dedicated vacuum line direct to the modulator from the intake. Also, the vacuum drops off upon acceleration, and the springs for the governor were not changed. Any other suggestions?
When the shift kit was installed in the transmission they might have used the modulator valve spring that come in some shift kits on the modulator valve? That will make it shift earlier also, if theirs one in it remove it. It's located behind the modulator valve, to get to it remove the vacuum modulator then pull the modulator valve out with a pocket magnet & the spring will either come out with the valve or stay in the transmission case, if so use a pocket magnet to remove the spring from the transmission case.
Thanks for this info............i will definetly look at that!
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