'73 TH400 WOT upshift point question.
Thread Starter
Burning Brakes
Joined: Apr 2001
Posts: 752
Likes: 1
From: Springfield Oregon
'73 TH400 WOT upshift point question.
I have a rebuilt TH400 with what the shop refered to as a stage 2 shift kit. The shop is no longer in business, so I couldn't call them for any info. With the shifter in "D", gas pedal floored, it upshifts about 4400rpm on the stock tach. No 1-2 chirp, but good and firm. Now, if I pull it into "1", gas floored, and run it up, it does it's 1-2 on its own at about 4800 without me moving the lever to "2". Why? Plus, when it hits 2nd this way, it does a nice chirp. I tried adjusting the vacuum modulator, but it only raised the part throttle shift points. My real question is why can't I hold this thing in low until like 5300 or so. I have a pretty much stock rebuilt L-48 short block with rebuilt 1.94/1.50 heads, 204/214 Performer cam and intake, Holley 1850-600, and 2 1-2" exhaust through Sanderson 1 1-2" primary manifolds. Any thoughts appreciated.
Le Mans Master
Joined: Aug 2000
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From: Semper ubi, sub ubi
Cruise-In VI Veteran
Cruise-In VII Veteran
Re: '73 TH400 WOT upshift point question. (ORVette)
The only way to adjust the shift points is by changing the springs and weights in the governor. Unfortunately the governor is on the top of the transmission and can only be reached by dropping the transmission, or cutting a hole in the floor of your car.
My car also shifts at the exact same points as your does. Standard rebuild, with standard passenger car/truck governor settings. You have to request a performance governor to be installed to get WOT shift points near your redline.
My car also shifts at the exact same points as your does. Standard rebuild, with standard passenger car/truck governor settings. You have to request a performance governor to be installed to get WOT shift points near your redline.
Intermediate
Joined: Jan 2000
Posts: 37
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From: FLORIDA
Re: '73 TH400 WOT upshift point question. (ORVette)
When I had my TH400 rebuilt I had the shop put in a B&M Street/Strip kit.
Under normal acceleration in D it shifts very smooth. Put the pedal down and
it will bark from 2nd to 3rd. Hold the shifter in 1st and it will shift on its own
to 2nd at 5500.
We don't know how My brother inlaws 71 TH400 was rebuilt but it will snap
your neck and bark from 1st to 2nd at almost any speed. We have run it
up to 6000 holding it in first and it still would not shift. That one is getting
a B&M put in it also.
Under normal acceleration in D it shifts very smooth. Put the pedal down and
it will bark from 2nd to 3rd. Hold the shifter in 1st and it will shift on its own
to 2nd at 5500.
We don't know how My brother inlaws 71 TH400 was rebuilt but it will snap
your neck and bark from 1st to 2nd at almost any speed. We have run it
up to 6000 holding it in first and it still would not shift. That one is getting
a B&M put in it also.
Safety Car
Joined: Jun 2001
Posts: 4,004
Likes: 2
From: Severna Park Md
Re: '73 TH400 WOT upshift point question. (DRAGON)
Thats strange. I thought if its manually held in first it should stay in first , indefinatly, till you manually shift it to second. Sounds like maybe the linkage is not pulling it all the way into first. I could be wrong but it still sounds wrong for it to upshift on its own when held down in first. If the actual posistion of the valve in the valve body is in between first and second, damage can be done to the trans. I would pull it into first gear then disconnect the cable from the lever and see if you can move the lever back further. If no there could be a problem in the valve body with pressure crossing two circuts at the same time. Again bad for the trans. This is just my guess. My 73 TH400 would stay in first as long as i held it there. Good luck with it. :chevy
[Modified by Jvette73, 4:22 PM 9/6/2001]
[Modified by Jvette73, 4:22 PM 9/6/2001]
Thread Starter
Burning Brakes
Joined: Apr 2001
Posts: 752
Likes: 1
From: Springfield Oregon
Re: '73 TH400 WOT upshift point question. (Jvette73)
I did some hunting and found the guy who did my trans. I was just talking to him and he didn't have a definate answer either. He thought that probably what is happening is what Jvette73 said. He suggested pulling it back into low and holding back on the lever while I ran it up. I will go give it a try in a few minutes and write back with the results. Oh, he also happened to be friends with the parents of the kid who owned the car before me. With permission, he proceeded to tell me of the abuse this thing had suffered at the hands of that little brat. 2 blown converters in 6mo.! ETC! I know $7500 was too much to give for this car when I bought it last April, and I know it was probably not the smartest investment to spend the extra $4500(to date) that I have getting it fixed up. But after hearing the tale of terror I heard this afternoon, I'm proud in a way to have taken this poor, molested '73 and turn it back into what it was meant to be. At least to the best of my ability and available finances so far. It's a noble thing to rescue an abused Corvette and do your best to make right what Bubba tried to destroy, whether it's a '63 fuelie or a non-matching #'s '73 L48 Coupe. Thanks for listening.
Troy
Troy
Thread Starter
Burning Brakes
Joined: Apr 2001
Posts: 752
Likes: 1
From: Springfield Oregon
Re: '73 TH400 WOT upshift point question. (ORVette)
Well, that didn't do anything. When I pull it into low, it locks right in, no play. Wonder if maybe there's some adjustment in the linkage to make sure it's totally in first.
Drifting
Joined: Dec 2000
Posts: 1,851
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From: Liberal Kansas
Re: '73 TH400 WOT upshift point question. (ORVette)
In a stock TH350 and I believe the TH400 is the same, A "safety" feature is that the trans will shift out of low on its own even if held in low. Usually the shift is a much higher rpm than 4800rpm. Some shift kits will change or eliminate this feature. Come to think of it my 700R4 shifts on its own even when held in low gear.
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Safety Car
Joined: Aug 2000
Posts: 3,711
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From: San Antonio TX
Re: '73 TH400 WOT upshift point question. (ORVette)
Smokehouse69 is still correct. The governor overrides the shift points even if you are shifting manually. You need a full-manual valve body to overcome the governor's control. You can put it in first and leave it there, stomp the throttle, and the transmission will still go through an automatic upshift to second. The governor also controls at what point the trans will downshift. This means that you can select first gear, but if the governor thinks the drivetrain speed is too high, it will delay that downshift until it is safe. If the forced upshift is way too low, the vacuum modulator may be bad or not correct for your application. But if it is right, the only way to raise the upshift point and retain automatic features is to tune the governor. Unto itself, it's not hard, but at Mark wrote -how are you going to get to it?
C6 the C5 of tomorrow
Joined: Nov 2000
Posts: 6,665
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From: Twin Cities Minnesota
Re: '73 TH400 WOT upshift point question. (gerry72)
I had my trans guy raise or eliminate the manual shift override. I just can't remember if there is a point where it will over ride the manual shifting or not.
I do know that I can spin it up to 7000 rpm without it shifting! ( rev limiter :o )
I do know that I can spin it up to 7000 rpm without it shifting! ( rev limiter :o )
Pro
Joined: Jun 2000
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From: phoenix az
Re: '73 TH400 WOT upshift point question. (427V8)
I have two TH400, one in my 75 vette and one in a 73 camaro. The one in the camaro will shift by itself at about 5000. And it's a necksnapper for sure. The vettes 400 won't shift by itself, at least not out to a hair over 6000. I haven't gone any higher than that. The trans guy that rebuilt my vettes 400 said that you have to adjust (or do something or another) the regulator in the valve body to make it stop self-shifting. He didn't seem to think it was that hard cause he was prepared to walk me through it on the phone. I chickened out.
Race Director
Joined: Oct 2000
Posts: 15,055
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From: central FL, near the beach.
Re: '73 TH400 WOT upshift point question. (BubbaJJ)
i found it very easy to get the gov. out. i used a 3.5" hole saw under the rug on the console tunnel. took 10 minutes, and another 10 minutes to make a plate to cover the hole.
Thread Starter
Burning Brakes
Joined: Apr 2001
Posts: 752
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From: Springfield Oregon
Re: '73 TH400 WOT upshift point question. (drives61)
Where exactly is this access plate on the tranny. Can anyone explain in a nutshell what has to happen to change the governer setting or eliminate it. I'd like to at least be able to spin this thing to the old 5300 factory redline. Thanks again for all the responses.
Safety Car
Joined: Jun 2001
Posts: 4,004
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From: Severna Park Md
Re: '73 TH400 WOT upshift point question. (ORVette)
Gotta jump in again. Again I could be wrong but here go's. I still believe that if the tranny is manually held in first from a dead stop it should stay in first as long as its held there. This is assuming that the valve body piston is actually in the 1st posistion. Dont go by the shifter cause it ciuld be outa wack. The manual overide that others have mentioned, is designed to keep the tranny from being manually "downshifted" into 1st, whenever the govenor determines the vehicle speed could damage the tranny if manually downshifted way to early. I can hold my 400 in forever...as well as every other th400 and 350 ive ever owned.
Race Director
Joined: Oct 2000
Posts: 15,055
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From: central FL, near the beach.
Re: '73 TH400 WOT upshift point question. (Randy 71)
OR
is the kickdown circuit working? i don't think any vette shifts under 5000 normally. except when the circuit fails, then mine too shifts at 4400.
it's easy to recalibrate the gov: costs $28 https://www.corvetteforum.com/review...ernrecalib.htm
is the kickdown circuit working? i don't think any vette shifts under 5000 normally. except when the circuit fails, then mine too shifts at 4400.
it's easy to recalibrate the gov: costs $28 https://www.corvetteforum.com/review...ernrecalib.htm
Thread Starter
Burning Brakes
Joined: Apr 2001
Posts: 752
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From: Springfield Oregon
Re: '73 TH400 WOT upshift point question. (drives61)
The kickdown switch works. It downshifts just like it should. I guess I have to agree that every TH350,400 I've had would hold first until I shifted. Is there some way to adjust the linkage.
Safety Car
Joined: Jun 2001
Posts: 4,004
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From: Severna Park Md
Re: '73 TH400 WOT upshift point question. (ORVette)
ORVette,
If you floor the pedal from a 3rd gear 30mph roll, will it downshift into 2nd or 1st? The point im making is the modulator will also cause a downshift. Just because the modulator will cause a downshift does not mean that your gas pedal downshift switch is working. :chevy
If you floor the pedal from a 3rd gear 30mph roll, will it downshift into 2nd or 1st? The point im making is the modulator will also cause a downshift. Just because the modulator will cause a downshift does not mean that your gas pedal downshift switch is working. :chevy
Thread Starter
Burning Brakes
Joined: Apr 2001
Posts: 752
Likes: 1
From: Springfield Oregon
Re: '73 TH400 WOT upshift point question. (Jvette73)
I shoulda been more clear. From 30 it will shift into 1st. Also, when I hook a probe into the trans. lead and one to ground, I get 12V when the switch is tripped(key on, engine off). I can hear the hood door open too. One thing, I've really gotta cram the pedal to get the switch to trip. I bent the tabs and elongated the holes in the bracket to adjust it to open sooner. I could try to get it a little closer. I'll keep tinkering.
