72 Auto transmission adjust shift

[Ted K.]

Transmisson was rebuilt, works good, but stays in lower ranges too long and down shifts to a lower gear with only half pedal.
Is this adjustable? (linkage, vaccume, electronic)?
If yes how?
Thanks, Ted

[John H.]

Re: 72 Auto transmission adjust shift

Transmisson was rebuilt, works good, but stays in lower ranges too long and down shifts to a lower gear with only half pedal.
Is this adjustable? (linkage, vaccume, electronic)?
If yes how?
Thanks, Ted

Ted -

I'm not a C3 guy, but your TH-400 kickdown/downshift is controlled by a switch located above the accelerator pedal. Dunno about the upshift points.

[Edward J.]

Re: 72 Auto transmission adjust shift

Ted, Typically the transmisson rebuilders sometimes put shift kits into the transmissons, which increases the fluid pressure to have a later up shift, the downshifts will also be more sensative. These transmissons typically have early shift points and are sluggish under light throttle.

The THM400 transmissons do not have a detent cable which allow for you to taylor the shift points as the thm 350 transmisson does,It maybe a possiable the shift modulator maybe the wrong one, sometimes theres a flat screw head in the end of modulator vaccum tube that is adjustable and can sometimes help taylor the shift points, also make a check of the vaccum hose to the modulator and see if it has manifold vaccum.

[Terry M.]

Re: 72 Auto transmission adjust shift

Ted, Typically the transmisson rebuilders sometimes put shift kits into the transmissons, which increases the fluid pressure to have a later up shift, the downshifts will also be more sensative. These transmissons typically have early shift points and are sluggish under light throttle.

The THM400 transmissons do not have a detent cable which allow for you to taylor the shift points as the thm 350 transmisson does,It maybe a possiable the shift modulator maybe the wrong one, sometimes theres a flat screw head in the end of modulator vaccum tube that is adjustable and can sometimes help taylor the shift points, also make a check of the vaccum hose to the modulator and see if it has manifold vaccum.

I have never seen the adjustable modulator on a stock Corvette TH400, but those are common aftermarket items. Who knows what a rebuilder will install.

Not only does the modulator need full manifold vacuum, but there should be NO vacuum leaks in the line or fittings going to the modulator. The rubber hose at each end of the line only seals the connection. The smaller diameter of the metal pipe should go inside the nipple at each end and the rubber hose slides over the junction. I have seen so many of these installed improperly, or Bubbaed and that will always result in a functional failure.

[Edward J.]

Re: 72 Auto transmission adjust shift

Terry, you are correct I should have specified that the after market ones are adjustable. factory vaccum modulators are easy to tell apart, from the after market ones. canisters on the aftermarket are about 1/3 the size.and may have a colored strip on the modulator to help tell which one is correct for each application.

[Terry M.]

Re: 72 Auto transmission adjust shift

Transmisson was rebuilt, works good, but stays in lower ranges too long and down shifts to a lower gear with only half pedal.
Is this adjustable? (linkage, vaccume, electronic)?
If yes how?
Thanks, Ted

Ted,
For TH400 shift points are controlled (as best I remember from transmission class, and that is a long time ago) by oil pressure pushing against springs. So there is no external method of controlling shift points other than the possibility of an adjustable vacuum modulator that Ed mentioned.

If you had the rebuild done why not go back to the rebuilder, and take up the issue with them?

[Steven B.]

Re: 72 Auto transmission adjust shift

Ted,
For TH400 shift points are controlled (as best I remember from transmission class, and that is a long time ago) by oil pressure pushing against springs. So there is no external method of controlling shift points other than the possibility of an adjustable vacuum modulator that Ed mentioned.

If you had the rebuild done why not go back to the rebuilder, and take up the issue with them?

Correct, change springs in governor for lower speed shift point control under about 40 mph IIRC and weights for higher speed shift control--- they work together.

[Terry M.]

Re: 72 Auto transmission adjust shift

Correct, change springs in governor for lower speed shift point control under about 40 mph IIRC and weights for higher speed shift control--- they work together.

I was thinking some more about this as I attempted to get some parts from a Chevrolet dealer today.

The weights you are talking about would be on the governor that controls line pressure, I think. Depending on the inclination of the rebuilder they may have believed increasing the line pressure would be a good thing -- and to some extent it is -- but the rule of unintended consequences applies.

[Edward J.]

Re: 72 Auto transmission adjust shift

The problem with changing out the springs now would be a chore because the exhaust pipe bend up the rear area of the transmisson, getting the cover off with the cross member and exhaust there, would require some work.

As Steve says the rebuilder should repair this problem, I have done shift kits in the turbo 400 trans. and theres much more than a few changes. the kits I have done only boost line pressure a 10 PSI which relates to about a 4-5 mph increase between points and that would be at light throttle. the 400 thm really needs this much boost because it just shifts way to early.

[Ted K.]

Re: 72 Auto transmission adjust shift

Hard to complain too much to rebuilder since it was rebuilt over two years ago. I took it out and had it rebuilt when I had the engine rebuilt. Finally got it back together last fall and only drove it around the block. Spring is here and have been driving it a bit and noticed the shifitng problem.
Thanks for all the info I will let you know what it takes to get it right.
Ted

[Jim T.]

Re: 72 Auto transmission adjust shift

Under very light application on the gas pedal my 70's turbo 400 will shift to third about 25 mph. Knowing this, if I want more control of the driving situation, for almost 40 years now I have used my own shift control, pull it back in second before it shifts to third rather than the gas pedal application to activate the downshift.

[Ted K.]

Re: 72 Auto transmission adjust shift

My problem is just the opposite. Driving 50MPH and step a little on the gas and it downshifts to 2nd range, step a little more and it goes into 1st range which is way to low for that speed and rpm.

[Tom L.]

Re: 72 Auto transmission adjust shift

Your description of upshifts under light accelleration are the same on my '72's THM400. Also, I don't have downshifts as Ted describes, I have to push way down to make it downshift. However, the upshifts under heavy accelleration are significantly higher. I havn't mapped them out, too busy smiling. While reading this thread I was wondering what the up-shifts should be for a THM400 under heavy accelleration and would the engine option matter. Mine is an LS-5.

[Edward J.]

Re: 72 Auto transmission adjust shift

Your description of upshifts under light accelleration are the same on my '72's THM400. Also, I don't have downshifts as Ted describes, I have to push way down to make it downshift. However, the upshifts under heavy accelleration are significantly higher. I havn't mapped them out, too busy smiling. While reading this thread I was wondering what the up-shifts should be for a THM400 under heavy accelleration and would the engine option matter. Mine is an LS-5.

Lynn, I have a chart that tell the up shift points and vary only a few miles per hour between the big and small blocks. it goes 1-2 shift wide open throttle 35-50 mph, 2-3 shift wide open throttle 67-83 mph.

I would say that these are not true with cars with modifacations to the engine or transmisson, and will vary slightly with all cars.as transmissons get a few years behind them, the pressures vary, internal springs get older and weaker.

[Tom L.]

Re: 72 Auto transmission adjust shift

What you posted seems to be about how my transmission performs. The reason I asked is my current vette is the only one I've ever had. Very shortly after I purchased the car the transmission needed rebuilding. I did it using myself a B&M kit (stock, no mods) and was never sure if it worked as it should. I guess I did OK. Thanks!!