I've got a 75 L-82. This is a Sunday cruiser that sems to shift fine and I rarely punch it hard. But when I do ty to accelerate quickly from a dead stop, the trans stays in 1st gear way too long and I have to back off the accelerator to let it shift into 2nd. This has been an issue for many years, but now I want to finally solve this problem. This must be adjustable somehow. How can I make the trans shift from 1st to 2nd gear sooner. I think all this is controlled by vacuum --- isn't it?

Would like to get this working properly once and for all

Thanks

 

Mine does that too but maybe not as bad as yours. I believe there's an adjustment in the transducer on the tranny where the vacuum line goes.

 

hi, im not sure but the modulator must be adjustable, un plug the vacuum hose and there must be a screw if you got a ajustable modulator

 

I have a '72 auto TH400 tranny and had the same problem. Purchase a transmission modulator that is adjustable - you can get them anywhere. Adjustment allows for a smooth or hard shift at different speeds. It is an easy fix.

 

you may need to adjust the governer

 

What RPM does 1st shift at before going into 2nd and what RPM to 3rd. Thinking it's to long does not help us, help you we need the RPM

 

yes!
if it is 5000 it is normal

 

I think it is possible for the 1-2 Accumulator Spring to be broken which would cause hard shifting as well... see link below.

http://forums.corvetteforum.com/show...nsmission+leak

 

If the detent cable is adjusted too short it will cause it to hold 1st gear too long. And then shift hard

 

My car is a stock 1975 L-82 which only has 220 hp. My transmission is a stock th400. This engine's pulling power is maxed out at around 4500 rpm and I would want it to shift around 4500 rpm or a little earlier. If I keep then car floored --- i don't know were it would shift, maybe when the engine blows up. If I let my foot off the gas a little, then if shifts. Also, even if I try to shift it manually, it will still stay in 1st gear while I have the car floored even though I've moved the shift lever to 2nd gear.

Hope this is enough info.

Thanks

 

I think my stock th400 tranny works on vacuum and not a detent cable. That is, i don't think I have a dedent cable ---- of if I do, I'm not aware of it ---- where is it. Seems like the only stuff going to the tranny are the shift cable, the trans fluid lines and a vacuum line. Maybe I need to look again --- I'm not at all familiar with how this tranny decides to do things. By the way, this tranny downshifts perfectly from 3rd to 2nd when I hit the gas and I'm doing under, say 45 mph. At 50mph and above, no downshift ---- but at 50mph I'm grinding 2,500 rpm which is right were the engine seems to have it's max pulling power (max power seems to be between 2,500-4,000 rpm's with this engine and my gear ratios).

Any more ideas would be helpful

Thanks

 

I have a TH400 tranny too. Where did you buy your adjustable modulator and is it a simple swap without adaptors, mods, retubing of trans fluid lines etc.?

Never seen these devices in any ecklers or Mid America catelogs

Thanks

 

Here's how a transmission works, 3 components control shift points.

Governor, thats connected to the output shaft and regulates hydraulic pressure based on vehicle speed. This is spring operated weights, as the weights are pulled further out against the springs more oil pressure is allowed past the governor to act on the shift valves inside the tranny to signal the correct shift points.

Throttle Cable, simply monitors the position of the gas pedal through a cable that runs from the gas pedal to the throttle valve in the valve body. ( you do not have this type)

Vacuum Modulator, monitors engine vacuum by a rubber vacuum hose which is connected to the engine. Engine vacuum reacts very accurately to engine load with high vacuum when the engine is under load and diminishing down to zero vacuum when the engine is under heavy load. the modulator is attached to the outside of the tranny case and has a small shaft which passes through the case and attaches to the throttle valve body. When the engine is under load or no load, high vacuum act on the modulator which moves the throttle valve in one direction to allow the tranny to shift early and soft. As the engine load increases, vacuum is diminshed which moves the valve in the other direction causing the tranny to shift later and more firmly.

First I'd check your vacuum line, to see if there vacuum there, just pull the hose with the car running (have a helper with their foot on the brake) If you have vacuum, replace the modulator valve, if no vacuum, check the connections and hoses for leaks.Modulator valves can become dirty, so don't be afraid to remove it and clean it. your modulator valve is adjustable, but unless you know what your doing I won't explain that part.

Lets us know what you find at this point..

 

You ARE correct- a 400 is 100% vacuum and has electric detent downshift. Sorry, I'm still in the THM 350 mode. If it's shifting that late, you have a modulator problem. Either the modulator has gone to wherever they go when the die, or you have a vacuum problem. Easy to change the modulator- if you get an adjustable one, it can be tweaked a bit to raise/lower the shift points- don't go over 1 turn either way from it's "out-of-the-box" condition or you may need another new one.

 

 

Before I modded my th350 that had 50k on it, it would shift at 5000 rpm with the stock L-48 and pulled nicely till it shifted on it's own. I would be wondering why your engine stops making power at 4500.