Exdeath

I have a 79 Vette with a TH400 transmission.

Last night I installed a 2300-2700 stall converter from Jegs (I am on a wife and new baby budget, so price was the driving force here)

http://www.jegs.com/i/JEGS/555/60400/10002/-1

When I am getting on the highway now, the car shifts from 1st to 2nd, then goes to 5-6,000 RPMs (Not at WoT) and just sits there wanting to continue to wind out.

I let off the gas and it DOES NOT SHIFT like I have seen other people experience. I can let off, hit the gas, let off and it just continues to run back up to 6,000 RPM without shifting.

I have to pop the car into neutral and then back into drive and it shifts to third and stays there.

Cruising around at lower speeds, the car shifts through its gears perfectly.

I have adjusted the vacuum modulator out about 6 turns and it does not seem to change the issue, I was hoping it would cause the car to shift at a lower RPM but it doesn't.

Any tips on how to fix this, the car was working fine before the upgrade.

So again I am good at low speeds, but does not shift into 3rd on the highway, sits at 6,000RPM and won't shift, without kicking into neutral and then back into drive.

Any help would be greatly apprecaited.

bobs77vet

thats the range of the torque converter that i have.....mine is on a 700r4 though. i dont see how this is a torque converter issue....i dont know the th400 but are you sure its not something simple like a broken vacuum line on the modulator, maybe it broke or slipped off of the vacuum fitting? are you sure the shift linkage is fully engaging third?


edit you know the more i think about this.....make sure you are getting full engagement when you put it in drive and its not hanging up in 2nd......put the car in drive and then lie on your side with heavy leather gloves on and reach up under the car and grab the shift linkage and move it forward....if it moves forward you need to adjust the linkage

Exdeath

I will try it, but I don't understand why it works and shifts into third under less than highway speeds, when I am just cruising around.

I can make it occur everytime when I take off to get on the highway, not WoT but on it pretty good, and it just keeps winding up and up and never shifts until I pop it in neutral and back down.

I will see what I find out by pushing on the lever manually.

Thanks for the input

snoopykissedlucy

Um.....am I the only one picturing you underneath the car and pushing the shift lever into drive and having the car run over you?

I'm sure you know what you're doing but please be careful.

My two cents:
Trans operates under two different modes....part throttle and full throttle.
Part throttle shifts are determined via the vacuum modulator.
Full throttle shifts are determined via the spinning governor.

I'm willing to bet there's a line pressure issue related to the governor....or you're just low on fluid.

Let us know what it turns out to be.........I'm putting in a new torque converter soon....probably the same one you have!

Project80

It really sounds like a modulator problem.

Exdeath

I know it is not a total failure or disconnect of the module, because if I disconnect it, the transmission will not even shift out of 1st, just red lines the engine.

The fluid level is perfect, I have checked that countless times.

I am ordering a new B&M red stripe modulator to try out, there doesn't seem to be any slack in the linkage so I don't think that is it.

http://www.summitracing.com/parts/BMM-20234/


Please keep the ideas coming, I am open to any suggestions.

Thanks again guys

Exdeath

Does the governor come into effect when you are not at WoT?

Because when it is not shifting and running about 6,000 RPM the car is not floored, just maybe a little over half throttle.

snoopykissedlucy

So at part throttle, you get 1st and 2nd gear but no 3rd?

Check that the vacuum rubber/steel line between the intake manifold and the vacuum modulator is not plugged or has a hole. Also, you can call the Summit racing tech line. They are great and sold me my transmission so they can help you out.....the tech line is only open 8-5 m-f.

Exdeath

We checked the vacuum on the line and it is fine, the car has 30 pounds of vacuum at idle and no fluctuations.

We found out through testing tonight that it gets worse after the car heats up.... Strange, we just can't put the pieces together yet on what is happeneing.

Any ideas why popping into neutral and back down into third would cause the car to shift? We are wondering if this issue is deeper in the tranny and just got worse or noticeable.

Would loss of hydraulic pressure in the transmission cause the problem if there was a bad seal or something?

Project80

What about the kickdown linkage, did you check to see if it's adjusted properly?

Exdeath

The linkage seemed to be fine, snaps right in gear with no problem.

I kind of doubted this was the issue in the first place because it doesn't have the problem while cruising (sub-50 mph) and mainly shows up after the car is hot and getting on the highway / cruising on the highway.

snoopykissedlucy

Just a shot in the dark................

When you installed the torque converter.........did you rotate it enough times to hear two thunks? Many rookies just rotate it once.

Exdeath

Don't remember the "thunks" specifically, but it did engage and feel firmer when it was turning.

We safety wired it back after it engaged during the install.


If the pump was not engaged I don't see how the car would work at all, much less being able to get it into the car if it was a couple inches too far forward.

Exdeath

Just to be safe I am dropping the pan, installing a new filter / O-Ring / Gasket on the pan and changing out all the fluid I can.

I think it could be a pressure problem so I want to rule out as much as I can.

bobs77vet

i guess i misunderstood , if it goes into third under some conditions and not others i dont think its the linkage. and the car doesnt have to be running to see if the linkages work.....any flat spot will work fine. hills and running engines would not be safe.....

Exdeath

My TH400 filter is just like this...

http://www.youtube.com/watch?v=OQRKU...layer_embedded

Here is an entire post on whether or not is supposed to be this way.

http://www.pirate4x4.com/forum/showthread.php?t=893584

Anyone had any experience with this?

Exdeath

So we dropped the pan, drained the fluid, and found out that the pick up tube only had a single O-ring at the top, which was supposed to have 2.

We installed the 2nd O-Ring at the top of the pick up tube, new filter, and poured in full synthetic ATF.

When entering the highway, at about 3/4 throttle I no longer have to manually switch from 3rd to neutral and back to get the car into third, I just have to let off the gas a little to get it to shift from 2nd to third.

Any other ideas now that the car just requires you to let off the gas when it is red lining to get it to shift to third? (NOT WIDE OPEN THROTTLE, about 3/4)

I am guessing it is a misbehaving vacuum modulator and have a red stripe B&M on its way.

Thanks again for all the help

Project80

Modulator problems that I have had in the pass usually had hard, delayed shifting. Between 1st, 2nd and 3rd, is the shifting smooth?

Exdeath

Keeping in mind that low speeds / cruising everything is just fine, and only the problems occur when you are taking off hard, but not necessarily full throttle.

When taking off hard it does shift hard between 1st and 2nd, then doesn't want to between 2nd and 3rd unless I just briefly let off the gas then it immediately shifts.

Exdeath

It was a bad vacuum modulator, I replaced it with the B&M red stripe and everything is working great now.

Now I just have to work on adjusting the governor which is shifting at about 4000-4200RPM at WOT, and I want it to wind out to at least 5500.

Thanks for all the help and suggestions.