Harsh shift from 1st to 2nd

[87sierra]

I have a th400 and when im driving down the road it shifts hard into second once the truck hits 35mph and going into 3rd it has to reach 45mph before it shifts can anyone help

 

[crazy4offroad]

1-2 shift accumulator spring broke, is what it sounds like to me.

EDIT: sorry misread that, not the same on a TH400. :/

 

[89Suburban]

TV cable adjustment.


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[HotRodPC]

No TV cable on a Th400. And it does also have an accumulator, it's just inside the pan. I'd have to find my Th400 flowchart to see what operates 2nd. Does it do this also when you manually shift it? Find out. That can help with diagnosis. If could even be a band servo, and it could be the intermediate roller clutch failing.

 

[87sierra]

Yes it does it when i manually shift it it shifts smoother when at higher rpms at 35mph than at low rpms at same speed

 

[Maineguy10]

I'm having a very similar problem, my 6.2 w the th400 is shifting really hard from 1-2 and 2-D. I have really good vacuum by the injection pump, but at idle I cannot feel any vacuum at the modulator, which I replaced. With both ends of the vacuum line disconnected I can blow compressed air through the line. I also turned the modulator out 4 turns ( one turn at a time then test) and this didnt help my problem either.
Any and all ideas/help would be greatly appreciated.

 

[crazy4offroad]

I have really good vacuum by the injection pump, but at idle I cannot feel any vacuum at the modulator, which I replaced.

This tells me your hard line is either plugged or crimped. It would be the first thing I would try to fix since it's the first verified thing wrong without having to get too deep into it. I replaced mine with some cheap 1/4" soft copper line.

 

[Maineguy10]

I had someone give it some throttle and I still couldn't feel any vacuum by the tranny, I'm off to replace my hard line now.

 

[crazy4offroad]

When you give it throttle the vacuum will stop. But you should definitely replace the line, find out why there's no vac at idle.

 

[HotRodPC]

Vacuum has not a thing to do with hard shifts. Vacuum will help decide shift points with the governor is all. Can the modulator firm up a shift??? Yes it can, but not by vacuum. It can firm up the shift by delaying the shift and allowing fluid pressure to be higher when it does shift therefore the higher fluid pressure is what firms up the shift.

So you're saying it's shift hard in all forward gears. But what about from P to D, or P to R. Does it bang when you put it in gear. Now Th400 does have a fix for a hard Reverse. You'd go from P to R and BAM it goes into gear. But all others were fine. IF it's just hard shifts from 1-2 and 2-3, I'd ASSUME and not swear, that the pressure regulator valve is hung up for high pressure and not coming back home.

Also, how is the shift points? Are they normal? We want to make sure it's not a stuck kickdown solenoid. If this was the case, it should be shifting a little late, maybe even a whole bunch. If it were a Th350 and the detent valve was hung up, no matter what it would shift way late and probably wouldn't shift until about ready to blow the motor. This being a Th400 has a bit different set of charachteristics though.

 

[Maineguy10]

Maybe I wrote that wrong, it's not so much shifting hard, but has to be high rpm to shift. I had the transmission out to replace the flywheel, and it has been acting this way since I put it back in, I replaced the fluid and filter as well, I have to wait till the morning to get some line to replace the vacuum line, this may not be anything to do with my problem, but at least I can make sure it's not the problem.

From P to D or P to R it's normal, not a big 'clunk' or anything. It's just 1-2 and 2-3 I have to be moving along at high rpm to make it shift. Is there a way to check or inspect the pressure regulator valve?

 

[89Suburban]

Check your TV cable adjustment, I went through the same thing with mine, cable was adjusted wrong. You got to push the button in on the side with the throttle in the idle position and let it retract. Then let the button go and go into the cab and push the pedal to the floor/WOT to reset it. It will click into place. Then if you have a bent bracket like me you can fine adjust it to where it should shift right.

 

[HotRodPC]

This is exactly why it's critical to be precise in the problem for troubleshooting. So you don't have a hard shift at all. What you have is a late shift. On a Th400, it's only 1 of 3 things. Vac Modulator, or not getting vacuum. Governor and it's possible, during the R&R of the trans, and clean fluid, you may have broke a piece of crud loose and it's hung up the governor, or the kickdown solenoid switch or wiring going to it. Did you maybe pinch the kickdown switch wire in the bell housing on reinstall? To try, just disconnect it from the trans and go for a drive. If that corrected the problem, then you need to trace that wire and fix it. If no change, hook that back up, and take the 4 bolts off the governor cover, pull the governor out, and see if the valve is falling back on the weight. To do this, pointing the plastic gear up to the sky, and hold the weights in, then pull them out and watch the valve in the cylinder if it's moving freely. Whether it is or not, get some carb cleaner and clean out that valve and cylinder best you can. Put it back and go for a drive. If that solves it, then you had junk stuck in your governor. If it doesn't, then the issue IS with the vacuum line and/or the modulator. You shouldn't need a gasket for the governor cover. Just use the same one. They are durable. If it's tears by chance, just use some RTV silicone with the torn gasket. It's not a high pressure spot. You also should not have to drain the pan either to do that. You might lose a 1/4 cup of fluid at the very most, but I'd expect just a few drops.

 

[HotRodPC]

Check your TV cable adjustment, I went through the same thing with mine, cable was adjusted wrong. You got to push the button in on the side with the throttle in the idle position and let it retract. Then let the button go and go into the cab and push the pedal to the floor/WOT to reset it. It will click into place. Then if you have a bent bracket like me you can fine adjust it to where it should shift right.

NO TV/Detent cable on a Th400. Has an electric kickdown solenoid and that's all it does is downshift.

 

[Maineguy10]

Thanks hotrod, I was hoping you would chime in, you seem to know your stuff and I appreciate the detailed explanation, I will try that!