Wah! Off the line shudder with new trans

[davepl]

Well, the 4L80E is in and everything is jiggy... almost.

When you take off from a stop, if you use about 1/4 throttle at least, once you roll out maybe a truck length a "shudder" develops that lasts through about half of first gear.

The only way I can get it to "sustain" that effect is up a hill. Like up a 15% grade or so it'll shudder the whole way if you find that combination of load and speed.

It's like the driveshaft is winding up and releasing almost, or (more likely but I hope not) something amiss in the transmission itself.

The posi is very tight, and around corners you can feel it, but this is in a straight line.

On the highway, around town, no other symptoms of it. Fine at speed, no vibrations or noise of any kind.

I -think- I tried a second gear start (the TCU allows for that) and that it was still there, but I don't remember for certain at the moment, will re-check that on Monday.

Anyone know enough about transmissions to make a guess? I'd like to have a few theories for the tech to chase on Monday. He did the install and now he's going to chase this issue...

[geezer#99]

Is the new tranny longer than the original?
Have you checked your pinion angle?

[hugger6933]

Dave, this sounds crazy but have you put it in low gear at the shifter and tried it? Just to make sure you are starting in first gear is what I'm getting at. You said you did a second gear start could you feel the gearing difference between then and first? Does the TCM throw a code? It seems as it should if something is wrong there. Seems like I remember you not being happy with the plug at the case did it get changed out?

[davepl]

The trans is 2-3 inches longer than original, but we shortened the front driveshaft and the carrier bearing position is unchanged, so really, the back part of the driveshaft is unchanged as well. The pinion angle, since it's set by the location of the carrier bearing and the "nub" on the axle that's non-adjustable, can't easily be changed (or screwed up).

There is a noticeable difference between starting in first or second, so I know I'm getting first.

Changed out the plug on the case, no codes that I'm aware of.

Wish I was more certain it persisted in 2nd. I was chasing another issue at the time that turned out to be the tight posi, so I've sort of forgotten which test was done when.

I -hope- it's not a problem with the 1-2 shift application.

[The Chainsaw]

I think I might have asked this already, but what controller are you using?

[davepl]

I'm using the TCI-TCU controller. That's why I can do a second gear start (or so the documentation makes it sounds like that's a feature they added).

[The Chainsaw]

Have you done any experimenting with the line pressure option? Probably won't change anything, but it might narrow down a few options.

[Shelby0099]

One of the first items I would think of for shuddering under load, 1st and 2nd gear is the torque converter.

[indymachinist]

Originally Posted by Shelby0099

One of the first items I would think of for shuddering under load, 1st and 2nd gear is the torque converter.

Agreed. First thing I thought of. Can you manually lock/unlock the converter with your transmission controller?

[davepl]

I haven't changed the line pressure, though I don't recall a setting for that - only shift aggressiveness and shift firmness. Do you mean shift firmness?

Short of R&R and swapping the converter, is there a way to diagnose it as the converter or rule it out somehow?

[kwmech]

Did the complete drive shaft get re-balanced when you shortened the front section?

[Acorn]

Are you 100% sure you got your spark plug wire situation figured out? Did you look at the spark plugs themselves? More often than not, engine problems are misdiagnosed as tranny problems.
My second guess (key word: guess) would be torque converter shudder. I'm not good at giving step by step instructions but Google search torque converter shudder.

[davepl]

100% sure its not the engine. Honest. Trust me. I can get that load/rpm window once moving without a problem.

We didn't rebalance the shaft because its 20" long or so, they said no point. And its not a vibration, it's WAAAY more than that would be. It's physical, like a rub, not a shake, if that makes any sense.

The only thing I can think of that's not internal to the trans would be that we shortened the driveshaft, but what if we didn't quite shorten it enough? There's like 1/8" of free end play, maybe less. I'm wondering if maybe we need to move the carrier bearing back a 1/4" to give the front shaft more room.

Is there any way the rear end could cause this? It only happens under some load, but not enough that I'd think it was spinning a tire and catching it. I have a helical locker. It's pretty aggressive around corners, so I wonder. I went and did a bunch of circles in each direction to loosen it up, but it was OK that way.

On the Ford forums guys are talking about spring wrap causing an acceleration shudder, but I don't recall one before in this truck, though I didn't drive it much before.

And to complement Acron's sig:
"The quality remains long after the price is forgotten. Thought I do have a long memory."

[Acorn]

Are you saying you only have 1/8" of slip yoke travel? That's seems short. Ideally it should be like 1" sticking out. How long is your yoke and how far into the trans is it? Just throwin out ideas....
My sig quote was on the wall of a supply house I used to use. But I prefer my dads redneck version when your knee deep into a project, "now ain't the time to be cheap."

[davepl]

Yes, the front driveshaft only can move back and forth a tiny bit. The rear, since the suspension has to travel, has lots of room to slip on its yoke, but the front doesn't.

It shouldn't move around anyway, but I wondered if the whole truck "bowed" a bit under acceleration if that causes it to bind up. That'd be ideal because I could fix that!

[Livrat]

After shortening is it possible when the drive shaft was reassembled the tech got the center slip yoke off a tooth or two making the U-joints out of phase. that will cause a shudder as the u-joints will work against each other and bind.

[Joe Pass]

Have you tried the friction modifier for torque converter shudder? I use it in my overdrives and its cured a shudder in my 4L80...http://www.summitracing.com/parts/lbg-19680/overview/
The GM dealer also sells their brand

[The Chainsaw]

Does the torque converter act normal when locking up?

[davepl]

I just don't think the converter should even be trying to lock up at 5 mph, would it?

Livrat, I'm going to open a new thread about driveshaft phasing, as that's new to me!

[geunther]

When you shortened the front portion of the two piece drive line, you likely changed the angle of the u-joint at the transmission output.

If the carrier bearing is in the same location, then the angles of the u-joint at the transmission output and the carrier bearing are likely different than each other. My assumption (if you did not change any angles) is the angle at the transmission is now greater than it was. This can cause a shudder if this angle is different than the corresponding angle at the carrier.

The angles of the transmission output and the carrier need to be the same (essentially offsetting each other). and the angles between the carrier and pinion should be the same. Optimally, the pinion can stand to be about 2 degrees different (down), so as the torque twists the rear axle upward, the angles comes into sync with the carrier joint angle.

[The Chainsaw]

No it won't be locking up that early, but I have had lockup converters do what you describe which was somehow related to the clutches according to my transmission guy.

[davepl]

I think the angles have stayed the same - it's pretty much a perfect straight shot back out of the trans to the carrier bearing.

My concern is rotational alignment - is the front shaft keyed so that they could have only put it in one way, and would that matter if it was rotated 60 or 90 or 150 degrees from that?

[geunther]

Yes, that would matter. 180 degrees out would not matter, but anything different would. The cross of the u-joints need to be in line. That way as one speeds up, the other is slowing at the same rate. it is when these rates are different that causes a vibration. Initial shutter tends to be something different however, because the speed of the driveline is slow enough that out of time driveshaft won't be enough to cause a notable affect.

Stupid question. Have you checked the tightness of the axle mount (u bolts). I have had shudder when they have been loose. Initiation torque is greater and can cause the axle to rotate within the mounting. Just something to check.

Also, did you change anything about the pinion nut, such as new seal installed? Retightened? anything like that?

[davepl]

Thanks, good point on the axle bolts! You'd imagine I tightened them up but in a 3-year restoration anything's possible, I'll check!

[davepl]

All the local trans shops and the local reputable driveline shop say to get the driveshaft balanced as a unit, so I'll do that today.

I kinda think its a waste of time, as imbalance wouldn't be load dependent, but it'll rule that out.

Still wish there was some way I could rule out the posi, but I guess the fact that it happens in a perfectly straight line rules it out mostly.

Apparently they can duplicate it in the shop up on jack stands by applying the brake and throttle, loading it up a bit. The whole back of the trans jump up and down I'm told.