richsanto

Hi folks,

1996 LT1 A4, 3.07 rear, 82k miles, purchased 2 months ago. Been experiencing a vibration in drive at approximately 2000 rpm (about 47-51 mph). No vibration in 2nd or OD, can't really tell if there is a vibration in 1st. Already checked U-Joints on main drive shaft and half shafts, and for any imbalance or damage to the shafts. I have a sneaking suspicion that the car may have been autocrossed or otherwise driven in competition at some point in the past, as it has VBP Sport Springs and Bilstein sport shocks, non-stock front sway bar, and some type of competition brake pads (quite bad cold, grab like crazy when hot). Otherwise, no slippage in trans. After reading up on other forum posts and internet posts on this subject, it appears there is a possibility that the vibration is coming from an imbalance in the 3-4 clutch plates that sit in the sun shell (from wear), which shows up in drive since the sun shell is rotating. Assuming that, is there any issue with just "living with it" until the eventual trans rebuild?

Thanks in advance!

Rich

arbee

Confab, you can have a go at this diagnosis!

richsanto

Update/More Info: Vibration comes in around 48 mph with the converter locked, closer to 50-51 if the converter is unlocked. Stronger with converter locked. Pretty much gone by 55 mph. A quick shift to neutral while vibrating - vibration is gone. Vibration does not appear in 4th or 2nd; can't really tell in 1st.

Thanks.

confab

Very, very unlikely it is your transmission. The input drum assembly isn't balanced from the factory and neither is the sun. TCC Slippage is unlikely because it happens with the TCC unlocked.

That is a key, btw. If you have an engine miss or performance issue, it can go unnoticed till the converter locks and the vibration comes right back to the driver, who then associates it with the TCC application and blames the transmission. People drop their cars off at transmission shops ALL THE TIME because of this. It is very common.

But, it is very unlikely you have a transmission issue causing this. 99% chance it's something else.

The fact that you can put it in Neutral and the vibration stops would seem to rule out things like tires, etc..

I'd be looking at the engine and the driveline.

Good Luck!!!!

richsanto

Thanks! Appreciate the response! Couple of questions....
1. Does the PCM tell the engine to do anything differently in Drive vs. any other gear? The vibration doesn't show up at 2000 rpm in 4th, 2nd, or 1st, nor at 50 or so mph in 2nd or 4th, just 2000 rpm/50 mph in Drive
2. I haven't dropped the converter cover yet to check for a missing weight on the flexplate or a cracked flexplate itself (since it isn't RPM specific). Is there a chance that a weight could be missing or the flexplate cracked, and the harmonic only shows up in direct drive (Drive) under load (vs. revving engine to 2000 rpm in neutral or 2000 rpm in any other gear).

Regards,

Rich

confab

50 MPH should be 4th.. Or close to it in the OD position. A vibration in 3rd (drive) only is really problematic on the 4L60, because there's nothing to vibrate. In 3rd, the whole internal assembly is locked together and the transmission runs straight through, 1 to 1. Drive does engage the coast clutch, which gives you engine braking.. But I've never seen anything that would cause the condition you describe.

Broken flexplates are possible, but they typically drag or vibrate at all speeds. You can inspect it easily enough from underneath by removing the cover and rotating the engine.

PS: on every other 96 chevrolet product, you can test the association with the TCC clutch by holding your foot on the gas, steady.. Then take your other foot and touch the brake pedal hard enough to activate the brake lights and the TCC clutch should come off. You should be able to see an RPM jump associated with this. Remove your foot from the brake and it will re-lock shortly thereafter.

I have seen bad u-joints that were tight enough they passed the "shake test" but still caused problems.

If this is your transmission, it is an unusual condition for sure. I would still look elsewhere first.

confab

PS: Under moderate throttle, it should upshift to third at (about) 30 to 35 MPH. If it is associated with third/drive gear, and you don't feel it then? I don't think it is in the rotating assembly.

It could be a weird TCC issue, but those typically get worse, not better, with speed and load.

richsanto

Thanks again, Confab! Yes, i know this is strange. I only found it by running each gear up the rpm range slowly when I got the car (obviously, checking for misses, vibrations, etc.). The U-joints pass my "prying with the screwdriver, no rust on the caps" test, but like you say, they might still be a problem.

Converter unlocks and locks up fine (actually, I had to adjust the brake pedal switch when i first got the car) - that's how i checked for the vibration unlocked vs. locked - i just feathered the brake pedal at steady speed.

The only reason i suspected the trans was an on-line article (non-Corvette related) where people were experiencing vibrations in Drive in a 4L60/700R4 that were traced to either the "Beast" sun shell or uneven wear on the clutch plates where they actually wore eccentrically and once they locked up in Drive, it unbalanced the transmission. Fact or fiction, I don't know. Only trans I've had my hands in side are Turbo 350s (a LONG time ago).

That said, I will keep plugging away at the engine and driveline.

Thanks again,

Rich

confab

Interesting.. I do the "Beast" here as an upgrade. I offer it in all the 4L60 type units, and I've never experienced that. It's actually an old part, been on the market for 15 years I know of. Never happened to me, but.. If their QC department had a hard headed day? I guess anything is possible. It is an aftermarket offering, and was never offered stock.

Never seen offset wear on clutches either, they spin in the drum when unlocked and damage like that would be REALLY unlikely, unicorn type stuff.. Probably associated with broken parts if they dug deeper.

I've done, literally, thousands of 4L60's here.. And I still see things that surprise me, so anything is possible? But, that doesn't match my experience with the unit at all.

I hope you find it. When you do, let me know what happened. I'm curious about it and it sucks because I hear things like this and I could probably tell you more with 5 minute test drive, but that's not possible over the internet!

good luck!!

confab

And third only makes it bad because you can say.. "Well, maybe it's a bad bearing?" Or a planet? Or something like that.. But none of that stuff moves in third. It just rolls straight through.

confab

PSS: I don't know about the 96.. But my 85 had a metal damper ring on the front of the transmission driveshaft yolk, and it was loose and causing me problems when I first got it. It looked like a little, teenie, harmonic balancer.

I removed it and didn't notice any ill effects from its absence.. If you have one, maybe grab that and twist it, see if it is loose.

richsanto

richsanto

Thanks for the responses.....appreciate it!

confab

Can I interject without upsetting anyone?

confab

A reverse input drum that loose would clank. It would clank on reverse engagement and it would clank on a shift to second. It would clank a LOT, and you would hear it.

It would destroy the sun shell in short order. You would see the results in the pan.

It would be easily testable from the pan area. You can see it. Just push on it..

I like everyone here.. There's no **** contest.

richsanto

Sure, Confab, please do!

In response to "ihatebarkingdogs", the vibration is only from about 47-53 or so mph in Drive only. Lower or higher, no vibration. In fact, if you leave the shifter in Overdrive, you never feel it since it is already in 4th gear around 35 to 40 mph.

richsanto

Thanks, Confab! More info - no noise from trans, very quiet, reasonably firm shifts. I changed the fluid and flushed as much as I could when i got the car. Old fluid was a bit pinkish-brown, some silvery friction material in pan (really fine powder, no chunks), a bit of "fuzzies" on the pan magnet, nothing more.

confab

Again.. Lemme know what you find, bro..

I'm intensely curious about it now.

richsanto

Will do, Confab. I'm just **** about these things, 'cause I don't want to wind it out and break something. Also, 3rd is really nice to cruise in on twisty back roads with the 3.07 rear. Thanks again!

confab

Normal.. Transmissions don't run as clean as engines.

Gears turn together. Bushings wear..

Just what you would expect.