gta_knight

Hey guys,

My cars been doing this for a while and i think ive been kidding myself its not happening... so time to sort it out ! the trouble is im not totally sure what the clunk is.

Heres what happens. Whenever i slow down, usually from 30mph, not exactly harsh on the brakes, but not just a gradual stop, the transmission obviously shifts down the gears and when it gets to 2nd or 1st when im going pretty slow i get a clunk from the rear end in time with the gear change

The transmission shifts gears smoothly so i dont think its a problem with the actual gearbox. When i looked into it a few years ago i ended up replacing the the transmission mount and torque arm bush as they both looked well past it, and what do you know....it made no difference.

I was talking to a mechanic friend last night at a meet and he said my Universal joint on the driveshaft might be worn out, so when its shifting that extra movement is causing a little bit of slap ?

OrangeBird

iTrader: (1)

If it is your U joints , and your U joints are original , here's a little tip to pass on to whomever you have replace them ;

Most U joints of the pressed in variety have some sort of metal retaining clip to hold the U joint "cups" into the driveshaft and yoke . GM decided , however , that we mechanics needed just a bit more frustration in our lives and crap in our lungs so they eliminated the metal clips and instead hold the cups in with some kind of nasty glue injected into the area where the cup fits to the shaft (and transmission yoke) . Now , someone can press on those cups till their blue in the face and the cups won't move till the glue is dealt with . The way the glue is eliminated is by taking a plumber's torch and heating up the appropriate area till the glue comes squeezing out , looking like a Cheetos cheese snack and smelling like all Hell . Once the glue has been boiled out of the joint it's then a simple matter of pressing out the old and pressing in the new just as any other pressed in U joint is done , The replacements will even come with their own metal retaining clips to hold them in place , eliminating the need for any kinds of re gluing the new one .

I will enclose a couple of shots taken when I replaced mine and if you look at the cups you'll see on six of them the area where the recess is that the glue was injected into . The reason the other two don't have the whitish looking area is because they were the ones that got bolted to the rear differential's input yoke and are held in with bolts & metal straps , and have metal clips on them like the new ones will use with all the cups . Also , If you search U tube , I had found a video where a guy was showing exactly how to replace the factory glued in U joints .



Best of Luck .....

gta_knight

Thats real helpful mate thanks ! so they are glued in.....great, thanks GM ! urgh

Is there anyway to test the U joint to see if its worn, im guessing it would be a case of jacking the rear up and trying to move the propshaft to see how much play is in it ?

don_roberto_z28

my car did the same thing, it turned out to be the torque arm bushing. if you lift your car up by the center of the diff, you will be able to see the torque arm clunk against the tail shaft of the transmission "IF" the torque arm bushing is bad. I am pretty sure it will be this as my car just went through the same thing.

gta_knight

Mine was bad, so i replaced it with a poly urethane bush, but i still get that clunk ! its driving me crazy. I replaced the transmission mount too but no difference

Night rider327

iTrader: (13)

Yeah sounds like it could very well be a U joint.

Jack car up, rear wheels blocks, trans in park. Grab driveshaft at the front end and yank/push/pull/twist it. Then at the rear end of shaft. You should hear and see the clunk and freeplay

gta_knight

Cool, i will get under the car at the weekend then and have a look. Its hard to pinpoint too as i took the car out last night to have a proper listen and it usually sounds like its coming from the back of the car, then when i pulled up outside the house it sounded like it came from the center console area ! which i guess would be the front of the driveshaft where it goes into the transmission.

OrangeBird

iTrader: (1)

Don't forget , it could be either (or both) of the U joints having play , and sounds can travel and be heard in weird areas VS where they're actually being created .

Anyway , what happens is , those little cups in my earlier photos have needle bearings in them that the center piece (the cross looking thing) resides in . Those little pins need occasional greasing or they'll eventually dry out and begin wearing . The rub is that original equipment U joints don't have any provisions for a grease fitting like the aftermarket ones do ! Yeppers , the General not only glued em in , he assured their eventual death by providing no means to properly lube them ! This "Lubed for life" notion that the manufacturers try to pass off as better engineering is nothing but smoke and mirrors to keep the average dolt returning to the service department on all too frequent occasions . If ALL U joints could be regularly greased , how many do ya think the dealership would be able to gouge folks on the replacement of ? Built to fail is a wonderful way of assuring captive customers , eh ?

So , in closing , lift it up a bit , block the wheels real good , put it in neutral , and get under there and try to wiggle the drive shaft around . Your specifically looking to see if the drive shaft can be moved any little bit at all that doesn't move the yoke at the other side of the U joint a corresponding amount . If you can move the drive shaft even 1 MM without moving the yoke it's attached to it means that the pins are worn and new U joints are in order . As the wear progresses the clunk gets worse , drive line vibration is begun to be noticed , and if the pins totally turn to dust , the cups crack and the drive shaft falls out .

-=Z28=-

iTrader: (3)

Make sure to get u-joints that have zerts on them.... if you get u joints... How can anything be sealed and not require grease.... Come on!!

sofakingdom

iTrader: (1)

Usual cause of the "clunk", is overlap in the shift timing.

In the 700, it's between 3rd and 2nd... one set of clutches has to release, while the other has to apply. (clutches & band actually) Timing of the 2 events is critical: if the one lets go before the other grabs, you get the engine RPM "flare"; if the one stays engaged too long while the other also engages, the trans is basically shifting into 2 gears at the same time, which is the equivalent of "locking up". The "clunk" is the sudden application of torque in opposite directions as that occurs, and it takes up whatever slack in the drive train.

So yeah, replacing U-joints and tightening up the pinion tolerances and all such as that, is a "fix"; but getting the shift timing right, is the "cure". MUCH eeeeezier talked about than done I might add, as things like fluid exhaust orifices for the clutches and other esoteric things must often be fine-tuned.

Night rider327

iTrader: (13)

The good solid body ones are much stronger and comes prefilled with a synthetic grease, body is solid forged.

I really like the neapco brute force solid U joints. I use them on my car. 11 years now on the front U joint, 6 years on the rear U joint (changed rear end to ford 9", needed diff. joint) and that's on a 10 and low 11 second car

gta_knight

So it could be the transmission too then ? damnit. is that a case of adjusting the cable and test driving the car?

Anyway i still havnt been able to get under the car to check as ive been off work all week with a bad back, so lying under the car just isnt happening at the minute !

Ive found the U joint on rockauto no problem for the rear of the driveshaft but what is the front one called exactly, the yoke ?

Night rider327

iTrader: (13)

With the stock steel driveshaft, slip yoke, transmission, pinion yoke and rear end your front and rear U joint will be the same size, same part number

gta_knight

ok i got under the car tonight and tried moving the driveshaft, not much going on to be honest, i video'd it while i was under there. what do you guys think ?

gta_knight

No further forward with this guys.

If it is the cable that needs adjusting then ive got no idea how to do that going off Sofakingdoms reply :/

I may have to bite the bullet and do something i never do with my car.....take it to a garage and let them have a look.

sofakingdom

iTrader: (1)

Adjusting the cable won't help.

A garage won't be much help either; it is internal to the transmission, having to do with the sizes of fluid feed orifices, spring tensions, etc. Not something for your average "garage", and likely to require complete disassembly, tuning, and reassembly of the transmission, by someone very technically adept with this specific trans model, to clear up.

Most likely not worth fixing. Better to just put up with the extremely minor slightly annoying sound.

gta_knight

Well they aren't a regular garage, they specialise in American cars so they know their stuff.

It might be worth letting them have a look incase they can see something else on their ramp that I can't lying under it

Pro Built Automatics

The downshift clunk is very common and is nothing to be concerned with, just annoying. If you want to get rid of most of it, follow the TransGo Junior Shift kit instructions about grinding the "four slots" on the steel cover on the 2nd apply servo. Add the Corvette servo at the same time if you do not already have one as it will improve the 2-3 and 1-2 shift. This will usually lessen the downshift clunk, but not rid of it completely.

gta_knight

Took the car to a transmission place and the guy adjusted the cable and its much better ! shifts smoother and the clunk is nearly all gone actually so im pretty happy now

PurelyPMD

iTrader: (3)

Next time you get under the car, use jack stands, not just the jack. Just sayin....

Pro Built Automatics

It now shifts much smoother??? Yes, lessening the tension on the TV cable will make the downshift smoother. Just make sure you still can get a low speed 2-1 downshift, otherwise it might be too loose here. Check the TV cable to make sure that it is not too loose. There should be no more than a 1/16" of a inch play at WOT. Are the WOT shifts still the same feelwise and are happening at the same rpm as before the cable adjustment? If they changed noticeably, it is probably too loose, unless it was "over tight" to begin with. You want to make sure here as the wrong adjustment can take out the transmission.

gta_knight

he showed me the cable at WOT and there was a lot of play, he unhooked the cable, put it to wide open throttle and the cable couldnt even hook back on so he adjusted it so i can actually achieve WOT.

I havnt actually driven the car since taking it home from the garage as ive been busy doing other stuff so i will have really concentrate on the shifts next time im out in the car, this week hopefully

sofakingdom

iTrader: (1)

Sounds to me like 2 things are about to happen:

1. You're about to burn up your transmission; and
2. The "clunk" is unchanged, you're just in a form of mental denial formally known as "confirmation bias", wherein you adopt a "belief" (see "religion") before having any physical evidence in its favor whatsoever, and then as physical evidence appears, you selectively ignore what doesn't fit what you WANT TO "believe", and invent scenarios where whatever you ACTUALLY observe is at least somewhat compatible with what you "believe" even if it requires arbitrary contortion of the meanings of words. http://en.wikipedia.org/wiki/Confirmation_bias

You haven't "fixed" your problem. All you've done is cover it up in such a way that a few months/weeks/days on down the road, when your trans burns up, you'll be able to convince yourself that mis-adjusting your TV cable wasn't the cause. You'll of course be wrong.

Put the TV cable back where it belongs, and learn to accept the "clunk". Your days driving the car instead of looking at it sitting in your garage as you save up for your next transmission will be longer and less unhappy that way.

gta_knight

Ha ha what a reply! Just so happens that's exactly what I've been doing tonight as the car war revving in 1st gear when putting your foot down and wasn't really moving off quickly.

I've adjusted the cable myself and took it up and down the road and it takes off like it used to again, shifts fine and the clunk was minimal the three times I slowed down to check, feels good again

Stauf

iTrader: (2)

Sorry, but I have nothing to add. But I love Sofa's way of dumbing it down for the rest of us.