Mechanics

VERY ODD DRIVETRAIN PROBLEM.

GMC Other

Hello people. I am new to this forum and first would like to say this looks like a very informative, helpful site.
Now to my question--1982 trans am, 305 cross-fire injection, automatic transmission, odometer says 252? Miles. Car is pretty much original. 200 metric tranny, 2.93 or? 2.92 rear-end. Car is in very good shape (body, interior, and many, many parts are either new or rebuilt.) THE PROBLEM IS--when first accelerating from a complete stop, or when accelerator is pushed enough to cause a fairly hard downshift there is a loud knocking noise (and thud on the bottom of car feeling.) What I have found is that both front and rear u-joints are apparently slamming and gouging into the floorpan, l (not sure what you would call it, basically the metal on the bottom of the car.) For this to be happening apparently the whole driveshaft is RAISING a good 3 inches to make this happen. By looking at this it seems as if the rear of the transmission and the front of the rear differential must be lifting too--otherwise I do not see how this contact would be made? I can't imagine simply bad u-joints would cause this, but I am at a complete loss when it comes to what the problem is. ANY IDEAS ARE GREATLY APPRECIATED! THANK YOU VERY MUCH FOR ANY REPLIES!
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Recaroguy
May 4, 2006.




For it to be doing that it's hard to imagine it not binding up but the first places I would look are the tranny mount, motor mounts and the leaf spring pins which should be sticking out about the middle of the set of leafs. Something has to be really loose or missing alltogether for this to be happening. You are right, this is really weird and not something I myself have run into but it seems that any or all of the above have to play a part somewhere along the line. When you find out please let us all know. I'm sure you have caught a lot of peoples interest with this one : shock: Also, it would seem that the rearend would have to rotate for the rear joint to hit, unless your springs are totally collapsed. Like you said, WEIRD!

Tiny
Burrbrian
May 4, 2006.
Hey burrbrian--thank you for your ideas. When you say something binding do you mean a u-joint? And if so do you think possibly a u-joint would cause all this havoc? To add a little more info. This car does not have leaf springs in the back. I am looking at a picture and this is what it does have--" rear stabalizer bar, lower control arms, torque arm, track bar @ track bar brace" --which all seem to be tightly bolted in place with no missing bolts or nuts, and all rubber bushings related/in contact with all these parts are in good shape (rubber is intact, not missing or rotted.) Also there is a " crossmember" thing mounted under the rear of the tranny that is tight, with no missing bolts, and rubber bushing/mount is in good shape. Also I now have found a weight mounted about 3 inches towards the front of the car on the driveshaft just ahead of the rear u-joint that apparently is also slamming into the underbody while spinning. Just to make sure I'm not losing my mind I chalked the ends of the u-joints and the driveshaft weight and took the car for a little spin. And. You guessed it chalk marks slammed/ground in to the under body in all 3 spots. I have yet to check the motor mounts as burrbrian mentioned. Also rear springs are fine--not broke or collapsed, and rear shocks seem to be doing there job. I STILL NEED HELP! By the way, when AND IF this problem gets resolved I will definitely let you know what was found. But at this point I'm not holding my breath. : (

Tiny
Recaroguy
May 4, 2006.
When you chalked everything did you check the tranny mount and see if the bolts were there and tight? It seems like when you give it gas it has to be raising up at tail end of tranny, lifting the the driveshaft up to meet underbody and making joints hit. How much clearance is there with the car just sitting? My money would be in this area as I can't see anything else that would let it raise up like that. The weight you mentioned would just be for driveshaft balance. Crossmember thing sounds like a sway bar.

Tiny
Burrbrian
May 6, 2006.
Burrbrian--you put your money on the right thing--it WAS the transmission mount! YOU DA MAN! Well, a combination of the tranny mount being bad AND the last guy that installed it screwing up (the last guy is yours truly!) I had replaced that a long time ago (car doesn't get driven too much.) But, when I had done this I had stripped the nut and simply double-nutted it without realizing that the rear of the tranny was nowhere close to where it SHOULD have been ( it was raised up a couple of inches higher than it should have been, making the nut SEEM like it was tight, but in fact was not close.) After installing the new mount and tightening it the rear of the tranny pulled down a couple of inches making a tight fit between the mount and the crossmember (?Sway bar?) After a test drive (hard test drive : lol: ) all problems were solved ( no clunking, no banging---no parts grinding their way into the underbody of the car!) There apparently is a LOT of torque on these parts, cause as I said before--the rear u-joint was hitting too--which I believe means the whole rear-end system was lifting/twisting under a load.
Anyway, I would not have removed and checked this mount (as it LOOKED and FELT tight)---if it hadn't been for your replies.I THANK YOU VERY MUCHas stated earlier----YOU ARE THE MANTHANKS AGAIN----recaroguy.
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Tiny
Recaroguy
May 6, 2006.
How about that? I got something right! Well, accidents happen. Really, just glad I could help and hope everything is working good now.I think I would keep an eye on the joints for a while to make sure that they didn't get a little messed up by all this.
But it sounds like you are road ready now. Happy motoring! : Twisted:

Tiny
Burrbrian
May 7, 2006.
Burrbrian--yes, the motoring is going very well! : Twisted: : twisted: Thanks again and I WILL keep an eye on the u-joints--there was some extreme twisting going on under there, so I'm sure all those parts were giving it there best to not explode into tiny pieces--I really appreciate all your help and this extremely helpful website. : ) Take care, recaroguy.

Tiny
Recaroguy
May 7, 2006.

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