K heres what I've got: 1984 Berlinetta, 305 LG4+700r4, CCQuadrajet. 193,000 miles, nearly everything has been replaced mechanically. Timing bumped about 8 degrees. Suspension is nearly all new and tight. Damn near perfect alignment
Alright heres my problem. When taking off the line, or when I stomp on it from a slower speed, I get a big vibration under the console/shifter area. I had read here that this indicates bad u-joints, so I replaced those(still had the stock ones). And I'm still having the same problem. The only other things I think it could be are the tranny mount, motor mounts, or a bad first gear...HELP!
Thanks
Kengo

 

possibly loose torque converter bolts or maybe cracked flexplate. is it a vibration that changes with just RPM or speed?

 

Trans mount....

Your trans is coming up off the crossmember and your drive shaft is rubbing on the tunnel.

 

So trans mount, to swap a new one in can i just use a floor jack? Or will i need a trans jack?
My flywheel is missing a few teeth, but otherwise i dont get any funky vibrations at other speeds (like highway). I doubt the TC would be loose, I havent touched it, and shifts are smooth.

 

with the trans mount broke you will get one hell of a bang when the tailshaft/yolk hits the tunnel not a vibration!

 

I vote front motor mounts or front torque arm mount.
Front mounts are a major pain to change, and thus never get changed.
If you don't plan to ever go major HP, just use new stock ones instead of poly.
Poly is however essential for handling the torque roll-up from a hi-perf application with the low 1st gear ratio of the 700R4.

 

I have to agree with 84, I just replaced the trans mount in my V6. You would think the tranny is coming through the floor if it went bad. You still could have a U Joint problem if one of the needle bearings came out or the littlest bit of grit got into them. Doubt a bad first gear you would get more of a grind and intermittent shift problems if it was going out. Drive shaft could be out of balance or have a dent..the littlest of dents will throw them out of balance, and it could deffinately be the torque arm bushing I'd inspect it well also. If none of that works check your rear tires and rims could be something there. Vibrations can be a big pain..good luck, let us know what worked.

 

Thanks for all the input. So I was looking around underneath, working on something else, and I noticed scratches on the tunnel right above the yolk u joint...so the driveshaft is definitly jumping up there. I plan on doing the old 350 swap, so I'll put in new motor mounts when I do that, but could just a bad mount for the torque arm/trans cause that too?

 

The torque arm mount bolts to the tranny. If the tranny taishaft goes up, so does the torque arm.

Tranny mounts are EASY to diagnose. Slide under there and push up on the tranny tailshaft with both hands. It's heavy but not that heavy. If the mount is broken you'll see the rubber part of the mount separate like opening up a sandwich. It'll be ripped clean in half. When you let the tranny back down the mount will look just fine again as both halves will seat back together again perfectly.

I've replaced MANY of them on 3rd gen cars over the years, as have MANY others on this board. Best idea for a replacement- Energy Suspension poly mount. It bolts in and works almost like a stock rubber mount (no real vibrations or other nastyness) but it will NEVER rip in half.

After you check the trans mount then check the torque arm mount. Likewise, it's pretty easy to tell if it's shot.

 

The trans mount is like $18 or something, and takes about 5 minutes to change out.

Just do it. Don't use the "I'm waiting till I put in the 350" excuse. That's just lame, especially for a non-motor part. Like, yeah my car needs brakes, I'm waiting till I put the ("the" ) 350 in. Yeah right. No point in tearing stuff up while you're saving up for the next year or 2 for your next motor.

The torque arm bushing is one thing, and the trans mount is another. You need a trans mount. The torque arm bushing won't do that; it will mostly make it make loud banging noises all the time. Not that your torque arm bushing isn't bad or wouldn't benefit from a new one, only that it won't cause the problem you're complaining about.