My 02 scsb has been sitting up for 2 years because I’m afraid if I drive it some else will fall apart. Two years ago I swapped to the 4l80 behind my lq9 with heads and cam and it has vibrated since put in. I am trying to decide what route to go because I really want to race this season. The truck only vibrates from 70-75 and it shakes everything. It has broken my entire exhaust system including my headers, and Im afraid of what else will fall off. I know it's not the drive shaft, but what in the tranny can cause this, also can it be the converter? Like I say it only happens starting at 70 and stops at 75. Thanks for any Ideas. (let me also add the tranny has been rebuilt twice)

 

Wheel/tire balancing is the first thing to check. Secondly, if you can get it up on a hoist and have somebody run the drivetrain to 70-75 mph, you might see or hear the source of the vibration. If it's shaking the whole truck that badly, it should be pretty easy to find.

Good luck on the hunt!

 

When you did the swap did you keep the crossmember level the exact same as stock?

 

+1, are you sure there isn't a driveshaft angle that is off?

 

Sounds like your pinion angle is out of whack. If you're sure the driveshaft is balanced, then it's most likely going to be the pinion angle. Unbalanced wheels/tires that would cause your truck to vibrate that bad at 70-75 would have most likely vibrated at other speeds as well, and would have been noticeable by looking at the tire.

 

my cross member is a modified stock member, but it was put together using a level. There is a possibility it could have messed up but I can check that out. The only reason I think it may be something else is the only time it happens is between 70-75. I would think it would get worse as the speed gets higher if it were due to cross member angle

 

Could be this, could be that.....

Tire balance is the first thing you should check by the description you gave. Or you could check alot of other complicated things first.....

Example; If one or two of your tires is 1/2 ounce out of balance(or worse, you have a tire beginning to seperate), you won't notice the vibration until 65-70 mph, and it usually goes away at just over that speed. A completely non-balanced tire, without any wheel weights at all, will not even begin to vibrate until 40-45 mph. This is what I do for a living man, get it checked first.

 

Yea, I guess it's easy enough to check the tire balance first, but it makes no sense that swapping a trans in would knock the tire balance out of whack lol . Unless the truck sat for six months during the swap and the tires got flat spotted or something Since the trans was removed, swapped with a completely diffrent model of trans, and crossmember modified, I'd be leaning more towards something in the drivetrain such as pinion angle, tailshaft angle, or even a bad bushing in the tailshaft of the 80E.

*EDIT* but you are correct AKlowriderZ71, it is probably best to eliminate the obvious first

 

I had the same thing and it was obvious that it was the tires. If u feel it in the steering wheel then its the tires. If it was tranny, d-shaft ud feel it in SOTP. I assume u checked all the mounting brackets...etc? Motor mounts, joints...R/

 

I will start with the tires, and we will see