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Okay, ever since I swapped over to a new aluminum DS and FRPP rear end I've had bad driveline vibration starting around 65-70 mph. The vibration isn't completely continuous, either, as it will come and go from time to time, but it's usually lpresent. I've torqued the bolts on the pumpkin and used urethane bushings on it. The really weird thing is that a lot of the vibration is transmitted through the steering wheel. I can't for the life of me figure out how driveline vibration could make the steering wheel shake up and down, but that's the case. I can't seem to solve this problem, so please tell me if you have any input. Here's my exact setup:

New, not used aluminum dynotech driveshaft
JustinH's FRPP pumpkin w/3.73 gears
MN12performance urethane diff bushings
Stock rubber rear diff mount, but it seems in fair condition

And here are my thoughts on what could be wrong:
Out of balance DS (seems unlikely, balancing weights are there and the thing is brand new)
Torque converter not liking the 3.73 gears
Movement in the pumpkin (seems unlikely since it's torqued to specs front and rear with brand new bushings)
Bad tranny/motor mounts causing the whole engine/trans to move (seems unlikely because the problem only popped up after the rear end/DS swap)

This thing is driving me crazy. Please help me if you can.
 

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what makes you think its the driveline and not something like hub runout or rotor runout in the front end. Rotor runout will cause a vibration even when you are not on the brakes.

It sounds like you have everything in the driveline covered, might want to look elsewhere.

Tire balace is good i'm sure? Wheel torque? Search vibration, the mn12 chassis is very suspect to bad vibes. I am chasing down one in my car that I will have to work on next season.

My 95 had bad vibes which turned out to be loose bolts on the subframe.


JH
 

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JustinH said:
My 95 had bad vibes which turned out to be loose bolts on the subframe.
:eek: Good thing you found those!

If your steering wheel is shaking, chances are pretty good that it's some sort of front-end suspension or steering problem. I've never heard of a driveline vibration being transmitted through the steering column like that. The first thing I'd check is the brake rotors and tire/wheel balance.
 

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Either front end issues, or a tire out of round/thrown belt inside/out of balance. Had this issue on my 95, and it had a high-spot in the tire tread itself, so it kinda made a skipping noise on the highway, kinda like the sound big 4x4 tires make. Hard to explain I guess, but that's what it sounded like to me.

Jay
 

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Here's a story to make you "think".

buddy takes his car in for a new rear end/gears/posi, etc

drives home, and has funny front end vibration.

comes to find out the shop doing the work took the car for a test drive. They got a flat tires. When they went to get his spare out, they found his can of fix-a-flat, and used it. The mechanic then stopped at the part's store and bought another can to replace his.

They never told him until later when he bought new tires. The tech at the tire shop used to work at the driveline shop, and came clean. (fix-a-flat "pools" in a different spot everytime, like water in a bucket being slung upside down.-causes vibes)

wont fix your car...but...you just never know. Have you checked with the shop that did the driveline work and ask them if they had to mess with motor mounts, tranny mounts, etc?

strange things happen to cars in shops.
 

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Discussion Starter #6
I did the work myself, so I know that no one got any flat tires or anything like that. I have obviously considered that the steering wheel vibration could be exclusive to the front end, but it would be an incredible coincidence, since I never had that problem at all before I swapped the pumpkin and DS. That's why I'm convinced there's a link, because the problem showed up immediately after that swap and with no other changes. The only real difference I've made is increased RPM between the engine and the differential. The tires, wheels and brakes themselves should see no additional forces that would cause problems, at least not that I can think of. And it's not a side to side vibration that you'd expect with an out of balance tire--it feels as if the entire steering column is shaking up and down, and I also feel vibration feels as if it's directly beneath the driver's seat. This one has me stumped, please continue with your suggestions.
 

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Have somone stick their head out the door and take a look while it's vibrating, Maybe they can see something out of whack :D

Jay
 

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Check the K-member bolts. The steering column on a Mark VIII actually passes right next to the left exhaust manifold. if anything has moved, perhaps it is now touching SOMETHING in the steering linkage.

Your exhaust, something has to be touching the front end.

motor mount, trans mount, something.

The only other thing I can recommend is to try rotating your tires. front to rear, if only for a day.

typically, if a vibration is in the steering-wheel, it is from the seat-forward if it is in the seat, it is from the seat-back

so, draw an imaginary line(cut your car in half), and check everything from the seat to the front of the motor. it doesnt take much...
 

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Discussion Starter #9
Thanks, I'll see if I can figure something out that way. I do have a new trans mount and crossmember bushings awaiting installation, so we'll see if that helps any, too.
 

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Possibly a couple of things..
Bad setup in the FRPP diff-mine had it,and I've got the bearing races to prove it.
Bad wheel bearings...
JL
 

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Discussion Starter #11
Thanks JL, I was hoping you'd have a thought on this one. It is possible the diff isn't set up correctly. That's something I didn't check, and I let one aftermarket company test fit one of their differentials in the housing, so I can only hope they installed the TL diff correctly. The rear wheel bearings are roughly 30,000 miles old, what's the life expectancy on them?
 

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mercutio said:
Thanks JL, I was hoping you'd have a thought on this one. It is possible the diff isn't set up correctly. That's something I didn't check, and I let one aftermarket company test fit one of their differentials in the housing, so I can only hope they installed the TL diff correctly. The rear wheel bearings are roughly 30,000 miles old, what's the life expectancy on them?
The rear bearings..I've seen them go for 220K miles on my car(really due for a change now),and others have them fail with 20-30K miles on them.
JL
 

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I'll bet you're still running 15" rims with some combo. 17" rims with Z rated tires and ecentric rings are the way to go if you want smooth highway rides with the driveshaft/ rearend combo's. Take two in the morning and call me about noon. Ed.
 

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Hollywood Ed said:
I'll bet you're still running 15" rims with some combo. 17" rims with Z rated tires and ecentric rings are the way to go if you want smooth highway rides with the driveshaft/ rearend combo's. Take two in the morning and call me about noon. Ed.
Aftermarket wheels and hubcentric rings are a piss-poor idea to cure vibration.
I never had a problem with 3.73's/dynotech shaft,and the stock 15" wheels on my car-that's absurd.
JL
 

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Discussion Starter #15
I'm with JL on the wheel issue, but I am running 17x9 wheels with 275/40s in back and 255/45s in front. Other specs, just in case anyone is wondering:

rear disc swap
front UCA's replaced under a year ago
front upper shock mounts replaced under two months ago
originally used Steeda diff bushings but replaced them with MN12perf. bushings after the vibration destroyed one of them. Will post pics of that whole situation. Basically, if you look at the way the Steeda bushings work, the middle bushing tried to suck its way out past the lower bushing. I assumed that CAUSED the vibration, but apparently not, since those parts are gone but the vibration is still there.
 

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I'm chasing a vibration too................keep us updated. I haven't made anyprogress with mine............:mad:
 

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I have the same symptoms. Ever since I installed the FRPP pumpkin, there has been vibration at highway speeds. I haven't even begun to try to identify the problem. But from what was said here, it looks like I paid $600 for a diff that wasn't set right?

There is another problem on my 'Bird...whenever I reverse, there is a noise about every 1/4 revolution of the wheel. I wonder if it's a symptom of the same problem.

When I get ready, I'm gonna have the diff fluid checked/changed and finally get that custom driveshaft. I hope that will fix the problem, but from reading this topic, it doesn't sound as promising.
 

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i have a frpp diff too, checked it before installation, well within spec. I also checked the driveline while everything was spinning up to 80+ mph, no vibs there........
 

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Put the car in the air ( Carefully !! ) and gently take it up to 60mph that will
rule out the front wheels then take the rears off and install the bolts with
some spacers to keep the rear rotors snug. If it still does it then it's the drive train.

Do you have mags ? If so do you have wheel centering rings ?

They do help, or they did for me.
 

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In addition, I think that you need to raise the wheels up too so that the CV joints don't fail too quickly.
 
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