1991 Mustang coupe thinking the driveshaft is bad?
my 91 coupe has a pretty good vibration and is definitely being made by the drivetrain. replaced almost all the front end, and tires. I haven't had a chance to get under the car and see if there is play in the universal, but around 65 mph the vibration becomes noticeable and the faster i go the worse it gets. Under 65 and its pretty quiet. everything vibrates though the shifter handle feels like its a "females toy ". I just put a new clutch kit and pilot bearing in too thinking that was it cause the pilot bearing was toast, but that wasn't it. I really dont feel like messing with the rear or the tranny again so i hope its just a universal, it will give me a reason to get an aluminum driveshaft
I have a very similar problem, though I have an AOD trans. I'm hoping a new harmonic balancer will cure it (or some of it, anyway). I replaced the rear and universal joints, and it's still there.
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Stock 1992 Vermillion Red Mustang LX 5.0 Hatchback wAOD and new 3:73 gears
Pull the drive shaft off, Its really easy and send it to a drive line shop. They put new joints and balanced it for like $80. It probally needs to be done anyway.
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91 GT convert. GT40P's, Off Road H-PIPE, Flowmasters, 4.10s, posi rear, welded subframes, 70mm TB, TFS Stage 1 cam, 155lph holley pump, Cobra intake mani, ford racing headers, Allum. Radiator, No smog, Freashly rebuilt......
If you have 3:73's and the stock driveshaft it will vibrate right up until the moment you swap it out for an aluminum one. Depending on their material and construction, driveshafts have a critical speed, and like any object, various harmonics which they will resonate at. This means that any driveshaft will have a certain speed at which it resonates naturally. For aluminum this may be much higher than steel, however. What you guys are experiencing is most likely whats referred to as the critical speed of the driveshaft. This is a point at which the driveshaft reaches a certain rpm, and begins vibrating. From that point on, it will only get worse until it actually fails. Something you really dont want with a steel driveshaft.
Try an aluminum driveshaft. Its like $200 and it will almost certainly solve your problem. I tried everything from balancing tires, to changing the position of the driveshaft over and over again. Just buy aluminum.
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Cardinal Red '88 GT 5 speed, Cobra Upper/Lower, Dynomax complete Super-Turbo cat-back system, Turbo Coupe Rear 10" discs, 3:73 gears, SN95 Master Cylinder, Summit Racing adjustable proportioning valve, Spec Stage II kevlar clutch, steeda adjustable cable/quadrant, 73mm front calipers, P.A. Performance 130 amp alternator, 99 Contour Electric Dual Fans, Battery relocated, rebuilt 91 T-5, Mass Air conversion
If you have 3:73's and the stock driveshaft it will vibrate right up until the moment you swap it out for an aluminum one. ....
What? That is simply not true. I had 3.73's in my '90 LX with a stock driveshaft and it was smooth as silk. Never once did I have a vibration.
By the way....changing a driveshaft over to an aluminum one when you have a vibration does not fix or correct the vibration issue(unless the vibration is the result of a bad driveshaft, in which case, changing it out for another steel one would correct the issue aas well), it simply masks it as aluminum is better for absorbing the vibration vs. steel. Bottom line, if you have a driveline vibration and you switch to a aluminum D/S, guess what..... you still have a driveline vibration, you just can't feel it as much. If it was going to fail as you say, then it is going to fail anyway whether or not you have steel or aluminum.
You left out a couple of key components with regards to explaining critical speed.
Critical speed is based on tube length, tube diameter and rpm. In other words, you don't want a driveshaft that is 72" long, 3" tube diameter, turning 8000rpm's but in the same token, a driveshaft that is only 51" long, 3" tube won't reach critical speed until 115mph in 4th gear with a 3.73 rear end.
To the OP.....Was the vibration present before you changed clutch?
Have you checked the rubber isolator mount on the cross member? Are you certain you got the marks on the driveshaft lined up with the mating marks on the pinion yoke flange when you re-installed the driveshaft (after changing the clutch)?
What happens if you push the clutch in during the vibration, does it go away or get worse?
Richard
Tech Support
Tremec TKO, T45 & T56 Transmission Systems
Obamacare my a$$ We are all going to pay for electing this tool.
Remember, it is almost impossible for anyone, myself included, to diagnose a problem over the internet without seeing the car. My advice is simply that, advice based on years of experience. I don't want you or anyone taking my word as gospel. I am trying to give some direction as to what I think may be happening.
First off, saying Aluminum absorbs vibration better than steel, and that that is why it goes away is completely inaccurate. The dampening properties of Aluminum vs. Steel is not that large. Were not talking about urethane here. Aluminum will vibrate less harshly given all of the same circumstances because it is less dense, and therefore will have a lower moment of inertia. Just like a composite shaft will have a lower moment of inertia than aluminum.
He cant change the length of his drive shaft, its set, so im unsure as to why we would even mention it.
An aluminum driveshaft has fixed hundreds of peoples vibration problems on this board, including mine.
However i will agree with this: If you have any bad bushings ie: motor, trans, trans x member bushings this problem can become suddenly noticeable. For instance when i changed to urethane trans bushings i created a horrendous vibration that increased with speed of the engine. Turned out I had a bad motor mount that I couldnt detect at all with the rubber x member mounts in.
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Cardinal Red '88 GT 5 speed, Cobra Upper/Lower, Dynomax complete Super-Turbo cat-back system, Turbo Coupe Rear 10" discs, 3:73 gears, SN95 Master Cylinder, Summit Racing adjustable proportioning valve, Spec Stage II kevlar clutch, steeda adjustable cable/quadrant, 73mm front calipers, P.A. Performance 130 amp alternator, 99 Contour Electric Dual Fans, Battery relocated, rebuilt 91 T-5, Mass Air conversion
vibration was there before the clutch, and as long as I'm going above 65 or so mph then the vibrations present whether its clutched in or not, feels like its generated from the rear of the car, thats why i'm going with a bad universal, tomorrow i'm going to crawl under the car and check it out. I have done HUNDREDS of clutches, it was slipping thats why i changed it, rubber mount is brand new also
First off, saying Aluminum absorbs vibration better than steel, and that that is why it goes away is completely inaccurate. The dampening properties of Aluminum vs. Steel is not that large. Were not talking about urethane here. Aluminum will vibrate less harshly given all of the same circumstances because it is less dense, and therefore will have a lower moment of inertia. Just like a composite shaft will have a lower moment of inertia than aluminum.
He cant change the length of his drive shaft, its set, so im unsure as to why we would even mention it.
An aluminum driveshaft has fixed hundreds of peoples vibration problems on this board, including mine.
However i will agree with this: If you have any bad bushings ie: motor, trans, trans x member bushings this problem can become suddenly noticeable. For instance when i changed to urethane trans bushings i created a horrendous vibration that increased with speed of the engine. Turned out I had a bad motor mount that I couldnt detect at all with the rubber x member mounts in.
I am not the one who said, and I quote..." If you have 3:73's and the stock driveshaft it will vibrate right up until the moment you swap it out for an aluminum one."
I did say that aluminum DOES absorb a vibration better than steel AS IT IS SOFTER.
It does not "fix" a vibration that is caused by something other than the driveshaft, it is impossible. It will mask it but it won't fix it. Now if the problem is in the driveshaft, then yes, switching to another steel or aluminum driveshaft will replace the defective driveshaft but that is it.
OP.....
1. Be sure to check that there are weights present on the tubing. Usually one weight on each end (make sure one didn't come off and throw the driveshaft out of balance).
2. Did you check (or make sure you check) that the line up marks are present, and in line to each other, on the driveshaft and the mating flange on the pinion.
3. Definitely a bad u-joint can cause this kind of vibration, let us know what you find.
Richard
Tech Support
Tremec TKO, T45 & T56 Transmission Systems
Obamacare my a$$ We are all going to pay for electing this tool.
Remember, it is almost impossible for anyone, myself included, to diagnose a problem over the internet without seeing the car. My advice is simply that, advice based on years of experience. I don't want you or anyone taking my word as gospel. I am trying to give some direction as to what I think may be happening.
I HAVE 4-10 GEARS.I just changed my tranny mount now i do have vibration.could i have put my shaft on wrong i did not mark it before i took it off.I looked at joints(and smoked a couple)they looked good .well i will change them and try spinning shaft to another bolt pattern on rear yoke.at least i will have starting point so i can try it at different possitions if it still vibrates.
1991 Mustang coupe thinking the driveshaft is bad? 
". I just put a new clutch kit and pilot bearing in too thinking that was it cause the pilot bearing was toast, but that wasn't it. I really dont feel like messing with the rear or the tranny again so i hope its just a universal, it will give me a reason to get an aluminum driveshaft
What? That is simply not true. I had 3.73's in my '90 LX with a stock driveshaft and it was smooth as silk. Never once did I have a vibration.
