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My rear end vibrates.

RagTop

RagTop

Old Grumpy
Lucky me, huh? Well, actually my Mustang has a monster vibration coming from the rear of the car at speeds of 70+ mph. Way back when dinosaurs roamed the earth I swapped the stock 2.79 8" open pumpkin for a set of 3.55s given to me by a kid at a car show. He just said "Take em". I took them to Mustangs Only that used to be on Old Oakland Road in San Jose, and Chuck sent them to his guy to rebuild them. Since that time there has been a suspicious vibration coming from the back of the car and I have succeeded in ignoring it for lotsa years. We even drove it to Birmingham and back, for the 45th Mustang Anniversary, with the vibration. I finally decided to do something about it and took it to a local guy here in Lincoln. He said the right rear axle was bent and the U-joints needed replacing. I said the problem showed up right after I installed the rear chunk and the axle was the same one that was with the car when I bought it. Anyway, after a week he contacts me to tell me the car is all fixed. I take it out for a test drive and the vibration is still there. He comes with me and drives the car at 70+ mph and agrees it aint fixed. The first thing that crossed my mind when the vibration showed up was that I had installed the drive shaft 180* out and it was imbalanced. Then I convinced myself that the drive shaft is balanced as a unit and that wasn't the problem. That allowed me to avoid getting under the car and actually doing anything about it. The drive shaft seems to be the mechanic's best guess now. I'm suspicious of the 3.55s. Do any of you Mustang rear end wizards have any wisdom to impart on possible causes of my rear seat vibrator chairs?
 
I'm just wondering if it could be something internal to the 3.55 chunk that I installed in the car. I had it rebuilt, but who knows? Maybe that's why the kid just gave it to me. The noise appeared right after I installed that rear end. Does anyone know if the drive lines are balanced as a unit or can they be thrown out of balance by being installed 180* out?
 
Something as small as the gears in the axle shouldn't cause it to vibrate like that. Usually bad U-joints are really noticed when you start and stop, or let off the gas... Since it is a speed dependent issue id tend to lean towards the transmission or the drive line (angle of transmission to rear axle) being off. Possibly the wheels not being balanced, maybe a weight fell off when they installed your new gears?
Is it a clunking vibration or a soft humming?
Since the outputs of the gears in the rear end are encased and don't really have alot of mass to them i doubt that its the gear assembly that's causing the issue. And if it was a manufacturing defect where the gears weren't machined correctly, then it should of been very noticeable when it was being bench-checked for proper gear meshing.
You can bend a shaft when installing new axle bearings, you can also damage the bearings during install as well, but that would give you more of a grinding sound/sensation than a vibration per say...
 
I'd look at the driveshaft. with the 3.55's it's spinning faster. With the 2.79's you weren't driving fast enough to get the shaft to it's vibration point.
 
Mike:
I'm the one who installed the 3.55s and the drive shaft, but that was a bunch of years ago. I'm 70 now and don't feel like laying around on the garage floor anymore. The car is coming home from the local shop this afternoon. The owner, a good old boy wrench twister, says that the car drives "like a new car" now. He used to have an S code 69 Mach 1 back in the day, so he's familiar with and favorable to the car. We'll see. So far he has replaced the passenger side axle, both U-joints, and the transmission mount (it was apparently torn). He also sent the drive shaft out for balancing. I believe that the passenger side axle might have been bent. From having the car stripped to bare metal the last time I had it painted, I know that it took a hit in the right rear at some time prior to my buying it. The U-joints were last replaced when I did the rear end replacement, but that was a very long time ago. Who knows how long the transmission mount might have been torn? It could have happened when we pulled the engine on its last rebuild. He also says he had all four wheels re-balanced because he thought that might be the source of some of the vibration. I'm hoping that he isn't just overselling how improved the car is to get me out of his shop. You know, daffy old man in need of companionship. Either way, I'm done chasing this issue. I will let you guys know how a new 69 Mustang drives, however.;)
 
I was suspicious of the local mechanic when he reported it drives "like a new car". I figured he was just perfuming the pig. He wasn't kidding. The vibration is almost completely gone and it will get to well over 80 mph without the dreadful vibration from the back of the car. Still just a touch of vibe when you come off the throttle at those speeds, but barely noticeable. Must have been either the drive shaft balance or the transmission mount separation, or a combination of both, that was causing all the rumbles. I'm betting on the drive shaft because the problem arose right after I put in the lower gears. The drive shaft hadn't seen the revs previously that the 3.55s put on it and that would explain the sudden appearance of the problem with the rear end swap. B67FSTB and Huskinhano were right in their assessment. I believe the transmission mount might have been torn when we had the engine out. I had put my smaller floor jack under the bell housing to support the tranny and, when I came into my shop the next morning, the jack had dropped and the tranny was hanging in space. Or the rubber could have just rotted with old age. I know how that works. So a big thumbs up to George's Friendly Auto Service in Lincoln, CA.
 
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