Drivetrain Drivetrain Shudder/Vibration

5.0specialist

Active Member
Jul 10, 2016
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pembroke pines, fl
Hello everyone. My 91' mustang gt convertible developed a shudder after getting polyurethane motor mounts installed. The mounts raised my engine a bit and I am still using the stock transmission mount. When I would take off from a dead stop at part throttle, I would feel a shudder (feels like a wobble type vibration) and sometimes when my aod shifts into 3rd gear (feels like bumps on a road). When it happens it is brief and if I give it hard acceleration, it does not do it. There was zero shudder before I have installed the engine mounts, so it should not be the transmission. My dad had a 98' jeep XJ that had a similar issue/symptoms after he raised the jeep and it was fixed by installing spacers underneath the transmission mount. I wonder if I need to do the same thing or just buy a polyurethane transmission mount that would raise the transmission and restore the original drivetrain/shaft angle.
 
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08GT500

Active Member
Jul 12, 2018
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Massachusetts
Hi,
The Poly tradeoff..Based on the Durometer rating/type/Mfg, Polyurethane mounts transmit and deliver normal driveline harmonics/ vibration(s) to the Chassis much greater than forgiving rubber mounts that isolate it.
If fighting an incorrect pinion to Transmission angle, mounts being stiffer now, it will damage UV Joints, Seals, Trans Tailshaft & Pinion input bearing(s), leaks and then Diff./Trans damage..
So, any Handicaps must be avoided/ corrected. You’re aware of the problem, and know the solution, just need to do it. Really not giving you a hard time, only reminding you to trust your good intuition & instincts... :nice:
Your Text....
“ just buy a polyurethane transmission mount that would raise the transmission and restore the original drivetrain/shaft angle.”
Yes!
Can you use Shims (washers) to raise the Trans mount height? ..Sure.... But Is that the level of craftsmanship you want to utilize?
You’re better than that, and would tell another another to do it once..correctly.
If you’re running Poly Motor mounts, you should certainly also be running a Poly Tranny Mount. And vise versa.
If circumstances present and you ever need to shim a Tranny mount, cut a piece of correct thickness steel plate the same size as the rear (Tranny side) of the mount with the correct fasteners, drill the mounting holes and bolt it solidly between the mount and Tranny.
All applicable Vehicle’s Pinion angle needs to be aligned correctly both laterally and vertically, Driveshaft being a straight shot from Trans to Differential at Vehicle ride height.
CV joints are purposed only to prevent driveshaft binding during Suspension travel- not to correct an improper Tranny to Pinion angle.
If your motor is raised with Poly mounts, using the Poly Trans.mount meant to be used with them for the application, your Tranny will obviously also sit higher, but should retain the same correct pitch for Driveshaft alignment from Trans. to Differential.
You can check it with a generic tube 5mw 650nm laser, seating it against the center of the Trans output shaft, laser dot should be dead on center with the Diff.input shaft stud.
Should have no UV Joint angles when installed, sitting on all 4 tires.
Once confirmed, be sure to check that no wiring, connectors, cables, fluid lines are rubbing or pinched, close to exhaust, etc.
Don’t want those headaches!lol!
Good luck with it!
Best!
-John
 
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08GT500

Active Member
Jul 12, 2018
896
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Massachusetts
Thank you John! I just wanted to confirm my intuition. I'll definitely go with the poly tranny mounts to match the front engine mounts.
Hi,
The way I’d typed up the Differential angle, Transmission angle post could be misunderstood & may be misleading, some editing was needed.
The Differential and the Transmission should be lined up straight and square from the vehicle left to the right (laterally), but significant height difference exists between the Transmission and Differential.
Though the Transmission output shaft and the Differential each have a target working angle they sit at, the remaining angle between the two (front and rear working angle) is what the Driveshafts universal joints need to compensate for..
Universals are continually compensating.
any questions, don’t hesitate.
-John
 
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