Driveline vibration problem - Vintage Mustang Forums
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post #1 of 9 (permalink) Old 02-10-2019, 02:49 PM Thread Starter
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Driveline vibration problem

I've had this vibration since assembling this car. Starts at about 40 and I can hear it more when I let off the throttle at that speed. By 60-70 it is really strong, like a repeating harmonic. It chills out a little by 80mph.

I bought this driveshaft new and I shortened the explorer 8.8 that is in there now so it could be either one. I also have custom trans mount and RMS lowered motor mounts, so my angles might be wrong too.
I already had the tires rebalanced (but it doesn't feel like any of the numerous tire imbalances i've had). I then measured my driveline angles and got motor down 2 degrees at the trans output side, driveshaft is front-up one degree, and the pinion is 1-2 up. Maybe too straight?

Any ideas before I take it in to the shop? Is there a way to tell if it's a janky axle or if the driveshaft is just out of balance?

1966 Mustang 302ci 4r70w, csrp front discs, 8.8 rear, EPAS, Fitech for fuel
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post #2 of 9 (permalink) Old 02-10-2019, 03:12 PM
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You have a lot of things going on but you could try to flip the driveshaft 180 degrees. No cost there but no guarantee's. Good luck, Jim
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post #3 of 9 (permalink) Old 02-10-2019, 03:49 PM
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If turning the driveshaft 180 doesn't solve the problem, check the U joint angles. The crankshaft centerline should be 3 down and the pinion should be 3 up. This will put the U joints in the center of their rotation. It doesn't have to be exactly 3, just anything close but the other end U joint has to be the same angle just opposite.

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post #4 of 9 (permalink) Old 02-10-2019, 05:42 PM
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^ What he said.
There are a couple of schools of thought actually on DL angle but "motor down
2 degrees at the trans output side, driveshaft is front-up one degree and the pinion
is 1-2 up" isn't one of those.
I'm not surprised you have harmonics.
Edit- You know, unless I'm misunderstanding, looks like around
a 1 degree operating angle.


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Last edited by GT289; 02-10-2019 at 06:10 PM.
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post #5 of 9 (permalink) Old 02-10-2019, 06:14 PM
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I think something else is going on.
Maybe u-joint wear...... which makes no sense because I would think
they're new?

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Last edited by GT289; 02-10-2019 at 06:25 PM.
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post #6 of 9 (permalink) Old 02-10-2019, 06:49 PM
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Where do you feel it most? In your butt/seat?

Have you tried putting it in neutral at speed and see if anything changes?

Not to add places to check just for the heck of it, but you mentioned its been there since you put it together - it could be in the torque converter or flex plate.

Safe to assume its not there if you rev the engine in park? What happens if you shift the trans down a gear (out of overdrive)?

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post #7 of 9 (permalink) Old 02-10-2019, 08:02 PM Thread Starter
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Quote:
Originally Posted by bbmach View Post
Where do you feel it most? In your butt/seat?

Have you tried putting it in neutral at speed and see if anything changes?

Not to add places to check just for the heck of it, but you mentioned its been there since you put it together - it could be in the torque converter or flex plate.

Safe to assume its not there if you rev the engine in park? What happens if you shift the trans down a gear (out of overdrive)?
If I kill the motor and shift to neutral on the highway it is still there, All the U-joints are new and double checked. The U joints have no play and move freely when I took the driveshaft out to check it. I feel it everywhere and hear it coming from under/behind me.

I took it out for a spin today and it was barely there. It was 20 degrees out though. I had a 99 F250 with a two piece driveshaft that I accidentally took apart and never got clocked right again. It would vibrate like hell on the highway except when it was very cold outside. Maybe driveshaft?

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post #8 of 9 (permalink) Old 02-11-2019, 03:10 PM
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I imagine that if temps had anything to do with it it must be in the gear oil/ rear end or axles
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post #9 of 9 (permalink) Old 02-13-2019, 10:13 AM
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Quote:
Originally Posted by GT289 View Post
^ What he said.
There are a couple of schools of thought actually on DL angle but "motor down
2 degrees at the trans output side, driveshaft is front-up one degree and the pinion
is 1-2 up" isn't one of those.
I'm not surprised you have harmonics.
Edit- You know, unless I'm misunderstanding, looks like around
a 1 degree operating angle.
Agreed. His front operating angle is 1 degree and the rear is 3 degrees. The operating angles need to be equal, opposite, and as small as possible, but not zero. Sounds like he needs to install some wedges to point the pinion down.

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