TheAMCForum.com Homepage
Forum Home Forum Home > The Garage > Transmission & Drivetrain
  New Posts New Posts RSS Feed - 76 Pacer - Rear End Vibration
  FAQ FAQ  Forum Search   Events   Register Register  Login Login

Click for TheAMCForum Rules / Click for PDF version of Forum Rules
Your donations help keep this valuable resource free and growing. Thank you.

76 Pacer - Rear End Vibration

 Post Reply Post Reply Page  123>
Author
Message
mantonas View Drop Down
AMC Apprentice
AMC Apprentice
Avatar

Joined: Mar/21/2015
Location: Knoxville, TN
Status: Offline
Points: 74
Post Options Post Options   Thanks (0) Thanks(0)   Quote mantonas Quote  Post ReplyReply Direct Link To This Post Topic: 76 Pacer - Rear End Vibration
    Posted: Jul/29/2017 at 8:13pm
I just got this car on the road after two and a half years of turning it from a total pile of junk to a pile of junk that can move under its own power. I am now in the process of driving it until something breaks, then fixing that, then driving it until something else breaks, fixing that, etc., hopefully eventually getting to the point where the breakage rate slows to something approaching that of a normal car. Right now, the problem I'm most concerned with is a drivetrain vibration that I am guessing is coming from the rear axle. The reason I think this is so is that I don't feel the vibration in my hands coming through the steering wheel, I feel it in another part of my body, coming through the seat. 

It's a rhythmic vibration that begins around 45 mile per hour, that lasts for maybe half a second and repeats every second or so. I immediately assumed u-joints, so I replaced those. I drove it around for a half hour or so today just to see if it improved, and it kind of did, but it's still there. 

This doesn't make sense to me; if it was the u-joints, I would think replacing them would solve the problem completely. Instead, it seems to have improved things, but the vibration is still there sometimes. Towards the end of my test drive, I developed the theory that it only happened when I was driving completely straight: if I was following a curve, it didn't happen. Maybe; I didn't drive it enought to fully test this theory.

Before anyone asks, I was careful to put the driveshaft yoke back onto the driveshaft exactly like it was before I removed the old u-joints, and the yoke can only go back into the transmission one way because there is one irregular spline, and I even kept track of where the driveshaft was in relation to the rear axle and strapped the u-joint back into that spot to retain that relationship. I guess it could be that my driveshaft is out-of-balance due to age: is that something that happens? I don't know.

Anybody out there have a clue what this might be? 
1976 AMC Pacer X
1978 AMC Pacer V8
1987 Jeep Grand Wagoneer
Back to Top
mantonas View Drop Down
AMC Apprentice
AMC Apprentice
Avatar

Joined: Mar/21/2015
Location: Knoxville, TN
Status: Offline
Points: 74
Post Options Post Options   Thanks (0) Thanks(0)   Quote mantonas Quote  Post ReplyReply Direct Link To This Post Posted: Jul/29/2017 at 8:27pm
Some more information I thought to add: 

1. The motor mounts have been recently replaced. 

2. The transmission mount has not been replaced.

3. The differential has fluid in it (just checked today).

4. The front wheel bearings are new (again, I don't think this has anything to do with the front suspension, but thought I'd add that in anyway).
1976 AMC Pacer X
1978 AMC Pacer V8
1987 Jeep Grand Wagoneer
Back to Top
tomslik View Drop Down
AMC Addicted
AMC Addicted
Avatar

Joined: Mar/07/2008
Location: Colorado
Status: Offline
Points: 535
Post Options Post Options   Thanks (0) Thanks(0)   Quote tomslik Quote  Post ReplyReply Direct Link To This Post Posted: Jul/30/2017 at 8:41am
drive shaft bent by some chance?

might want to have it checked and balanced

67 american 290/4speed
73 hornet X 360/auto



Back to Top
1970390amx View Drop Down
AMC Addicted
AMC Addicted
Avatar

Joined: Jul/12/2008
Location: colorado
Status: Offline
Points: 2361
Post Options Post Options   Thanks (0) Thanks(0)   Quote 1970390amx Quote  Post ReplyReply Direct Link To This Post Posted: Jul/30/2017 at 10:12am
The fact that it seems to be worse when you are not going straight sounds like it might be a rear axle bearing. If you find a road with a long sweeping curve and drive the road both ways is it loud both ways or just one direction?
1970 390 4speed Bittersweet shadow mask AMX
1970 Amx missing most everything, or in a box
Back to Top
73hornut View Drop Down
AMC Addicted
AMC Addicted
Avatar

Joined: Jul/03/2007
Location: Southern Oregon
Status: Offline
Points: 2585
Post Options Post Options   Thanks (0) Thanks(0)   Quote 73hornut Quote  Post ReplyReply Direct Link To This Post Posted: Jul/30/2017 at 10:42am
Have you rechecked the wheel balance?
71 Javelin
74 Gremlin
86 Eagle wagon
Rogue Valley Rumblers
Like Us on FB
https://www.facebook.com/groups/1602825606650796
Back to Top
vinny View Drop Down
Supporter of TheAMCForum
Supporter of TheAMCForum


Joined: Jan/05/2012
Location: Calgary
Status: Offline
Points: 2127
Post Options Post Options   Thanks (0) Thanks(0)   Quote vinny Quote  Post ReplyReply Direct Link To This Post Posted: Jul/30/2017 at 11:19am
Wheel bearings, carrier bearings, bent driveshaft, tires out of balance are all possibilities. How about putting it on jack stands and getting under there with a mechanics stethoscope? Hopefully it won't vibrate so much it falls off the stands.
Back to Top
73Gremlin401 View Drop Down
Supporter of TheAMCForum
Supporter of TheAMCForum
Avatar

Joined: Mar/02/2013
Location: Stmbt Sprgs CO
Status: Offline
Points: 374
Post Options Post Options   Thanks (0) Thanks(0)   Quote 73Gremlin401 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/30/2017 at 11:33am
Originally posted by mantonas mantonas wrote:

Some more information I thought to add: 

1. The motor mounts have been recently replaced. 

2. The transmission mount has not been replaced.

3. The differential has fluid in it (just checked today).

4. The front wheel bearings are new (again, I don't think this has anything to do with the front suspension, but thought I'd add that in anyway).


That you are feeling it in the seat of the pants and not through the steering wheel is a good clue, and you are doing the right thing - replace 1 part at a time and see what happens.

Since you are feeling the cyclical vibration only in a corner, that's an important clue as well - as that eliminates things like tire balance and probably driveshaft.  What I'd check/do next is:

1: As another poster noted, DO check the driveshaft for balance and/or a kink.  
2: Transmission mount replace.  Something is moving when you are cornering, and this could be a solution
3: leaf-spring bushings - both the front eye bushing, as well as the rear shackles.  They all go bad eventually, and no one likes checking the eye bushings.  But again, since the problem appears in a corner, something is shifting, and these could be it.  The rear shackles are easy, so do them first.
3A: check the iso-clamp pad that sits between the leaf spring and the axle, and check the u-bolts for correct torque if they have been removed at some point in time.  If it looks like nothing has changed since the car was built, and that nothing is moving, you are probably OK there.  But if there's any indication that things are moving, then that's gotta get sorted out. 
4: Is your Pacer at stock ride height?  Or has it been mechanically raised/lowered.  If yes, then you've got to check pinion angle to the driveshaft.
5: check the rear wheels for run-out.  If they check out OK, then check the rear axle hub for run-out.  At some point in the cars history, if it slid into a curb, it is possible that the hub is slightly tweaked.
6: you mentioned that you recently replaced the front motor mounts/cushions - just for the heck of it, make sure those bolts are still tight.  Pacer engine mounts are unique and IIRC don't have the settling issues the typical AMC engine mounts can have, but I'd check it anyway.

hope this helps!


Edited by 73Gremlin401 - Jul/30/2017 at 11:39am
73 Gremlin 401/Richmond Gear 5spd. Semi-insane.
77 Matador 360/727
Hideous and staying that way.

Back to Top
mantonas View Drop Down
AMC Apprentice
AMC Apprentice
Avatar

Joined: Mar/21/2015
Location: Knoxville, TN
Status: Offline
Points: 74
Post Options Post Options   Thanks (0) Thanks(0)   Quote mantonas Quote  Post ReplyReply Direct Link To This Post Posted: Jul/31/2017 at 10:39pm
I found a driveshaft shop nearby (in Knoxville, Tennessee). I spoke to them on the phone today and they said it would cost $45 to balance the driveshaft if I bring it in to them. That sounds pretty reasonable to me; I am going to try and do that either this Friday or next. It's somewhere to start, at least.

I have a new transmission mount, I've just been too lazy to install it. Hopefully I will do that when the car is back on jack stands to get the driveshaft out. 

I will definitely check to see if the motor mounts are tightened up; those motor mounts were such a nightmare to replace that I almost think they hate me and I wouldn't be surprised if they were the cause of my problem.

I'm not really sure about the vibration only happening while driving straight; by the time I had begun to formulate that theory, I was getting close to home again and I didn't feel like driving it around any more (man, I must be getting old!). I probably won't get another chance to test that theory until the driveshaft comes back from the driveshaft shop, and I hope the problem is gone by then.

A friend of mine from work said he had a similar problem with his 1964 Olds 442. It turned out to be a worn bushing for the yoke in the transmission (his transmission is manual), which in turn caused the yoke to wear out. He had the bushing replaced, but the problem didn't go away completely until he put a new yoke on it. I think he said the bushing was so worn out that it let the yoke move too much and the splines on the inside had wallowed out.
1976 AMC Pacer X
1978 AMC Pacer V8
1987 Jeep Grand Wagoneer
Back to Top
73Gremlin401 View Drop Down
Supporter of TheAMCForum
Supporter of TheAMCForum
Avatar

Joined: Mar/02/2013
Location: Stmbt Sprgs CO
Status: Offline
Points: 374
Post Options Post Options   Thanks (0) Thanks(0)   Quote 73Gremlin401 Quote  Post ReplyReply Direct Link To This Post Posted: Aug/01/2017 at 10:38am
Originally posted by mantonas mantonas wrote:

I found a driveshaft shop nearby (in Knoxville, Tennessee). I spoke to them on the phone today and they said it would cost $45 to balance the driveshaft if I bring it in to them. That sounds pretty reasonable to me; I am going to try and do that either this Friday or next. It's somewhere to start, at least.

I have a new transmission mount, I've just been too lazy to install it. Hopefully I will do that when the car is back on jack stands to get the driveshaft out. 

I will definitely check to see if the motor mounts are tightened up; those motor mounts were such a nightmare to replace that I almost think they hate me and I wouldn't be surprised if they were the cause of my problem.

I'm not really sure about the vibration only happening while driving straight; by the time I had begun to formulate that theory, I was getting close to home again and I didn't feel like driving it around any more (man, I must be getting old!). I probably won't get another chance to test that theory until the driveshaft comes back from the driveshaft shop, and I hope the problem is gone by then.

A friend of mine from work said he had a similar problem with his 1964 Olds 442. It turned out to be a worn bushing for the yoke in the transmission (his transmission is manual), which in turn caused the yoke to wear out. He had the bushing replaced, but the problem didn't go away completely until he put a new yoke on it. I think he said the bushing was so worn out that it let the yoke move too much and the splines on the inside had wallowed out.


you actually bring up a good point, and reminded me of one of my earlier Gremlins with a 258/904 setup - I had a cyclical vibration in it, and it was caused by a worn driveshaft yoke.  What you need to do when you get the car up on jackstands, is to check for play in the yoke as it sits in the transmission.  If there is any vertical (or side to side, depending how you grab it) slop/wiggle/movement in the yoke, then yes, that is most certainly part of your vibration issue. (you gotta grab the yoke to check this - not the driveshaft or U-Joint) Usually, the slop is accompanied by leakage of the seal itself, so if you are getting some drip from the tailshaft seal, then it's almost a certainty that there will be some yoke wear/looseness as well.  The thing I learned is that just replacing the seal will not cure the problem - it may get rid of the leak temporarily, and make the yoke feel better slop wise, but at the end of the day, it's the outer surface of the yoke that is worn.it is something you can check before you pull the driveshaft.  The outer surface of the yoke should be perfectly smooth and consistent.  Given the age of all our cars, you won't find that - some wear is normal.  But if it is significantly grooved, then you have 2 options - find a new/newer one, or have your existing yoke sleeved to bring it's diameter back to spec.

I'd definitely check this before sending the driveshaft off.  If the yoke is loose/worn, ask the driveshaft shop if they can machine and sleeve your yoke.  That, along with replacing the transmission mount, could very well solve the problem, or at least a good chunk of it.  Do still take a good look at the leaf spring bushings as well, but I'm starting to think now that getting the yoke issue checked and sorted will help you a bunch. 
73 Gremlin 401/Richmond Gear 5spd. Semi-insane.
77 Matador 360/727
Hideous and staying that way.

Back to Top
tomj View Drop Down
AMC Addicted
AMC Addicted
Avatar

Joined: Jan/27/2010
Location: los angeles
Status: Offline
Points: 1696
Post Options Post Options   Thanks (0) Thanks(0)   Quote tomj Quote  Post ReplyReply Direct Link To This Post Posted: Aug/01/2017 at 11:48pm
good point! it's not a part on most people's minds, since it's not a typical wear part. but after many, many times past their design life, the oddest things wear out. the driveshaft and yoke on my T96 and 'big nut' axle didnt vibrate, but the yoke would nearly rattle in the transmission. (it also inserted only 2" - 3" where the '98 T5 goes in over 6" -- seems like they learned from experience). the driveshaft yoke holes for the U-joint bearing cups were belled, from half a century of use and repair. cups were loose in the yoke.

stuff does just wear out. 

1961 roadster american
195.6 OHV, modded
T5z, 3.42:1 mustang axle
Back to Top
 Post Reply Post Reply Page  123>
  Share Topic   

Forum Jump Forum Permissions View Drop Down



This page was generated in 0.063 seconds.
All content of this site Copyright © 2012 TheAMCForum unless otherwise noted, all rights reserved.
PROBLEMS LOGGING IN or REGISTERING:
If you have problems logging in or registering, then please contact a Moderator or