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76 Pacer - Rear End Vibration

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Forum Name: Transmission & Drivetrain
Forum Description: If it's between the engine and wheels, it goes here
URL: https://theamcforum.com/forum/forum_posts.asp?TID=88812
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Topic: 76 Pacer - Rear End Vibration
Posted By: mantonas
Subject: 76 Pacer - Rear End Vibration
Date Posted: Jul/29/2017 at 7:13pm
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? 


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1972 AMC Javelin SST
1973 AMC Ambassador 401
1975 AMC Pacer D/L
1976 AMC Pacer X
1976 AMC Matador sedan
1978 AMC Pacer V8 coupe
1987 Jeep Grand Wagoneer



Replies:
Posted By: mantonas
Date Posted: Jul/29/2017 at 7:27pm
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).


-------------
1972 AMC Javelin SST
1973 AMC Ambassador 401
1975 AMC Pacer D/L
1976 AMC Pacer X
1976 AMC Matador sedan
1978 AMC Pacer V8 coupe
1987 Jeep Grand Wagoneer


Posted By: tomslik
Date Posted: Jul/30/2017 at 7:41am
drive shaft bent by some chance?

might want to have it checked and balanced



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67 american 290/4speed


Posted By: 1970390amx
Date Posted: Jul/30/2017 at 9:12am
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?

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1970 390 4speed Bittersweet shadow mask AMX
1970 Amx missing most everything, or in a box


Posted By: 73hornut
Date Posted: Jul/30/2017 at 9:42am
Have you rechecked the wheel balance?

-------------
71 Javelin
74 Gremlin
79 Spirit AMX
Rogue Valley Rumblers
Like Us on FB
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Posted By: vinny
Date Posted: Jul/30/2017 at 10:19am
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.


Posted By: 73Gremlin401
Date Posted: Jul/30/2017 at 10:33am
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!


-------------
73 Gremlin 401/5-spd.
77 Matador Wagon 360/727.
81 Jeep J10 LWB 360/4-spd
83 Concord DL 4-dr 258/auto



Posted By: mantonas
Date Posted: Jul/31/2017 at 9:39pm
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.


-------------
1972 AMC Javelin SST
1973 AMC Ambassador 401
1975 AMC Pacer D/L
1976 AMC Pacer X
1976 AMC Matador sedan
1978 AMC Pacer V8 coupe
1987 Jeep Grand Wagoneer


Posted By: 73Gremlin401
Date Posted: Aug/01/2017 at 9:38am
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 401/5-spd.
77 Matador Wagon 360/727.
81 Jeep J10 LWB 360/4-spd
83 Concord DL 4-dr 258/auto



Posted By: tomj
Date Posted: Aug/01/2017 at 10:48pm
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. 



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1960 Rambler Super two-door wagon, OHV auto
1961 Roadster American, 195.6 OHV, T5
http://www.ramblerLore.com



Posted By: mantonas
Date Posted: Aug/02/2017 at 7:38pm
I had a chance to work on the car after work today, so I put the rear axle on jackstands. Before I unbolted the driveshaft, I grabbed the yoke and tried to move it by hand and it definitely rattled around a bit. I made sure that the rattle was not happening at the rear axle or at the u-joints (makes sense, they're new): it was definitely rattling around in the transmission. I inspected the outside of the yoke: it looked pretty good. Shiny, no scoring, a few discolored patches from minor corrosion, but no ridge at the point where it transitions to the part that is outside the transmission output seal. I looked into the end of the yoke at the splines, and i couldn't see anything bad, but it could be very worn out and I wouldn't be able to tell from looking at it. I'm thinking it's the extension housing bushing that the slip yoke fits into.

I'm taking it to the driveshaft shop tomorrow, I'll see what they say. I am also going to start calling around to find a transmission shop that can replace the extension housing bushing. 

I also want to get ready in case I need a new slip yoke. I have not had any luck finding one that is identified as a direct replacement for a 76 Pacer with an automatic transmission, but I'm sure they're out there. Does anybody know where I can get one? I'm thinking that I might be able to use one from a Chrysler product with a 904 transmission. I might have to use a different kind of u-joint, but that's okay.


-------------
1972 AMC Javelin SST
1973 AMC Ambassador 401
1975 AMC Pacer D/L
1976 AMC Pacer X
1976 AMC Matador sedan
1978 AMC Pacer V8 coupe
1987 Jeep Grand Wagoneer


Posted By: mantonas
Date Posted: Aug/05/2017 at 2:02pm
I took the driveshaft to the shop on Thursday and I heard back from them yesterday. Turns out the driveshaft has problems. It's apparently pretty significantly out of straight and out of balance; they said at about 2100 rpm it starts to vibrate. That's pretty low, and it may line up with the fact that I start feeling the vibration starting at around 45 mph. They gave me a few options and I picked having them fabricate a whole new driveshaft for me with new yokes and tubing, balanced, with new Spicer u-joints (lifetime warranty on the u-joints). They also said my slip yoke looked good and they will polish it for me.

They did not do any work with transmissions but gave me the name of a shop who might be able to replace my tailshaft housing bushing. Who knows, maybe a new driveshaft will solve the vibration problem and I won't need to replace it.

-------------
1972 AMC Javelin SST
1973 AMC Ambassador 401
1975 AMC Pacer D/L
1976 AMC Pacer X
1976 AMC Matador sedan
1978 AMC Pacer V8 coupe
1987 Jeep Grand Wagoneer


Posted By: farna
Date Posted: Aug/08/2017 at 7:00am
The bushing isn't too hard to replace. Hardest to get the old one out! Stick the new one in the freezer for a few hours with the housing in the sun getting warm (or in a 150-200 degree oven - just warm though!). Then drive it in with a wood block on the end of the bushing.

If you don't want to do it yourself take the housing off (easy since driveshaft is off) and just take it to the trans shop. Will be a lot cheaper that way, and quicker to get it back.


-------------
Frank Swygert


Posted By: mantonas
Date Posted: Sep/01/2017 at 8:30am
I was hoping that the driveshaft would solve the problems, but it didn't. I now have a driveshaft that's worth at least 10 percent of the total value of the car. Talk about spending your children's inheritance. Anyway, the vibration is still there but now it's different. It starts at about 45 mph and sort of ends around 55 mph. Previously it would just continue to get worse the faster I went. I have now had it up to around 80 mph and while it is loud and feels like the world is coming to an end at that speed it's not because of the vibration. So I'm back to the bushing.

I called a transmission shop and they told me it would cost between $250 and $300 for them to replace it. If you want to do it with the transmission still in the car, then you need the mother of all special tools. Knowing that this car essentially has a Chrysler 904 Torqueflite in it I looked for the tool and this appears to be it:

https://www.atsg.us/atsg/extension-housing-bushing-remover-installer-and-output-shaft-support-fixture-party-t-0160-a.html

Still expensive ($179, presumably plus shipping and maybe tax) and I don't know if I would trust myself to use it properly.

Then yesterday I had a bit of good luck. A friend of mine from work gave me the name of a transmission repair guy that he has used who he says is good. This friend of mine has 5 kids and is the sole earner for his family, so when he says somebody is good, there's an unspoken "and cheap" that follows. And he was right: this transmission repair guy said he would remove the extension housing, knock out the bushing and replace it, and, while he was there, replace the tailshaft bearing, for a grand total of $100! 

So that's going to be my next step. I need to get into his shop sometime after Labor Day and I will see what it's like after that. If it still vibrates, I will probably start thinking about replacing rear wheel bearings. I've done a somewhat half-assed search for parts in preparation for that and I haven't found any, so that could be a problem.

Another data point: I read on some forum that maybe a vibration like this could be from the engine. It could be as simple as an engine cooling fan that's out of balance. The post said to throw it in neutral while the vibration is happening and see if it stops. So I did that, and it did stop, for a few seconds, then it started to come back. I interpret that to mean that when the transmission stops applying torque to the slip yoke, it disrupts the vibration, sort of like stomping on the brakes in a solid front axle Jeep to stop death wobble (some of you know what I mean!). You stop the vibrations temporarily by disturbing the system, but it eventually comes back. So I don't think it's coming from the engine.


-------------
1972 AMC Javelin SST
1973 AMC Ambassador 401
1975 AMC Pacer D/L
1976 AMC Pacer X
1976 AMC Matador sedan
1978 AMC Pacer V8 coupe
1987 Jeep Grand Wagoneer


Posted By: CamJam
Date Posted: Sep/01/2017 at 9:32am
Did you ever install that new transmission mount you bought? I just spent two months chasing a similar problem. Turned out to be the tranny mount. $8.99 at O'Reilly's and took all of 15 minutes to install.

The clue was the pinion angle. Transmission angle measured at the yoke and rear diff angle measured at flat spot on bottom of case should be within a couple degrees of each other. Mine differed by about 6 degrees because missing rubber of the mount was causing the rear of tranny to droop by about two inches. Tranny mount can appear good until you look closely and realize that the metal "blade" part of the mount is no longer bonded to the rubber.

-------------
'73 Javelin 360 (current project)
'72 Baja Bronze Javelin SST
'69 Big Bad Orange AMX (2018 Teague Heritage Award) SOLD



Posted By: mantonas
Date Posted: Sep/01/2017 at 9:56am
No! I never did that! Thanks for reminding me. I think I have one, I just need to put it in. It's heck getting old and forgetting things all the time.

-------------
1972 AMC Javelin SST
1973 AMC Ambassador 401
1975 AMC Pacer D/L
1976 AMC Pacer X
1976 AMC Matador sedan
1978 AMC Pacer V8 coupe
1987 Jeep Grand Wagoneer


Posted By: CamJam
Date Posted: Sep/01/2017 at 10:38am
Originally posted by mantonas mantonas wrote:

No! I never did that! Thanks for reminding me. I think I have one, I just need to put it in. It's heck getting old and forgetting things all the time.

Well if not, they're only $8.99 at O'Reillys (assuming you have the 232 motor) same as what I paid for the Javelin with a 998 transmission. Different part number, but the design of yours is the same as mine-- just probably a different height.  You can see how the right angle metal "blade" is (barely) attached to the rubber.  In my case, half the rubber was gone entirely. 
 
https://hwww.oreillyauto.com/shop/transmission---transaxle-16780/mounts-17155/at-mount-12768/1976/american-motors/pacer?q=transmission%20mount" rel="nofollow - https://hwww.oreillyauto.com/shop/transmission---transaxle-16780/mounts-17155/at-mount-12768/1976/american-motors/pacer?q=transmission%20mount

Just FYI, the mount as pictured in the O'Reilly's photo is upside down.


-------------
'73 Javelin 360 (current project)
'72 Baja Bronze Javelin SST
'69 Big Bad Orange AMX (2018 Teague Heritage Award) SOLD



Posted By: 73Gremlin401
Date Posted: Sep/01/2017 at 4:22pm
Originally posted by CamJam CamJam wrote:

Originally posted by mantonas mantonas wrote:

No! I never did that! Thanks for reminding me. I think I have one, I just need to put it in. It's heck getting old and forgetting things all the time.

Well if not, they're only $8.99 at O'Reillys (assuming you have the 232 motor) same as what I paid for the Javelin with a 998 transmission. Different part number, but the design of yours is the same as mine-- just probably a different height.  You can see how the right angle metal "blade" is (barely) attached to the rubber.  In my case, half the rubber was gone entirely. 
 
https://hwww.oreillyauto.com/shop/transmission---transaxle-16780/mounts-17155/at-mount-12768/1976/american-motors/pacer?q=transmission%20mount" rel="nofollow - https://hwww.oreillyauto.com/shop/transmission---transaxle-16780/mounts-17155/at-mount-12768/1976/american-motors/pacer?q=transmission%20mount

Just FYI, the mount as pictured in the O'Reilly's photo is upside down.


What he said.  X2.  If that mount is toast, and from what you've written earlier it is, you can do all the balancing and parts changing you want...and you'll still have vibration.  Thing to remember is that an AMC 6 cylinder engine with an automatic transmission is roughly 1/2 the length of the entire car - which also means that the torque of the engine has a very long 'beam' to exert it's force on - and the only thing that is keeping it in check is the rear mount.  Can't tell you how many of those I've broken over the years, and many times not from abuse - a big ol block of rubber that has oil spraying on it all the time is only gonna last but just so long.


-------------
73 Gremlin 401/5-spd.
77 Matador Wagon 360/727.
81 Jeep J10 LWB 360/4-spd
83 Concord DL 4-dr 258/auto



Posted By: CamJam
Date Posted: Sep/01/2017 at 5:33pm
Yep, and those Torqueflites do like to leak!

-------------
'73 Javelin 360 (current project)
'72 Baja Bronze Javelin SST
'69 Big Bad Orange AMX (2018 Teague Heritage Award) SOLD



Posted By: farna
Date Posted: Sep/02/2017 at 7:38am
I had some driveline vibrations and couldn't figure them out. Took the driveshaft to a shop for balancing and making an inch longer, and they found the splines were damaged in the yoke and binding a bit. Said that was more likely my problem than length (had 2.50" inside trans, 2.25" out -- Jag IRS so not much in/out movement). Lengthened it anyway, but got a new yoke also. Problem solved! So check and see if the yoke slides in/out easily while you have the trans out. I was going to change the housing bushing again as I thought that was the source of a bit of bind. I'd changed it myself years ago, thought maybe I'd deformed it a bit when driving in. The new one slid in/out perfectly though. This was an AW4 trans, but that shouldn't matter.


-------------
Frank Swygert


Posted By: mantonas
Date Posted: Sep/13/2017 at 9:14pm
Do you know where I could buy a new yoke for my Pacer? i spent a little time looking but I couldn't find one. 

-------------
1972 AMC Javelin SST
1973 AMC Ambassador 401
1975 AMC Pacer D/L
1976 AMC Pacer X
1976 AMC Matador sedan
1978 AMC Pacer V8 coupe
1987 Jeep Grand Wagoneer


Posted By: farna
Date Posted: Sep/14/2017 at 5:59am
The yoke is the same as for any other AMC, and likely Chrylser also... for 1972 and up with Chrysler transmissions. Any trans shop can get one, or try http://www.dennysdriveshaft.com/c73_dodge.html" rel="nofollow - http://www.dennysdriveshaft.com/c73_dodge.html
Just tell the local shops it's a Chrysler trans. Would be a 904 for a six, 998 for the 304 V-8. All the bigger AMC V-8s used the 727, but you could have the smaller trans if someone has done an engine swap.


-------------
Frank Swygert


Posted By: vangremlin
Date Posted: Mar/12/2018 at 12:31pm
Hey, were you ever able to solve the vibration problem? I'm having similar issues with my Gremlin. Thanks!



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