Vibration!!

[bronze turbine]

My '68 300 has some vibration in the drive line. It does it when you are on the throttle or off the throttle, but at steady speed it seems to go away. I've already posted a thread on this forum about it and received a lot of advice. I'm leaning towards it being a problem with the differential and rebuilding one looks complicated. So, I was wondering, is it possible to get one already rebuilt and just swap it out? Kind of like a rebuilt transmission, engine, steering rack, etc.
Or would I be better off just taking it to a professional mechanic? Thanks!!

 

[polara71]

That depends on your skills. If you are asking these questions you likely won't touch it ( I'm with you).

The go to mopar rear end guy is
DoctorDiff Drivetrain Components

I've always stayed away from the used ones lying on the ground at shows so I took what I had to a guy.

I also at that point get my shaft balanced.

 

[Big_John]

My '68 300 has some vibration in the drive line. It does it when you are on the throttle or off the throttle, but at steady speed it seems to go away. I've already posted a thread on this forum about it and received a lot of advice. I'm leaning towards it being a problem with the differential and rebuilding one looks complicated. So, I was wondering, is it possible to get one already rebuilt and just swap it out? Kind of like a rebuilt transmission, engine, steering rack, etc.
Or would I be better off just taking it to a professional mechanic? Thanks!!

I tried going through your threads to read about your problem but I could not find it, so I'm going to give you my two cents based on what you've posted here.

While I can't say that it doesn't happen, that type of vibration sounds a lot more like a u-joint rather than a differential. A bent or twisted driveshaft can cause some hard to diagnose issues too, same for an out of balance shaft (read, lost a weight) . Bad differentials are noisy, but I've never seen one really cause vibration.

 

[Chrome58]

Don't forget also about misaligned transmission and engine.
There's a whole procedure to make sure they are properly aligned when reassembled.

 

[polara71]

Don't forget also about misaligned transmission and engine.
There's a whole procedure to make sure they are properly aligned when reassembled.

That's always puzzled me. How is that possible if all three mounts are in a fixed position?

 

[bronze turbine]

The original post may have been posted by my brother who is known as WissaMan. In a nutshell, we had already swapped out the driveshaft with another and then with a completely new one, and also installed new u-joints both times. The vibration may have gotten better but did not go away. We did have new leaf springs installed shortly after we got the car, but the vibration was there before that. Could it be motor mounts? When you are cruising at a steady speed on the highway, even 80mph, it's smooth.

 

[Big_John]

The original post may have been posted by my brother who is known as WissaMan. In a nutshell, we had already swapped out the driveshaft with another and then with a completely new one, and also installed new u-joints both times. The vibration may have gotten better but did not go away. We did have new leaf springs installed shortly after we got the car, but the vibration was there before that. Could it be motor mounts? When you are cruising at a steady speed on the highway, even 80mph, it's smooth.

OK, found that.. Troubleshooting a driveline vibration

Now we aren't starting from scratch.

 

[Big_John]

I'd look at the trans and motor mounts. They always can stand to be checked so it's not a waste of time. A bad mount might cause this... The trans/engine shifts as it accel/decelerate and gives you a little misalignment.

 

[Big_John]

That's always puzzled me. How is that possible if all three mounts are in a fixed position?

 

[polara71]

Informative John, I like it.

My question is, how do you verify the angle of the engine and transmission? I may be sounding stupid here but the engine sits on the mounts, no adjustment possible. The transmission sits on a mount, with little adjustment possible. Aside from a factory installed shim what's to check?

 

[Big_John]

Informative John, I like it.

My question is, how do you verify the angle of the engine and transmission? I may be sounding stupid here but the engine sits on the mounts, no adjustment possible. The transmission sits on a mount, with little adjustment possible. Aside from a factory installed shim what's to check?

I'll look for a good video on how to do it.

The angle of the rear end can be matched to the angle of the engine/trans using angular shims.

If the rear trans mount has sagged, that would drop the trans down, angling the rear down and that can be changed by replacing the mount. The mount can also be shimmed a little, but usually, you see the rear being adjusted.

These would go between the rear leaf plates and the rear axle perches.

 

[Big_John]

And this is how you check it. At about 4:40 he shows how to check, but he talks about vibrations, so it starts at 3:45. This is a great channel BTW. Yea, it's Ford stuff, but it's really good DIY mechanics.

 

[polara71]

Since you brought the rear up I'm not sure how you can install that wrong. Without customizing a bunch everything " falls" into place.

I will watch the video...

 

[furious70]

My 2 cents is that usually the center section isn't a constant problem, usually it changes on acceleration or deceleration

 

[CBODY67]

The FSM details how to measure the driveshaft angles at the u-joints. That special tool might be replicated by a magenitized level (reading in degrees) that can be found at Home Depot and such.

Are the new rear springs at "stock height" or were they raised from that level? Raising the rear of the car MIGHT significantly affect the angle between the trans slip yoke and the driveshaft itself, when assembled.

The angle at the rear u-joint can also affect judder on acceleration. This is adjusted by shims that go between the spring and the axle tube pads on each side. An angled shim, not a flat one.

Motor mounts can collapse with age and time, as can the rear trans mount.

I concur that it's most probably a driveshaft "area" issue, rather than a rear axle issue.

Just some thoughts,
CBODY67

 

[david hill]

Excessive run out is a first check on drive shaft inspection. Anything over 1/8 " is grounds for replacement, twisted drive shafts are possible. Easy check is to hook up your timing and aim it at each joint and compare orientation to each other. They should be spot on. Rotate the rear tires front to rear to eliminate them as a contributing source. Check pinion bearings and carrier bearings. any combination can cause major headaches.

 

[oldkimmer]

I had a bent rear diff in a car I bought cause some idiot torqued the u clamps to 100 ft lbs. They are supposed to be 45 ft lbs. Kim

 

[Chrome58]

That's always puzzled me. How is that possible if all three mounts are in a fixed position?

I wasn't talking about misalignment between engine/transmission and driveshaft, but misalignment between the engine and the transmission.

The torque converter housing has some adjustable dowels, that you need to rotate in order to make sure that the transmission's shaft is perfectly aligned with the engine's shaft. I know it's like that for the '57-'59 C bodies, I'm guessing it's the same for the '70s C bodies.

If they're not perfectly aligned, you might generate vibrations in the whole powertrain.

 

[polara71]

I wasn't talking about misalignment between engine/transmission and driveshaft, but misalignment between the engine and the transmission.

The torque converter housing has some adjustable dowels, that you need to rotate in order to make sure that the transmission's shaft is perfectly aligned with the engine's shaft. I know it's like that for the '57-'59 C bodies, I'm guessing it's the same for the '70s C bodies.

If they're not perfectly aligned, you might generate vibrations in the whole powertrain.

Those pins go in one way, not sure how there's an alternative. If they aren't set, the block and Bell won't squeeze together.

 

[CBODY67]

Vibrations/shudders are different for a mere mechanical imbalance of the driveshaft than for an alignment (driveshaft angles) at the front u-joint and/or at the rear u-joint area, where vibrations/shudders are caused by an engine torque situation (plus/power or minus/coast).

CBODY67