67 wildcat vibration

[vito_d]

hi group

still can not get rid of drive train vibration half way thru first gear

so far...repalced carrier bearing and bushing, new u joints, new factory correct engine and tranny mounts and balanced the driveshaft twice.....90% better but still there

suggestions please! its driving me nuts

thx vito

[Guest brh]

Some backround info please. How do you KNOW its in the drive train. At what speeds do you feel the vibration and when does it go away. What work was done on the car or what happened prior to the vibration. Maybe something was done that led to it. Just some thoughts.

[vito_d]

the vibration starts half way thru first gear and goes away as it shifts into second[under normal acceleration]

it started happening as a light vibration and just got progressively worse over time. apparently buicks with dual drive shafts had a poorly designed carrier bearing that needs replacing sooner than later because it could not be greased properly. so my mechanic replaced that bearing and the bushing including new engine and tranny mounts, new u joints and completely balanced driveshaft{twice].

im assuming that was the original propblen because the vibration is 90% gone but is definitly still there and my mechanic is stumped

that is where we are to date

this car has been family owned and only has 35000 miles

thx for your help in advance

[Guest brh]

this one does sound like a stumper--I'd take a hard look at the engine causing the vibration, as the car is traveling faster the driveline is spinning faster, which if out of balance would mean the vibration would get worse. But as the trans shifts your engine drops in rpm which may explain why it goes away. Once you clear second gear the car may still have the vibration but you just can't feel it as much. Harmonic balancer at the front of the crankshaft??

[vito_d]

ok...good thought, but wouldn't you feel the vibration if you were to power brake. yeh ive tried, mildly as if i were accelerating normal and there is no vibration.....thx vito

[NTX5467]

It's NOT that Buicks had a poorly-designed driveshaft mechanism (as most GM cars and light duty trucks of that era had two-piece driveshafts per se, but it IS more critical to have the angles between the u-joints be "in spec" with such an arrangement. When those angles are not "in range", then you can have "low speed shudder" upon acceleration. This has nothing to do with "balance" of the driveshaft parts and bearings.

When you replace the bushings around the bearing, you not only put newer "isolation" in the mix, but also reposition the bearing to the more "in spec" relationship with the two driveshaft halves, getting "the angles" more "in range" with each other. If there's still some vibration in low gear, you might borrow some of your friends and put them in the car, maybe even a couple of hundred pounds of something in the trunk, then recheck for the low-gear vibration you mention. If it gets worse or better, then you'll better know whether or not it's an "angularity" issue. To fix it, there usually are some shims/spacers that go between the carrier bearing mount to the chassis to change the angle (at normal ride height) to get a happier medium adjustment/setting in that respect.

Also, if the springs have sagged with age and use, although they might not look like it, a similar angularity issue can appear from the lowered ride height and resultant driveshaft angularity changes.

Making sure the driveshaft joint angles are as they should be is something I first learned about in my 1966 MOTOR Repair Manual, which detailed how to troubleshoot and fix it. The center carrier bearings were less complicated and more "open" on the Chevys back then, typically, but this was a GM-thing back then (the two-piece drive shafts when Ford and Chrysler usually got by with just one, which worked well).

Also make sure that you replaced the water shields on both sides of the "sealed bearing", which is not completely "sealed" as many might believe (as we found out when a mechanic discarded them as "Not needed, it's a sealed bearing", but a year later, and every year thereafter until we got some more for it, the "sealed bearing" would have enough crud in it to become noisy and start to fail).

You'll need a service manual that covers your vehicle to get the correct driveshaft angularity specs, typically.

Just some thoughts,

NTX5467

[vito_d]

just some thoughts?? just some 'great' thoughts and maybe on the money. i did weigh down the rear just today[funny eh?]

and the shutter was definitly less. those rear springs are 8 years new, so i think i will use coil spring lifters in the front before just replacing them....do you agree?

thx so much for the advice......vito

[NTX5467]

Thanks for the kind words. Personally, I'm not really in favor of coil spring "fixit-things" other than the solid rubber spacers (contoured to fit the spring and the seat the spring sits in on the control arm). The twist-in types, as it was pointed out to me, put extra stresses in certain areas of the spring, which can further damage it and make the sag you were trying to get rid of come back at a later date.

The best thing to do would be to put the car on a drive-on lift (rather than a chassis contact lift) and then check the driveshaft angles at the u-joints, and their relationship to each other. Rather than use the special tools in the service manual pictures, you can now buy similar things at Home Depot, with a magnet on the base side of it, in the tools section. Then you can use some body shims or front end alignment shims to space the carrier bearing to get the angles more inline with where the angles should be.

Take care,

NTX5467

[vito_d]

ok...i'll be on it in the next couple of days..thx again vito

[vito_d]

hi ntx

my mechanic is having trouble locating specs on ride height and driveshaft angles...can you help? thx mike

[NTX5467]

Welllll, after looking in my 1968 Buick Chassis Manual, the "check and adjust" procedure is a bit different than it is on other GM vehicles, but here are a few additional things to consider.

On the LeSabre (5000) and Wildcat (6000), the front and center u-joing angles (in the car) are not adjustable and are not critical. IF you do need to change the angle at the carrier bearing mount's joint assembly, you put a shim under the rear transmission mount (or take some "shim" out, possibly).

On the rear of the driveshaft, the "pinion angle" is the angle between the rear of the driveshaft and the pinion shaft in the rear axle. This angle IS adjustable. It is adjusted by adding or removing shims from the upper control arm mountings (arm-to-frame location).

Now, this brings up something else . . . IF the rubber in the upper control arms has deteriorated (as it probably has as these bushings are typically never changed or ever get any attention in rear suspension rebuilds), it can let the center of the bushing shift or sag or "take a set" in a position that will alter the effective length between the upper mount on the rear axle and the upper mount on the chassis. This affects the "pinion angle" AND can result in a low-speed acceleration shudder, as described. Loading of the vehicle also affects the pinion angle, too. The factory setting is a compromise of sorts between smoooth operation through a wide area of rear suspension travel (no load, full load, etc.).

Another thing mentioned and illustrated in the 1968 Buick Chassis Service Manual is the manner in which the two pieces of the driveshaft are assembled relative to each other. Key thing is that if the rear u-joint flange of the rear section of the driveshaft assembly is, for example, horizontal, the front u-joint flange of the front section of the front shaft segment should be vertical (i.e., 90 degrees different than from the rear). This way, the accelerations/decelerations of each joint as they turn will cancel each other out.

On the 9000 series, with a rear constant velocity joint at the rear axle, it notes that this set-up is immune to driveline vibration/shudder issues due to the rear pinion angle . . . unless there is some physical interference with a vehicle body part or similar. There is also a different method to adjust the pinion angle via an upper control arm adjustment.

As for the "trim" height of the vehicle, here's a condensed version of the chart in the service manual.

"Dimensions are for car with no load, but with a full fuel tank. Measure from bottom of rocker panel to level surface. Note: On cars with oversize tires add 1/4"

Series 5000 (LeSabre) Frt Rocker 10-7/64" Rr Rocker 9-51/64"

Series 6000 (Wildcat) Frt Rocker 9-9/16" Rr Rocker 9-3/4"

All dimensions are + or - 1/2" "

Now, in taking these measurement, we are dealing with a tire size (in current sizings) of about P225/75R-15, which would equate to the prior H78-15 and 8.45x15 in prior times. Therefore, what would be important, possibly, is the relationship between the front and rear measurements more than the stated mesurement values themselves. Usually, a few inches from either end of the rocker panel, there begins some flat areas which would be the measurement points. In my own observations, having radials where bias ply tires used to be can result in something like 1/2" lower ride height than if the vehicle had the "equipment" bias or bias-belted tires on it. To help minimize this situation, you might adjust the air pressure to about 35psi cold while taking these measurements, then adjusting the tire pressure downward to where you normally run it.

It's my suspicion that if you get the rubber bushings in the rear suspension's upper control arms replaced, it should put things back into the normal orientation of things and probably further minimize your shudder situation. For that matter, replacing all of the rubber in the upper and lower rear suspension control arms might not hurt, either--including the track bar bushings (which also has a special orientation in the bar for them to work correctly--where the slots in the rubber are positioned in relation to the main bar stock itself). Getting these things done/replaced should get the rear suspension to "like new feel" again, I suspect.

Unlike what I saw in my 1966 MOTOR Manual for GM two-piece driveshaft u-joint angle checks/adjustments, the full size Buicks used a spring loaded cable which attached to the front of the engine and the rear axle cover. A plate between the engine oil pan and the transmission bellhousing put a "level" on the cable, so that the dimensions between the cable and each segment of the drive shaft/bearing assembly could be checked and possibly adjusted to specs. I highly doubt that you could duplicate this procedure without the original Kent-Moore Special Tool. Kind of the same thing as the magnetic angle measurements, but using something totally different to "get there".

Maybe this will give you something to go on and "run with". Some new things to look at and consider, too, when all of the side issues are taken into account. I DO suspect that when the rear suspension rubber bushings are replaced and the production "factory" relationships are restored, it might well fix the problem you have and no further "adjustments" or "checks" will be needed. In this case, you're replacing things that are reasonably easy to do, rather than ordering some new springs and finding that they do not really fix the problem until the bushings are replaced too.

Take care,

NTX5467

[vito_d]

your time, knowlege and concideration is well taken and appreciated...thank you vito

[Gene Brink]

Vito,

You might want to rethink where the vibration is a bit more. You indicated it went away when you shifted to second BUT regardless of gear the driveshaft and rear end will be turning at the same speed it was at the shift point because they are not dependent upon the gear you are in for a given road speed and hence do not change speeds as shifts occur. I'd guess either an engine, transmission or tire problem and the first step I'd take would be to rotate the tires (this since I did have a tire problem on a '68 LeSabre I owned - vibration around 30-35 but smooth below and above that speed - turned our to be some tread separation).

Good luck!

[vito_d]

hi gene

thx for that input. and i was thinking along the same lines in the begining.

i started with new tires, then i found a shop that would balance the tire to the rim because non of these factory wheels are true. with all that, no change. but remember, there was a significant change after the carrier bearing, bushing and driveshaft balancing was done. its just not completely gone, plus the last thing we tried was the correct trani mount, and after that was put in the vibration was actully more pronounced, sooooooooo, im still exploring. thx alot vito