============ Draft Aug 28, 2001=============
Yes, the vibe can be fixed!. It is not always easy to get it fixed because dealers are not used to this problem.
The vibe occurs more often in the Cobra than in live axle cars. This is because the differential is bolted to the car rather than suspended as a large weight. A given amount of imbalance in the driveshaft assembly may be felt in the Cobra whereas it may go unnoticed in a GT.
The amount of imbalance in the assembly depends on several things. First, the imbalance left at each end of the driveshaft when the shaft was manufactured. Second, the dimensional runout in the transmission output shaft and the differential pinion flange.
What Ford tries to do with the "yellow dot" on each end of the driveshaft and on the pinion flange is match up the light side of the driveshaft (as determined in the balance machine when the shaft was made) with the max runout point of the pinion flange. This is so that the mass center of gravity of the driveshaft is as close to coincidental with the rotational center point as possible.
The keys here though are the tolerances that are allowed for the balance and the runout. One would hope that the worst acceptable shaft balance would still not vibe when paired with the worst runout and that the tolerances would be set accordingly. Apparently this is not so and some combinations result in a little vibration.
Here is why. A shaft on the high end of the imbalance tolerance may be vibe free when paired with a pinion flange with runout at the high end of the tolerance scale. This is because the two errors cancel out. But combine that same high end balance shaft with a perfect, no runout, pinion flange, and ......it vibes! No runout to help cancel out the imbalance!.
This is why the vibe may result after a gear change in a Cobra yet all the parts except the ring and pinion are the same. The runout on the pinion changes when the old pinion flange is installed on the new pinion gear, and the tolerance stackup works against you and the vibe results.
Ford spent a lot of time last year sending out a fellow named Bill Franco to measure and diagnose vibe problems. He first tried moving the driveshaft position to get a different match up. If he couldn't get any results, he replaced the driveshaft and the differential, and sometimes even the transmission.
There were some transmission output shaft problems in a few cars that complicated the problem. There was also a mixup at the factory in which some of the "matched sets" of driveshafts and pumpkins got mixed up. But, in general, every case of vibration that was examined by Bill Franco was fixed. The dealerships are just not as persistent or as well equipped to deal with the problem as Mr. Franco is.
Ford issued several TSBs (Technical Service Bulletins) on the vibe that any dealer should be able to follow and execute. If they are unsuccessful, then maybe Ford will still have a Bill Franco type person available to help.
I personally have had three different differentials, 4 different gear sets, and two different drive shafts in my car--none were changed as a response to a vibe problem!!. Only one combination has resulted in a significant vibration, and I solved that one by adding weight to field balance the assembly.
If you want to experiment a little yourself, you may be able to save yourself a trip to the dealer. Unbolt the rear end of the driveshaft and rotate it 90 degrees from where it is now relative to the pinion flange. Bolt it back up and go drive the car. If the vibe is not better, try another 1/4 turn, then another, find the position which gives the least vibe, then try 1/8 turn either side of the best position.
If there is not much change when moving the driveshaft at the rear, the imbalance may be in the front. Take the driveshaft loose at the rear, carefully pull it toward the rear so that the front yoke comes out of the trans. Look at the rear of the trans output shaft and place a mark where the yellow dot was when the shaft was removed so you can tell where you started from. Reinstall driveshaft 90 degrees from where you started, keeping the dots matched at the rear, and go drive the car. Try 1/4 turn changes at the front until the vibe improves.
For those of you who experience a vibration after a gear swap, have the gear installer disconnect the driveshaft and put a dial indicator on the pinion flange to measure the radial runout of the locator lip. Have him place a mark on the pinion flange at the point of rotation where the runout is max distance from the center.
Put the mark on the outside edge of the pinion flange so that it is visible when the driveshaft is installed. This is where the original yellow dot should be but may not be after the gear change. If the new mark is at a different location than the old one, move the driveshaft dot to line up with the new mark and road test the car.
If the vibe doesn't improve, take it to the dealer and tell them that you have tried "indexing" the driveshaft with no improvement and tell them you think you need a different driveshaft but to check the runout of the output shaft and the pinion flange with a dial indicator and see if they are within spec as well. The original spec was 0.011" runout max allowable. It may be difficult to field balance the assembly if the runout is more than 0.006" or 0.007", however. An SVT dealer who worked on a lot of 99s will know exactly what to do.
Some word of comfort. This is not a "design flaw". This is a tolerance stackup problem. It can be fixed easily and permanently if a competent and persistent person works it for you.
Field Balancing of Cobra Driveshaft Assembly If Trial and Error Indexing Doesn't Solve the Problem
1. Purchase a couple of SAE type washers that will fit the driveshaft bolts. Note--I used to recommend a couple of hose clamps but they are too heavy and too touchy for most vehicles.
2. Jack up the car or otherwise get safe access to the rear u-joint
area and put a 1 by the yellow dot at the rear of the
driveshaft. Place a 2 90 degrees rotation either direction from the 1, and a 3 at 180 degrees and a 4 at 270 degrees, so that
you have the shaft marked in 90 degree increments.
3. Install one washer under a driveshaft bolt nearest the
1 position. Go drive the car and see whether the vibe is
better or worse than before installing the washer.
4. Move the washer to the 2 position, then the 3 position, then
the 4 position. Determine the best location and so note. If
vibe is worse in all positions, the washer weight is too high and a lighter washer will have to be used.
5. If the vibe is better in one position but not smooth, then you will need to add some more washer weight.
This involves a lot of trial and error. Ideally, you would use
a quick lube pit or better yet, a chassis dynomometer, to allow you
easy access to the car underside.
You may want to put the car on stands or blocks to do the initial runs.
If this is done, support the car on the lower control
arms, not the chassis. If you choose to do this, you do so at your own risk and independent of any recommendation by me.
You are responsible for your safety and the safety of those around you, not me. You do not want the tires hanging down from
the chassis while spinning them to 55-70 mph. Also, clean any rocks out of the tire treads before running the car with the
wheels off the ground. Also, turn off your traction control or you won't be able to get the car up to speed.
OK, that's it. Let me know what you think and the results you
get. Believe it or not, many guys have gotten great results with
a single clamp or washer in one of the four 90 degree positions.
Let me know if I can help. I have some experience with this problem since I am an engineer and the closest SVT dealer is 120 miles away, LOL. You will love your car......once the vibe is gone.
=========Draft Aug 28, 2001============