I have recently developed a problem where my ZF6 is rising up upon application of throttle in 1st and 2nd gears. The shifter rises probably 1-2". I thought for sure I would craw underneath and see a loose bolt on the C-Bracket at either the rear end or the tranny. No dice. I replaced all the rear end bushings, including the diff carrier last winter - all that still seems tight. The motor mounts don't seem to be the culprit because when I rev the engine without it in gear it does not appear to be loose at all. Any ideas? Thanks!
--Calvin

 

If the motor mounts are ok and if the batwing bushings are ok then you will need to take the C-beam out and eyeball the holes looking for elongation.

 

You need to put some load on the car to check the motor mounts. Hold the brake and load it to check.

 

Did you just eyeball the C-beam bolts to see if they were loose? May want to put a wrench/socket on the bolts and make sure they are tight.

I had the same sort of thing happen with my '87 with the 4+3. The bolts appeared to be tight but I noticed there was a small area on the C-beam around the bolt head at the trans end that was shiny. Turned out the bolts were just a bit more than finger tight.

Tightened all the bolts down and the shifter no longer moves around. And the entire driveline feels tighter.

 

Thanks for the tips, guys. I will try to torque them with a wrench. I torqued them to the FSM specs when I did the clutch about 1 year ago. In case the holes are elongated, are the automatic and ZF6 c-beams the same, or are they different part numbers. Thanks.
--Calvin

 

They are different.

 

Easier to make your own beam plates;nothing fancy needed just a piece of plate on either side of beam so as to spread the clamping forces over a larger area.

 

If the C-beam bolts are loose (it sounds like this is in fact your issue) than you cannot just tighten the loose ones. You need to make sure the diff and trans are in line before tightening the bolts or they will come loose again. To this I loosen all bolts. Put a jack under the trans and a jack on the diff under the input pinion. Slowly move them up and down until you can freely move all 4 bolts up and down and spin them by hand. If you can do this then all the holes are lined up and the diff and trans are in line with each other. Tighten all the bolts back up and remove the jacks.

 

Not an uncommon problem, apparently. There is a lot of stress put on those two joints at either end of the C-beam. Under stress, the aluminum, being much more maluable than the steel washers/bolts, will migrate away from the relatively small pressure footprint offered by the washers alone. This will result in the clamping force degrading and the problem accellerates, characterized by a sharp little ridge of aluminum forming around the periphery of the steel washers.

As clamping force is reduced, there is slippage, which is also followed by an elongation of the bolt holes in the C-beam ends. Thus the "slop" results in the beam riding up, as indicated by the shifter moving.

As indicated, installing a pair of steel plates to spread the clamping force, aided perhaps with some blue Locktite between the beam and the plates will prevent further slippage and perhaps even stabilize the tendancey for the rear to kick to the right when launching the car under high torque situations (e.g., drag racing).

Bill Boudreau at ZF.doc sells them - complete with caged nuts and reinforcement ribs on the plates to further strengthen the plates. The caged nut are really great for those with big hands. (In fact, if for no other reason than to facilitate the removal and reinstallment of the beam, the plates (to me) are worth every penny!)

However, a warning! What ever you do, do not exceed the torque specifications on the C-beam bolts and don't use anything but petrolium gell or light oil to lube the bolts before torquing them to avoid excessive clamping force that could distort the diff or the trans bosses - a bad thing. No STP or assembly type pre-lube.

As mentioned, to avoid vibration and excessive U joint wear, you'll want to align the transmission tail as shown below. Measurements are taken between the beam itself and the tunnel.



Hope this helps.

P.

 

Thanks again for all the help, you guys. I will check the motor mounts soon and then its onto the C-Beam this weekend.
--Calvin

 

It was the C-Beam bolts. 3 of 4 were not torqued anymore. I put some locktite on to help a bit until I can get the beam 0plates over the winter, since I have to have my ZF rebuilt anyway - 3rd and 5th gear syncros are not happy. Thanks again.
--Calvin

 

This was a great thread.
I had noticed the my shifter was rising under acceleration as well. I got under my 92 to tighten the support beam bolts, but trying to get a wrench on the top nuts of the transmission was near impossible. So I modified a wrench to make the job easier. The nuts are actually 18mm but I did not want to waste a Snap On wrench, so a 3/4" was what I had to work with. It is a little loose but still works.

The top wrench in the image is stock and the bottom is modified. I heated up the shaft of the wrench and bent it to 90 degrees. I taped it to a metal ruler to give it some length. This way it would reach the top of the tunnel. And just to note, I was able to tighten my bolts without removing the exhaust.

 

With all due respect to the experts and the FSM, this is what I came upon attempting the "jig" method as described earlier in this thread. On my 92 Base model, I found at least one rib in the transmission tunnel (as indicated in the modified drawing) in the vicinity of where the measurements were to be taken. When I used the 45mm measurement as described (outside of the rib), I found that my bolts were bound and would not spin freely. When I began jacking my transmission upward, the bolts began to spin freely at 36mm. Now, maybe that 45mm measurement would have worked if measured inside the rib, but that was not how the procedure was described.

 

Here are the gauges I came up with to do the measurements:

 

ive also heard of a broken shifter support causing this as well (the curved magnesium? piece w/ rubber bushing that bolts to the trans tailshaft and the rubber bushing supports the back of trans.