- Jul 14, 2006
Autoblog Maintenance 301: Transmission R&R, Part I
Several of our readers have been requesting tech topics of a higher level of complexity, and so we went and broke a transmission just so we could show you how to fix it. OK, that's not quite how things went; the Tremec/BWA T56 six-speed manual in our '96 Impala SS started making some seriously bad noises during an autocross event last year, and we're finally getting around to doing something about it.
In the first part of this series, we're going to demonstrate the steps necessary to liberate the transmission from its confines. We'll then go through the disassembly process, at which point we'll be able to show the various parts that allow a modern manual transmission to function. Finally, we'll find and fix the problem, and get our car back on the road.
First, a bit of background. I originally performed the T56 swap when the stock 4L60E automatic died about 1200 miles into its marriage with the new 396 stroker motor. About 300 miles later, I brought the car to the strip, and managed to break the 27-spline mainshaft of that T56 (this also twisted up the driveshaft yoke quite nicely). In went a cut-down 30-spline mainshaft from the Dodge Viper, along with some custom machining on the tailshaft housing to accept the larger driveshaft yoke.
This time around, we were sure that we didn't break the same thing, as the car still moved under its own power - albeit with a lot of clunking and grinding in most of the gears. In fact, the transmission worked quite well in 4th gear, meaning that the problem probably lies somewhere in the interface between the input shaft and the countershaft (the reason for this diagnosis should become clear in future installment). Regardless of the issue, the transmission needed to come out.
We started this project in the interior, but the first step of this project should be to disconnect the battery. I didn't shoot a picture of that step, because frankly I forgot to do it. Somehow, I managed not to burn down the vehicle, but we highly suggest that our readers don't engage in such risk-taking.
The shift knob, center console, and shifter boot is removed to expose the shifter.
We then pull the four bolts that hold the shifter in place. In this case, they were only finger-tight, which means that we really need to use some threadlocker upon reassembly.
Upon crawling under the vehicle, the first priority is to start draining the transmission fluid. Ours came out with a metallic sheen, which, er, isn't a good sign.
Next, the driveshaft is detached from the axle by removing the two straps that clamp the u-joint caps to the pinion yoke.
Carefully supporting both ends of the driveshaft, we remove it from the back of the transmission with a gentle tug. Expect a bit of transmission fluid to weep out at this point.
We're pretty sure that our shaft is thicker and longer than other blogs - four inches in diameter and nearly five feet in length. It's also built from MMC (metal matrix composite - aluminum that's been blended with ceramic) with massive 1350-sized u-joints.
It's encouraging that the splines of the driveshaft yoke are straight - this is a first for us.
With the transmission in neutral, we can easily spin the tailshaft - yet another good sign.
The next step is to drop the exhaust. In this case, we can get away by disconnecting the system at the headers, which gives us enough room to swing the intermediate pipes out of the way. Other vehicles may require the complete removal of the exhaust system, including the catalytic converters and headers or exhaust manifolds.
The nuts retaining the clutch slave cylinder to the bellhousing are removed...
...And the slave cylinder is tied out of the way using nylon "zip ties".
Since the LT1 uses a unique pull-type clutch arrangement, it is extremely important to disengage the fork from the throwout bearing. If this is not done, the fork will prevent the complete removal of the transmission, but it'll come back far enough to disengage from the clutch splines. The plates will probably shift, preventing the trans from going back into place, and several hours of dorking around will result. Don't make the same mistake.
Remove the driveshaft loop or any other items that may get in the way as the transmission is lowered and pulled to the rear of the car.
Disconnect any electrical wiring. There are only two connections on this vehicle - the vehicle speed sensor (shown above), and the reverse lamp switch.
Now we need to raise the vehicle so that we have enough room to roll the transmission out from under the vehicle. The job will be made much easier if you can get the rear of the car at least as high as the front, or even just a bit higher - we'll show you why in a moment.
Remove the bolt that retains the transmission mount to the crossmember.
Place the jack under the transmission and lift it off the transmission crossmember. Remove the crossmember.
Lower the transmission using the jack until the bolts holding the transmission to the bellhousing can be accessed. We will be leaving the bellhousing attached to the transmission in this case. Using a long extension, loosen the bolts. When removing the bolts, leave two of more easily-accessible ones in place. This will keep the transmission from going anywhere until we're absolutely certain we want it to move.
Keep in mind that whenever the transmission is lowered, the engine is moving with it. We recommend that a helper be placed top-side to watch for any interferences in the engine bay, particularly around rear-mounted distributors or mechanical fans (this vehicle fortunately has neither). Below, keep an eye on the exhaust and the clearance between the oil pan and front crossmember. Also beware of placing any strain on the wiring harness; start pulling connectors if it looks like the harness is getting tugged in uncomfortable ways.
At this point, lower the transmission until its weight is supported by the jack. Hopefully at this point it is close to being level; if not, it may be necessary to adjust the height of the vehicle. Remove the remaining transmission bolts, and roll the transmission rearward. It may take some wiggling to free it from its home; the tightly-fitting dowels that locate the trans to the bellhousing are usually the culprit. Gentle prying is acceptable, but don't lay into it with your full weight behind a Stanley Wonderbar before figuring out why it isn't moving.
We can now roll the transmission out from under the vehicle. The total time for this step of the project was a bit more than one hour; in all fairness, we've had a bit of practice.