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Drivetrain Troubleshooting Guide


Rrrrr...Clink! Those of you who are four-wheeling veterans are probably familiar with evil sounds such as that emanating from drivetrain components. If you recognize this noise, you're also familiar with the distinct pop of a U-joint sending its caps and hundreds of tiny needle bearings off into the trees, or the crisp snap of an axleshaft expiring. Four-wheeling is about going places most other people don't get to see, but no one ever said this privilege is not without costs.

Drivetrain breakage can sour a good day of four-wheeling faster than the sound of steel breaking can echo through a canyon. Since drivetrain is the stuff that links engine power to the ground, a single failure can cut your truck's 'wheeling ability in half or stop it altogether. It's touch and go with drivetrain parts as to whether you'll need a tow back to the trailhead or a quick field fix to continue, and that's why accurate diagnosis is so important. So, to help point you in the right direction when you know something's broken but can't figure out what, we dug through the junk piles at Four X Doctor, Off Road Unlimited, and Tri-County Gear to find prime examples of what commonly broken drivetrain parts look like.

We'll point out what the breakage affects so you can pinpoint the probable cause without tearing everything apart, which failures can be fixed on the trail, and which breakdowns will require a long ride in a tow truck. We'll also point out the weaknesses in certain drivetrain components where applicable. Kick back, relax, and enjoy this pageant of expensive horrors so you can be better prepared when one of your driveline parts turns south.

Bring a Spare
As you may have noticed, much of the breakage we show you here requires a spare part in order for a proper field fix to be made, so being prepared with the right parts is the best defense against drivetrain failures. But there's also a dividing line between being prepared and being, well, paranoid. It's a little overboard to carry a spare transmission and transfer case, but you won't be adequately prepared without a spare U-joint. To help you maintain a healthy balance, we've ranked what we believe four-wheelers should carry on the trail depending on how extreme they are.

Occasional Weekend Wheeler

*U-joint (rear)
*U-joint straps or U-bolts
*Gear oil
*Silicone sealant
*Sealed container for used oil

More Serious Wheelers

*Everything previously listed
*Front U-joints (if different from rear)
*Rear axleshaft

Out of Control Extremists

*Everything previously listed
*Front axleshafts
*Stub shafts
*Knuckle joints

Case cracks (arrow) on trannies and transfer cases are typically caused by extreme stress under load or trail obstacles and are more prevalent in aluminum cases than in those made of cast iron. A sudden leak is a good indication of a possible crack, but it may be hard to spot if the case is covered with grime. These cracks will grow with transfer case or tranny use and eventually will allow the case to expand, which wreaks havoc on the internal parts.

Automatic-transmission bellhousings have been known to crack as well, but this is usually the result of improper transfer-case mounting or engine-to-tranny alignment. We've heard this is usually caused by extremely heavy transfer cases, such as the NP203, not being properly bolted to the tranny crossmember or frame, which creates an extreme amount of stress on the bellhousing where it meets the engine.

The clutches inside some full-time, four-wheel-drive transfer cases and automatic transmissions can wear rapidly with severe use or with lots of miles. In trannies, worn clutches create slippage during shifts and are most noticeable by an increase in engine rpm without an increase in acceleration. If the clutches are shot, the fluid from the tranny or the T-case will be discolored and smell burnt.

Shift collars in manual transmissions and transfer cases can expire, although this is fairly infrequent. A broken shift collar is easy to diagnose because it'll cause the truck to be stuck either in gear or out of gear. Sometimes a person familiar with the operation of these components can remove the top cover and manually shift the truck into a gear that gets the truck off the trail, but this is best left to someone who knows what he's doing.

Tranny and transfer-case input shafts can break in half under severe load, but more common are sheared teeth between the shaft and the rest of the tranny. We usually see this kind of breakage at tough-truck competitions and other high-speed events. This type of breakage sounds bad and expensive, and it is.

The NP208 transfer case is known for front-input-bearing failure, which in turn chews a hole in the front casing. This is commonly caused by poor maintenance and running the 'case when it's low on oil. This type of failure happens over time, and often there isn't any indication that something is wrong, until it's too late.

This is clearly the result of too much horsepower. The output shaft of this T176 tranny exploded through the back of the tranny's case. Violent breakage, such as this, is almost always the result of too much throttle at the wrong time and leads to outrageous towing bills.

U-joints are notorious for letting go. In fact, we have yet to attend a trail ride where we haven't seen at least one turned turtle, which is why they're number one on the Bring A Spare list. When the caps of the joint explode, they often take out the retaining straps or U-bolts on the yoke at the same time. U-joints are a piece of cake to replace anywhere, so this kind of breakage is no big deal if you're prepared.

Driveshaft dings from trail obstacles can weaken the strength of the 'shaft and often cause driveline vibrations when the truck is at speed. Also, note the small counterweight near the dent which could succumb to trail damage. If the damage is severe, just remove the driveshaft and drive home on the other axle assembly (provided the 'shaft is not equipped with slip yokes).

This is what usually happens to steering knuckle U-joints when they fail but go undetected. After one of the caps squirts out, the U-joint cross gradually wobbles out of the cap retainer, which requires a new axleshaft. If the knuckle joint shears completely, the stub shaft and axleshaft will bind and clang against one another in four-wheel drive, which can damage both pieces.

Broken rear axleshafts mean deep doo-doo if they're retained by C-clips and you don't have a spare. The 'shaft will separate from the housing, taking the tire with it. Replacing an axleshaft on the trail isn't hard if you're lucky enough to be able to remove all the pieces easily. This Wagoneer owner had to remove the carrier from the housing in order to remove the splined end of the broken shaft from the carrier, which is frequently the case.

In more severe cases, the ears of the axleshaft or stub shaft shear off completely and the the U-joint usually disappears. Although you can drive home with this type of breakage, there's no way to fix it unless you have a complete spare assembly. This is also more common with high-horsepower applications and high-speed driving.

Grenaded hubs are probably number two behind U-joints as the most common source of driveline failures. Sometimes this failure is the result of too much horsepower, but more frequently, hubs fail because of improper maintenance. With an open diff, hub failure means no more power to the front wheels without a spare. On a locker-equipped rig, however, only one wheel is affected and the opposite wheel will continue to receive engine power.

This is what's left of a Gov-Loc rearend after a heavy-throttle rocky hillclimb attempt. Depending on the type of differential and situation, the rearend may lock up, spit parts out the diff cover, or make bad crunching sounds when rotating. The best way to tell gearset breakage from carrier breakage is to pull off the diff cover.

If the carrier is broken, you have a couple options in order to limp home. To minimize the chance of damage to other parts, it's best to move the truck as little as possible and call a tow truck. If that's not an option, get as much shrapnel out of the pumpkin as possible to minimize damage to the gearset, fill the pumpkin with fresh oil, disconnect the driveshaft, and carefully drive home on the front axle assembly

Most, but not all, drivetrain failures are serious. On many late-model trucks without locking hubs, the front axle assembly is engaged by a vacuum-operated device on one of the axle tubes. The vacuum line can be snagged on trail obstacles or just pulled off the assembly from excessive axle articulation, which will disable the four-wheel drive. Simply reconnect the vacuum hose and you're on your way.

If you look closely at the photo, you can see that the upper axleshaft has a slight bend in it, which is bad. Bent axleshafts create strange wobbles and destroy bearings. Smiled axlehousings are caused by jumping and also will destroy bearings, but failed carriers and gearsets are more common symptoms of bent rearends.

Broken gearsets are usually caused by a throttle jockey traversing rough terrain, but poor maintenance and water in the gear oil can also destroy them. A gearset can snap, crackle, or pop; it just depends on the situation. Also remember that worn-out factory gearsets will howl like mad when they're at speed before they let go.

Where to Get the Goods
Now that you've learned how to diagnose the snap, crackle, and pop coming from under your rig, here are some companies that offer the parts you'll need to set things straight. These parts range from those you can carry with you on the trail to those that may be beyond a shadetree-mechanic's ability to install, but all of them may be required to repair your drivetrain woes.

Front Axle Rebuild
The 8.25-inch front axles under the '88-to-current GM four-wheel-drive fullsize trucks and '91-to-current Suburbans and Blazers have caused many headaches when the owner considers a ring-and-pinion swap. In addition to nearly everything else you could need for your truck's drivetrain, Drive Train Specialists (DTS) now offers a custom bearing installation kit designed just for this application. The kit includes new ring bolts, bearings, seals, shims, a pinion nut, and marking compound so you can get the job done correctly. For more information, contact Drive Train Specialists, Dept. 4x4, 26400 Groesbeck Hwy., Warren, MI 48089, 800/521-0628 or 313/778-0540.

Two-Piece Slip Yoke
For the off-road enthusiast who frequently removes the driveshafts the advantages of the new Strange two-piece slip yoke are obvious. Simply remove the four Grade 8 bolts, and drop the driveshaft. The slip yoke holds the rear U-joint's caps in place, thus preventing the tiny needle bearings from spilling onto the ground. The front slip yoke remains in the transfer case, thus eliminating the mess caused when the fluid leaks out. The yoke is constructed of 4140 hardened steel and is precision-broached and ground for perfect alignment and seal. The coupler is matched to the yoke with a tight-tolerance slip-fit and uses a super-duty 1350 U-joint. Contact Strange Engineering, Dept. 4x4, 1611 Church St., Evanston, IL 60201, 847/869-7010.

Anything You Need
Motive Gear stocks about any drivetrain related rebuild part you may need for popular applications such as the 7.5-, 8-, 8.8-, and 9-inch Ford and 10- and 12-bolt GM axle assemblies. It also offers ring-and-pinion master kits that net you the ring-and-pinion gears and an installation kit that provides all the parts you'll need for a swap. For more information, contact Motive Gear, Dept. 4x4, 4200 S. Morgan, Chicago, IL 60609, 800/934-2727.

Ford 9-Inch Rebuild Kits
Are you changing the gearset or completely rebuilding your Ford 9-inch rearend? If so, Moroso has packaged a complete kit of American-made components for Ford 9-inch rearends with 2.9687-inch bore cases such as those used in 4x4 applications. The kit (PN 84000) includes Grade 8 hardware, O-rings, gaskets, shims, a carrier-and-pinion support bearing, a pinion seal, an adjustable pinion spacer, thread-locking compound, gasket sealer, and gear-marking compound used to verify the gear contact pattern. Axle bearings are not included in the kit. Contact Moroso Performance Products, Dept. 4x4, 80 Carter Dr., Guilford, CT 06437, 203/453-6571.

Heavy-Duty U-joints
A machine is only as strong as its weakest link. You can't build a brutal engine and forget about the driveshafts between it and the axles.

U-joint explosions can easily be avoided with the installation of high-perf U-joints such as those offered by Lakewood. Designed for most Chrysler, Ford, and GM vehicles, the Lakewood units are constructed of chrome-nickel molybdenum and are heat-treated. For added strength, the U-joints are not cross-drilled. Contact Lakewood Industries, Dept. 4x4, 8700 Brookpark Rd., Brooklyn, OH 44129, 216/398-8300.

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