A transmission uses gears to transmit the engine’s power to the wheels. A manual transmission lets the driver select the gears, and an automatic transmission selects gears based on the car’s speed and weight. So what can you do if the transmission decides not to transmit? You can repair it yourself or knowledgeably have it repaired for you. This article offers basic instructions on repairing manual and automatic transmission systems, drivelines, and differentials. Refer to the car’s service manual or an aftermarket manual for specific instructions on repairing your car’s transmission (but you knew that). Technically, most front-wheel-drive cars don’t have transmissions. They have transaxles. A transaxle combines the transmission and differential into one unit. So, as you read this article, translate transmission to transaxle if you have one. Otherwise, whistle silently. Do me a favor? Work safely! Please don’t try working on your car’s transmission or related parts without first safely jacking up the car and installing safety stands. Or use a heavy-duty ramp. Place blocks on the ground behind other tires to keep the car from rolling. I really don’t like sending get well cards to readers. It’s Test Timet Get your pencil out.
Here are some tips for troubleshooting troublesome transmissions:
- If the car won’t go into any gear, trace the shifting linkage from the steering column or floor to the transmission. It might simply be loose.
- If your car’s clutch doesn’t operate smoothly, check to see whether it has a booster that requires hydraulic fluid. It’s located near the power brake booster and probably uses the same fluid. Check the car’s owner’s manual or service manual.
- If your car has a manual transmission and the engine sounds like it’s speeding up when it really isn’t (especially on a hill), the clutch might be slipping and need adjustment or replacement.
- If you hear rattling from your car’s transmission, first make sure it has lubrication. Then look for loose mounting bolts.
- If the clutch pedal stays on the floor, check for a broken release bearing or fork.
- If you hear a knocking noise at low speeds or during turns, check the constant velocity (CV) joints.
Shifting for Yourself
Manual transmissions (see the first figure) that have been regularly serviced might not need repair for 100,000 miles or more. And when they do, most mechanics and doit-yourselfers replace the transmission as a unit. They don’t tear the transmission apart and start replacing components. They leave that up to specialists. So if your car’s manual transmission needs repair, you simply remove it from the car and reinstall a new or rebuilt unit. The job requires bigger tools, but it isn’t particularly difficult-in most cases. The disclaimer is offered because some automotive designers have the transmission wedged into the car so deep that the engine and transmission must be removed together and then separated. Nice of them! Fortunately, most manual transmission problems aren’t caused by the transmission, but by the linkage and switches attached to it. The linkage translates the movement of the gearshift to the movement of the appropriate gears within the tranny (what transmissions let friends call them). Manual transmissions have one, two, or three levers on the side that slide the appropriate gears around inside. Adjusting the linkage can solve many manual transmission problems.If the service manual says the problem is the overdrive unit, check to see whether your car’s overdrive is mounted in or on the transmission case. Some overdrives are external and can be replaced without removing the entire tranny.
A manual transmission is typically repaired following these steps:
- Locate and identify your car’s manual transmission and linkage. It’s behind the engine and below the firewall on rear-wheel-drive cars and either to the right or left of the engine on front-wheel-drive cars. Identify the transmission and check it against instructions in the service manual.
- Adjust the shifting linkage. The linkage is composed of rods or cables between the shifter and transmission. Typically, the linkage is adjusted by moving adjustment nuts on a linkage rod or at the end of the linkage cable. If instructions aren’t available, some staring and thinking will probably tell you what the adjustments do. Move-or have someone move-the shift linkage slowly through the gears to help you figure out what’s needed. In many cases, a bent rod or a loose nut suggests an easy solution. (Note: Studying the transmission is another term for taking a nap.)
- If adjusting the linkage doesn’t solve the problem, consider replacing the transmission. On many cars, this means disconnecting the driveshaft at a U-joint or CV joint (described at the end of this page) and removing bolts that hold the front of the transmission to the bellhousing. Remove all linkage and drain the lubricant from the transmission first. The transmission must be slid away from the engine to remove it from the clutch. Depending on the weight of the transmission, you might need help or a jack to safely lower the unit after it’s unbolted. Your car’s service manual will probably tell you the transmission’s weight.
- If necessary, replace the old transmission with a new or rebuilt unit. Of course, make sure it’s an exact replacement and that the bolt holes are in the same position so that it easily remounts. Check this even if you had your old transmission rebuilt because the shop might have replaced the case or given you the wrong unit. It can happen!
- As needed, reinstall and adjust the linkage, and then check the transmission for lubricant. Test the transmission for correct operation. If you’re getting tired of ice cream runs (and runny ice cream), go for pizza. Hold the anchovies on my half.
The engine and transmission work together with the help of the clutch. A clutch connects and disconnects an engine and manual transmission. It does so by friction, pressing the clutch plate against the engine’s spinning flywheel. When you push down on the clutch pedal, you are forcing the clutch release bearing and fork to release the clutch plate from the pressure plate’s clamping force against the flywheel. That disconnects the transmission from the engine. You then move the gear shifter to select the next gear and release the clutch pedal to reconnect the transmission to the engine.
If you’ve performed clutch adjustments described in your car’s service manual and it still doesn’t work smoothly, consider replacing the clutch.
So what can go wrong with the clutch system? The linkage can be out of adjustment because of wear. The bearing or clutch plate (also called a friction plate) can be worn out, or other components can be damaged. Some clutch systems use a hydraulic booster to make engaging and disengaging the clutch easier on the driver. The clutch booster works similarly to a power brake booster. If your car has a clutch booster, refer to the service manual for more specifics.
To repair a clutch system, follow these steps:
- Adjust the clutch linkage to see whether that solves the problem. The adjustment is located between the clutch pedal and the clutch fork that does most of the work. The linkage may be a rod or a cable. Follow the service manual for specifics. In most cases, the linkage is adjusted until the pedal free play (the movement of the clutch pedal before the clutch bearing moves) is liz or 3/4 inch. The adjustment is made by turning the adjustment nut until freeplay is correct and then tightening a locking nut to make sure it doesn’t change.
- If adjustment doesn’t solve the problem, you might need to replace some clutch components. To disassemble the clutch, first disconnect the driveshaft at a U-joint or a CV-joint, disconnect the shift linkage, remove the bolts holding the front of the transmission to the bellhousing, slide the transmission away from the bellhousing, and then remove the bellhousing to expose the clutch components. Whew!
- Replace clutch components as needed. Typically, the only parts replaced during a clutch repair are the clutch release bearing, the pilot bearing, and the clutch plate. They can be purchased at larger auto parts stores or through the dealer’s parts department. On many cars, remove the fork and bearing, the clutch pressure plate, the clutch plate, and then the pilot bearing. Cussing is discouraged because clutches are notoriously sensitive.
- Reinstall the clutch components in the reverse order in which you installed them, making sure that the bearing and linkage are lubricated. Keep lubricants off the clutch plate or they won’t get the traction needed to rotate. Finally, adjust the clutch (as described in step 1).
What Your Converter Converts
Manual transmissions have manual clutches, so automatic transmissions should have automatic clutches. Good guess. They do! They are called by assorted names, most commonly torque converter or just plain converter. Like an automatic transmission, the converter is hydraulic and complicated. If your car’s service manual suggests that the converter be replaced, don’t try rebuilding it yourself. Replace it as a unit, typically at the same time the automatic transmission is replaced.
A converter is typically replaced following these steps:
- Disconnect the driveshaft at a U-joint or CV joint, remove bolts holding the front of the transmission to the engine, slide the transmission away from the converter, and then remove the converter from the engine.
- If necessary, replace the old converter with a new or rebuilt unit. Reinstall and adjust the linkage, and then check the transmission for lubricant.
- Test the converter with a visitation to your favorite fast food franchise or the zoo.
Separating Good Joints from Bad Joints
The joints on a driveline (the shaft and joints that connect the transmission with the differential) serve the same purpose as those on your body: They increase flexibility.
Rear-wheel-drive cars need a driveline from 3 to 6 feet long to deliver power from the transmission to the differential and rear axle. They use U-joints to compensate for flex in the driveline.
Front-wheel-drive cars have short drivelines, but they must be more flexible than those in rear-wheel-drive cars. Front-wheel-drive cars use CV joints.
With proper lubrication and maintenance, U-joints last 60,000 to 80,000 miles or more. CV joints, used in frontwheel-drive cars, typically last about 40,000 to 60,000 miles. However, if they get noisy or make a clunking sound as you put the car in gear, it may be time to replace them. Technically, when you replace a U-joint, you are replacing only the bearing unit at the middle of it, an X-shaped part called the trunnion. CV joints have a Y-shaped part called the tri-pot assembly, which serves as the joint.
To replace U-joints, follow these steps:
- Test U-joints for looseness. Under the car, identify the U-joints on the car’s driveline. They are at the joint between the transmission and the driveline (front), maybe somewhere along the drive line (intermediate), and at the end of the driveline where it attaches to the differential (rear). Some cars have double U-joints, two-in-one. With the car in neutral and the parking brake on, turn the driveline back and forth by hand. A clunking sound or lots of movement, called play, in the joint means that the U-joints should be checked and, if necessary, replaced.
- To replace the universal joint, first use chalk, tape, or the end of a nail to mark the current position of each component in the driveline, making reassembly easier. After it’s loosened, the drive line will fall, so have one hand on it or tie a sling around it and to the frame to keep it from falling. Remove the bolts on the center or yoke. Use a large screwdriver to carefully push the two halves apart.
- Remove the clips holding the X-shaped trunnion in the yoke. Be careful not to remove any of the four caps on the ends of the trunnion because they contain small bearings.
- Depending on your car, you might need to replace other U-joints. Follow the same instructions. The front U-joint is typically attached to the transmission through a sliding shaft called the slip yoke and may need a seal removed first. If so, place a pan under the seal to catch transmission fluid or lubricant. The rear U-joint is attached to the differential through a sliding pinion shaft.
- Reinstall the driveline.
To replace CV joints, follow these steps:
- Test the CV joints by listening for front-end clicking noises as the car turns or for vibrations as the car accelerates. The CV joints are located between the transaxle unit and the front wheels. Inspect the CV joint’s rubber cover called a boot. If the boot is damaged but the CV joint isn’t making noise, replace the boot. If the CV joint is noisy, replace the entire unit.
- On most front-wheel-drive cars, the CV joint is replaced as a unit. Follow instructions in the service manual for removing and replacing CV joints. Typically, this means removing the drive axle and then removing the boot clamp and boot to access the CV joint. Parts to the CV joint include the housing, internal tri-pot assembly with roller bearings, and the boot. Replace any part that shows damage or wear.
- Reinstall the CV joint on the drive axle. Then reinstall the drive axle on your car.
Rear End Problems
The differential (illustrated in the next figure) on a rear-wheel-drive car is pretty simple. The end of the driveline is attached to a pinion gear. The pinion gear turns a larger ring gear that, in turn, passes power to gears within the differential case. It’s these differential case gears that decide which wheel gets how much of the power. So what? Let me tell you. The rear wheels must rotate at different speeds during turns. The outside wheel in a turn travels farther than the one on the inside of the turn. The differential tries to give wheels the power they need in turns, which means you can navigate the turn accurately and safely. How do you know when your car’s differential needs repair? If you noticed bits of metal in the bottom of the differential when you last lubricated it, the gears are wearing out. Jack the rear end of the car up and rotate the wheels one direction and then the other, listening for clanking sounds. If you hear them, the differential may be ready for replacement-or at least a closer look.
To replace a differential, follow these steps:
- To inspect the differential gears, place a pan under the differential to catch lubricant. Then remove bolts on the rear side of the differential case. When the case is open, lubricant flows out of it. Carefully touch the lubricant and the bottom of the case with your fingers or a magnet to check for small metal bits that indicate worn gears. Visually inspect the gears for wear, broken teeth, or other damage.
- If repair is necessary, refer to your car’s service manual or an aftermarket manual for instructions. Some differentials are quite easy to repair by replacing one or more gears. Limited slip or other differentials are more complex. In many cases, the axle or at least the wheels must be removed to replace differential gears.
- Reinstall the differential and test it.
Your rear end problems should be resolved-or at least your car’s rear end problems should be resolved.