Common gearbox problems and diagnosis

Gears are characteristically reliable mechanisms. Unfortunately, however, like all mechanical things, transmissions eventually wear out and untimately fail to act as efficiently as they were designed to. The following points are some of the most common indicators and warning signs that your transmission has encountered problems, or is in imminent danger of failing.

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Noise is the first warning sign of a transmission problem.

The sooner the problem is caught and repaired, the better: fewer parts will be affected and any repairs will be less expensive.

Bearing, gear and shaft wear are the most common causes of noise. As gear tooth faces and bearing races wear, they develop groves and small pits. While these are only small at first, they will gradually become larger, and as they grow, these imperfections will cause even more noise to occur.

Until you've experienced it, it's hard to appreciate quite how much noise a failing bearing or gear can make. The way one person hears a sound is very different from the way another person hears the same sound, and there really is no substitute for an experienced ear.

If you have any concerns relating to the noise your transmission is making, please contact one of our experts immediately.

Whining and howling

Whining that appears relatively suddenly and becomes very loud over short period of time can be indicative of damage to the gear teeth or gear hub bearings.

The most likely source of this damage is a shortage of lubrication. If the oil film on the gear teeth becomes too thin, the teeth will wipe against each other, subsequently destroying the smooth surface on the face of each tooth and causing them to mesh roughly. Even if the oil is replaced or replenished to aid lubrication, the damaged gears will never recover.

Other common sources of gear tooth damage include corrosion and wear. Water in gearbox oil can attack the steel surface of each gear, which can ultimately lead to pitting and abnormal wear.

Changing the oil often is a proven method of avoiding corrosion. Wear of the gear teeth is something that is both inevitable and unavoidable. However, ensuring the gears have lots of clean and suitable lubricating oil can greatly decrease the rate of surface wear.

Rumbling and growling

Low-pitched growling or rumbling noises when the engine is running are usually the sign of a faulty rolling-element (also known as a ball or roller) bearing.

They are extremely sensitive to small bits of metal or dirt in the lubricant film between the bearing elements and cause the rollers to drag across the surface of the race. The more foreign material in the oil, the faster the bearing will wear.

The growling of a bad transmission bearing, like the whine of a bad gear, will become increasingly more noisy while it wears, until the bearing seizes or falls apart, with catastrophic results.

One of the fundamental functions of the bearings in a transmission is to maintain the correct clearance between pairs of gears. If the bearings begin to wear and loosen, the gears can start to mesh improperly and become damaged from the crooked tooth contact. If the bearing falls apart, the results are equally as bad. Small pieces of steel from the broken bearing will travel through the transmission and can get caught between gear teeth. Ultimately, this can smash gear teeth and damage the gears. If one of the bearings seizes, it is highly likely that, in turn, it will take the transmission case with it.

Buzzing and hissing

If the gear shifter or the shift linkage are making a hissing of buzzing noise while the car is moving, particularly while the vehicle is accelerating or decelerating, a loose bolt or worn rubber isolators in the shift linkage is the most common cause.

Other causes of these high-frequency noises include bent shift forks, shift rods, or interlocks, or even excessive movement in the synchronizer sleeves.

In all of these cases, the cause of the noise it is the shift forks contacting the grooves in the sleeves. This causes a hissing noise that travels through the shift linkage to the shift lever.

Banging and clunking

A banging and clunking felt in the shift lever and heard under the car usually indicates a broken or loose motor mount.

Commonly, the noise is loudest when letting out the clutch when taking off from a stop. Alternatively, if the rear transmission is bad in a rear-wheel-drive vehicle, the noise will occur when letting out the clutch in reverse as the tail shaft of the transmission rises up and slams back down.

Broken mounts may also result in a clunking noise when accelerating and decelerating gently.

Another frequent sign of a damaged motor mount is clutch chatter. Contrary to widespread belief, clutch chatter is almost never actually caused by the clutch disc or flywheel; it is usually the result of overly flexible motor mounts, which may be due to design or wear.

Grinding and shifting trouble

The most common shifting complaint is grinding or "crunching" when shifting into gear, which is usually felt as much as it is heard.

The noise itself is caused by the ends of the synchronizer sleeve internal splines banging against the external dog teeth splines because the gear and sleeve are rotating at different speeds. This occurs as a direct result of something failing in the synchronization process. Inefficient synchronizer performance can be caused by either a problem in the transmission, or by a separate clutch issue.

While cone-like-synchronizers are simple and reliable, they rely on friction to function, and as such, the blocking rings degrade over time. They are also easily damaged by "speed shifting" without using the clutch that does not release completely, and imcompatible lubricants.

Synchronizer blocking rings wear badly when the clutch doesn't fully disengage because they are forced to act against higher speed differences than they were designed for.

The grinding will also dull the sharp end of the synchronizer sleeve splines and the gear dog teeth, and wear here contributes to bad synchroniser performance and hard shifting.

Jumping out of gear may be the result of any number of the internal and external problems. End play or preload problems with individual gears on the mainshaft are two of the most common, although linkage issues, worn bearings, and worn synchronizers can also cause jumping out of gear in certain situations.


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