Understanding Manual Transmissions

In terms of construction, manual transmissions contain two shafts; a primary shaft that is split into two parts/sections that can rotate independently of each other, and a secondary shaft that is made up of a series of fixed gears of various diameters. Note that the secondary shaft is driven by the input shaft, which is the first part of the primary shaft that is driven by the engine via the clutch.

The primary shaft accommodates a series of gears of different diameters that can be moved to different positions on the shaft by slides known as selectors, which are all connected to the gear selector in various ways. Thus, when the engine is running but the transmission is in neutral, the engine drives the input shaft, which in turn, drives the secondary shaft, but since no gears are engaged on the primary and secondary shafts, no torque is passed through the transmission.

However, when say, first gear is selected (with the clutch disengaged), the relevant selector moves the smallest gear on the primary shaft into apposition where it can engage, or mesh, with the largest gear on the secondary shaft. Power now passes through the secondary to the output shaft via a gear on the output shaft that rotates independently of the primary shaft. From the output flange, power is transmitted to the driving wheels through a differential with one or more drive shafts.

When second gear is selected, first gear is deselected before two differently sized gears are meshed, with each successive gear deselected before a new ratio is selected. In practice, each pair of gears has a different ratio relative to each other, and it is these differences that make it possible to increase a vehicles’ road speed in a predictable manner. It should be noted though that there exists a direct correlation between gear ratios, road speed, and the amount of engine power that is delivered to the driving wheels.    

For instance, the high ratio between the two gears that represents first gear means that the vehicle’s road speed is limited, but most of the engine’s power is converted into forward motion. The second-gear ratio is somewhat smaller, but since the vehicle is already moving, less engine power is required to maintain, or increase the vehicle’s road speed. This pattern of progressively reducing gear ratios is continued through the higher gears until the vehicle cruises in its highest gear, where the vehicle’s high road speed and momentum obviates the need for all, or most of the engine’s power to be converted into forward motion.

Manual transmissions have no real disadvantages, except to say that in some situations, such as driving in slow or stop-start traffic, constantly changing gears can become tiring.

Most of the common symptoms of faulty manual transmissions are much the same across all applications, and could include one or more of the following. 

This could happen in cases where selectors either broken, or stuck for some reason. The effect of this is that one gear cannot be deselected before another is selected, and in most cases, the transmission will be stuck in one gear, or it cannot be moved from neutral into any gear.
However, certain clutch failure modes can also produce this symptom. In these cases, the clutch may not release completely when the engine is running, which makes it very difficult, if not always impossible for gears to be selected or deselected.

Mechanical noises could include whining sounds as the result of damaged or worn bearings, grating noises when changing gears as the result of failed synchronisers, or humming noises as the result of damaged gear teeth, which is usually caused by lack of lubrication or dirty/degraded lubricant. Rattling noises can result from a worn or damaged input shaft bearing, as well as from damaged or worn output shaft bushes. 

Fluid leaks on manual transmissions are fairly common, and can be the result of damaged radial oil seals, leaking gaskets, or over filling of the transmission with lubricant.