Manual vehicle transmissions require the operator to manually shift through a range of gears, starting from a low gear and progressing to the highest gear, in order to propel the vehicle. By using a foot pedal, a clutch mechanism engages with a pressure plate to transfer the engine power to the drive line then onto the wheels. Sometimes the vehicle will not go into gear properly, which means there is a problem somewhere between the stick shift and the output drive line.
An automotive manual transmission uses a special high viscosity gear oil, unlike that in the crankcase. When the viscosity level thins out, it loses its lubricating qualities, especially under high temperature. The gears cannot rotate on their shafts properly, producing friction which can promote hard shifting or a vehicle that will not go into gear. Dirty, low or contaminated gear oil can have the same effect.
Linkage and Cables
The stick shift on a manual transmission connects with linkage rods and cams attached to the side case of the transmission. The rods and cams have nut and bolt fasteners, or cottar pins, that hold them in place. If these fasteners become loose, the shifting dynamic changes, causing misalignment and non-activation of the levers. No lubrication and broken or detached linkage rods will make shifting difficult or impossible. Vehicles that have shifting cables, which pull on the shifting fork to change the gears, can refuse to shift when a dry cable binds within its sheath or stretches beyond its maximum tolerance.
Synchroniser gears have the same dimensions as the interior transmission gears but only thinner. They retard the speed of the main gear, by forming a "mesh" to bridge to another gear. They function as small transition gears to allow a smooth shift. If the small synchroniser teeth have become worn, cracked or broken off, it will result in a hard or no-shift condition, sometimes accompanied by an audible grinding noise. It is quite common for one synchroniser to fail, which causes a shifting problem in only one gear.
Clutch and Pressure Plate
When the clutch engages, it applies force on the pressure plate, which turns the flywheel. Broken pressure plate springs or a worn or misaligned plate can cause a hard or no-shift condition. A worn clutch disc, or one contaminated with grease or oil, can cause a general to severe slippage and prevent forward or reverse movement of the vehicle.
Motor and Transmission Mounts
Generally, two motor mounts hold the engine in place while a transmission mount secures the tail shaft or transmission case. The motor and transmission mounts have thick rubber dampers that allow for engine torque and stress while also keeping the motor aligned with the transmission. When the mounts break at any location, it causes a misalignment and binding. The binding can bend the transmission linkage, making shifting difficult in one or all gears. A heavy "thunk" sound, which accompanies the release of the clutch in low gear, usually indicates a bad mount.
A manual transmission that has broken or chipped main gear teeth can grind or refuse to engage in gear if the damage has been extensive. This problem involves the gears on the main cluster shaft and can be quite expensive to repair.
Clutch Master or Slave Cylinder
The clutch master or slave cylinder are hydraulically operated components that allow hydraulic force, via the clutch pedal, to push a rod against the clutch fork. The clutch fork then activates the pressure plate. If the clutch master or slave cylinder blows a seal and leaks fluid, it allows air to enter the system. The air produces a non-firm, spongy clutch pedal response. If too much air exists in place of the hydraulic fluid, the clutch will not engage at all, leading to a no-shift condition. Usually, all gears will be affected.