The manual transmission explained
Thursday, 27 June 2019
Since the invention of the manual transmission in late in the 18th century, the basic design hasn’t changed much.
And for many drivers today, a ‘stick shift’ still offers the most enjoyable experience on the road.
While the design sounds rather basic, there’s more to a manual transmission than meets the eye.
And to gain a greater understanding of its function, here’s a high-level explanation of the manual transmission.
WHAT A MANUAL TRANSMISSION DOES
Every transmission has the same purpose – to ‘transmit’ engine power to the drive wheels.
The engine’s rotating crankshaft must be harnessed and converted into controllable, scalable energy.
A range of gear ratios necessary to make this happen.
Commonly, you’d find five or six forward gears plus one dedicated to reverse.
Whether a front-wheel-drive or a rear-wheel-drive car, the transmission is always between the engine and the differential.
A manual transmission has a shift lever that links directly from the gearbox to the console for the driver to manipulate.
Unlike an automatic transmission, a manual or standard gearbox requires input to change gears every time.
HOW DOES IT WORK?
Let’s start from the front of the transmission and work our way back, exploring how the system works.
Connecting the transmission to the engine is a clutch system.
The clutch disc uses friction against a flywheel to match the transmission to the engine’s speed.
At rest, the clutch is engaged with the flywheel – the clutch pedal has to be pressed to release the clutch’s grip, allowing the transmission to ‘freewheel’.
To engage the gearshift into gear, the clutch needs to be disengaged – that means the clutch pedal has to be pressed.
When the clutch pedal is released, the bond against the flywheel is restored and the car begins to move.
Changing gears requires that the clutch is disengaged again, and the gearshift can be moved to the next gear you want.
Inside the transmission, every gear has a different ratio, allowing you to control your speed using the engine’s RPMs.
First gear is always a high gear ratio, and the ratios decrease until the highest gear which is usually a 1:1 ratio or less.
Gears are lined up between the input shaft and the output shaft, and a synchromesh matches the gear’s speed with the changing shaft speed.
Reverse can only be selected by coming to a stop first.
Because it’s moving in the opposite direction as the engine’s rotation, a separate countershaft and gear engages with the output shaft to change the direction of the output shaft.
Reverse is always at a very high gear ratio for low speeds.
POTENTIAL PROBLEMS WITH A MANUAL TRANSMISSION
There are fewer parts in a manual transmission than in an automatic tranny. Consequently, there are fewer issues that arise with stick shifts. However, when there are problems, they’re often serious and need to be dealt with sooner rather than later. Symptoms include:
- Grinding gears when shifting. If you’re grinding gears when you shift up or down, you could have problems with a synchromesh. This symptom is often associated with an individual gear, not all of them. If grinding happens with all shifts, the clutch cable could be out of adjustment or the clutch burnt out.
- Slipping on acceleration. If the engine speed is increasing disproportionately fast compared to your speedometer, the clutch could be slipping. It’s a sign that the clutch disc’s friction material is excessively worn, there’s a hot spot on the flywheel, or the clutch has been oil-contaminated from an input seal leak.
- Shifter popping out of gear. If the gearshift won’t stay engaged in the one of the gears, there’s an excellent chance that the transmission fluid is low. It could also be a syncromesh problem or, in rare cases, worn or broken teeth on the internal gear.
- Transmission won’t engage a gear. If one or more gears won’t engage at all, you may have a broken or damaged gear or shaft inside the transmission. It’s a serious problem.
- Internal noises. Whining noises could indicate low manual transmission fluid or a bearing issue, while rattling or grinding noises from inside the gearbox are a sign there are damaged hard parts.
- Oil underneath the gearbox. You have a leak, either from a hose or seal, or even a cracked transmission case.
WHAT DOES IT COST TO FIX?
Luckily, most manual transmission issues can be repaired economically – at least, compared to an automatic transmission!
A clutch replacement could be $400 to $750 while a leaking seal can range from $200 to $1,000.
If the transmission needs to be rebuilt, you may expect a repair bill of $1,500 to $2,500.
A manual transmission replacement, however, would be more.
Expect the replacement cost to be $2,500 to $4,000, depending on the make and model involved.
Your local expert mechanic on AutoGuru can diagnose, repair or replace your manual transmission.
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Jason is a Canadian automotive content writer with a background in the auto service industry, but he’s been hooked on cars and mechanics since childhood.
One of his first cars was an ’80 Mazda RX-7 that’s sorely missed to this day. A ’68 Ford Torino GT, a ’66 Ford Country Squire Woodie station wagon, and a ’96 Suzuki GSX-R 750 have spent time in his fleet of cars, bikes, and trucks over the past two decades.
Jason’s pride and joy is under construction – a turbocharged ’88 Mazda RX-7 convertible. Also on his resume is CASCAR official certification.