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The main parts of a car transmission are (starting from the engine) the clutch, gearbox, dispatch box (for 4x4 vehicles), the drive-shaft, the differential and the tires. This article will help you diagnose a failing transmission. In the tips section is an overview of symptoms that indicate a problem in these parts.
- Park your car with its front tyres touching the curb. Engage 1st and start slowly releasing the clutch pedal without applying any throttle. The engine should gradually fade out and bog down when the pedal is completely released. If the engine just bogs down at some point, or the fading is not gradual, the clutch is damaged. If the gearbox grinds when you try to shift in first from a standstill, there's a damage in the clutch too.
- Try to pull off in 3rd gear with the front wheels at the curb, and without applying throttle. If the engine doesn't die, it shows a complete clutch failure. In that case, do not drive this vehicle.
- Find a smooth, straight road to test the vehicle. Start from first, and slowly accelerate from second. As you do this use late-shifting, i.e. slightly over-rev the engine (approx. 500-1000 RPM faster than the revs you'd normally shift at). Up-shift to second without using double-clutching. Repeat the same procedure when shifting into 3rd. Now, with your car running at approximately 50 km/h (25 mph) try down-shifting to second without double-clutching. Both the up-shifting and the down-shifting must be done without grinding. Grinding of the gears indicates a gearbox malfunction, most likely in the sync gears ("synchronizers"). To make sure it's the synchronizers, try up-shifting and down-shifting with double-clutching. If the grinding stops, then it's the synchronization.
- Park your car with the front tyres touching the curb with the gearbox in Drive. Start applying throttle smoothly. The car must slowly climb over the curb without the engine fading.
- Shift into drive and hold the break pedal after making sure your brakes work. Press the gas pedal all the way down. The engine should not fade. If it does, it means the transmission (particularly the clutch) does not disengage completely.
- Check for smooth shifting. On an even and relatively horizontal road you should be able to accelerate without any tangible jolts. If there are such, the gearbox has malfunctioned.
- Check for vibrations. Driving at about 70 km/h (35 mph) switch to Neutral (both auto and manual). There shouldn't be any lateral vibrations. If there are, this is either due to a warping of the drive-shaft, or a suspension damage. Basically, drive-shaft warping is perceived as a vibration in both vertical and horizontal direction, whereas a suspension damage is felt as a vibration in only one direction (i.e. either horizontally or vertically).
- Test steering. When trying to enter a corner with approximately 30 km/h (15 mph) there shouldn't be any tangible under-steer. The presence of such may be due to a differential failure, especially in FWD cars. Novice drivers must never try and test their differentials by trying to induce under/over-steer!
Tips for preventing transmission damages
- Avoid prolonged driving by slipping the clutch.
- Avoid jerks and jolts while driving.
- Avoid "riding the clutch", i.e. needlessly keeping your foot on the clutch pedal.
- Never use clutch slipping for regulating the speed of a heavy truck!
- Make sure the clutch of a manual transmission is fully pressed when shifting
- Do not use excessive force when shifting a manual.
- For rear wheel drive (RWD) vehicles, avoid driving through places at the minimum of the vehicle's clearance.
- The clutch is designed to smoothly disconnect the engine from the rest of the drive-train.
- Automatic transmissions have the so-called "hydraulic clutch". It's basically a combination of a hydraulic pump, driven by the engine, and a hydraulic motor, linked to the rest of the drive-train. This allows for the hydraulic liquid to flow through the motor, even if its load is too big for the engine to rotate it. This eases operation, but results in poorer acceleration, greater fuel consumption and severely decreased ability of the driver to use engine braking, which can be very dangerous on long downward slopes. Hydraulic clutches are easier to operate in urban driving, but become a drawback on long roads
- Manual transmissions have a foot-operated friction clutch, where the torque is transmitted and throttled by varying the friction between a leading and a trailing disc. The trailing disc is subject to wear, and is replaceable.
- Malfunctions in a hydraulic clutch include incomplete disengaging (due to old hydraulic fluid, which has become thicker than specified by the manufacturer), or incomplete engaging (most often due to a leak of hydraulic fluid or presence of an air pocket within the hydraulic circuit. These are both dealt with by replacing the hydraulic fluid, bleeding (if necessary) of the hydraulic system, and removing any possible leaks.
- Incomplete disengaging in automatic transmissions is felt as a forward jolt when the gearbox changes gears, whereas incomplete engaging is felt as over-revving the engine without any significant change in speed, especially when stepping on the throttle at high speeds (over 50 km/h or 30 mph).
- Malfunctions in a friction clutch include incomplete disengaging or engaging, complete failure to disengage or engage, increased force necessary for activating the clutch pedal, disengaging too high or too low within the pedal move.
- The incomplete engaging is most often due to worn out trailing disc. Sometimes it's due to improper calibration of the mechanism after a trailing disc has been replaced.
- Incomplete disengaging is due to the trailing disc sticking to the leading one, e.g. because of mechanical soiling of the friction surfaces or worn out springs.
- Increased force necessary for operating the pedal is due to hardened springs of the clutch, or — if your vehicle has a hydraulic-assisted clutch activation — because of a malfunction in the hydraulic system.
- The clutch disengaging too low or too high is an indication of a worn out trailing disc.
- The gearbox is designed to implement the trade-off between speed and power, as to keep the engine within the optimal revs while operating the vehicle at different speeds. It's the second most complex unit in a vehicle after the engine itself.
- Gearboxes come in three types: manual, semi-automatic, and automatic
- A manual gearbox is — as the name implies — operated manually, by moving a lever to select a desired gear.
- Semi-automatic gearboxes are combined with a hydraulic clutch. They allow the driver to select a gear up or a gear down. These are most often seen in rally cars, where there are two levers on both sides of the steering wheel. Usually the right one switches a gear up, and the left one switches a gear down.
- Automatic gearboxes alter the power/speed ratio automatically, either by degrees, or gradually (the latter implement is technically known as a "variator") An automatic gearbox usually has several positions: Park (or Block) (gearbox locked, the drive wheels are blocked, equivalent to switching a manual gearbox in 1st or reverse with the engine off), Drive (automatic switching of gears as the speed changes), Neutral (gearbox disengaged, allowing motion by momentum), Reverse (this one is self-explanatory), 1 (lock in 1st gear), and 2 (start in 1st, then shift into 2nd and do not shift any further). The latter two are used for climbing steep slopes as to avoid the gearbox constantly changing gears.
- There are implements that allow an automatic gearbox to operate in semi-automatic mode, allowing the driver to manually shift gear up or gear down, but w/o using a clutch. These operate exclusively by aids of electronics. This is common in high-class German cars like the S-Klasse Mercedes. Usually the corresponding position of the lever is marked with T or M and the driver selects a gear down by moving the lever to the left, and a gear up by nudging it do the right.
- The most common malfunctions of an automatic gearbox are failing to change gears even with the gearbox put in Drive, improper selection of gear, resulting in over-revving the engine or loss of power, failing to engage at all, failing to disengage when the lever is at Neutral, or failing to block the drive-train when the lever is in Park or Block.
- The most common malfunctions of a manual gearbox are grinding when switching, self-disengaging gears (the lever fails to stay locked), too much free travel of the lever, impossible shifting into first (this is very common in old heavy German machines), impossible shifting into reverse.
- It is possible to shift into first by pressing the clutch, shifting into second, and then moving the lever to first without releasing the clutch.
- In manual gearboxes there's an implement that prevents the driver from shifting in reverse while the vehicle is moving forward. Especially in heavy German machines this fool-protection lock is often damaged due to wear, and it becomes impossible to shift in reverse even with the vehicle at standstill. To solve this, shift in 1st (if the fool-protection for the 1st gear has failed as well, use the previous step), then shift in reverse.
- Sometimes it's normal for a manual gearbox to forbid shifting in first or reverse when the cogs of the gears are facing. To solve this, release the clutch with the engine running, then try again. Whereas it's normal to happen from time to time, constantly being unable to shift into first or reverse is an indication of malfunction.
- Increased grinding when changing gears is most likely due to worn out synchronizers. To determine if this is the problem, try using double-clutching (if you're familiar with the technique). When down-shifting any vehicle weighing more than 1000 kg (2000 lbs), use double clutch as to prevent the synchronizers from wearing out.
- If your manual gearbox disengages by its own will, STOP USING THE VEHICLE! Refer to a repair shop as soon as possible!
- The gearbox is an extremely complex unit, and you should never try and repair it yourself! In most cases it is possible for the gearbox to be repaired, but only by a skilled mechanic. Remember: if you are skilled enough to repair the gearbox yourself, you wouldn't be reading this!
- The most common malfunction of the drive-shaft is evinced as increased vibrations. It's very common for a novice truck driver (e.g. C category according to the EU classification) to warp the drive-shaft due to poor judgment of the vehicle's clearance. A warped drive-shaft must be replaced, not fixed! Sometimes the "Cardan joint" is warped or knocked out of balance. In some cases it can be mended rather than replaced.
- The front and rear tires never rotate at the same speed. The dispatch box in 4x4 vehicles is designed to cope with this. It acts like a differential between the front and rear axles, allowing for the engine to drive both axles at different speeds.
- The most common malfunction of a dispatch box is the inability to change its function (e.g. switch between 4x2 and 4x4) If this happens, refer to a repair shop.
- Increased tyre wear and tyre noise while driving is another indication of a dispatch box malfunction. Same rules as with gearboxes apply. if your dispatch box has malfunctioned, stop using the vehicle!
- The differential is designed to allow rotating of both drive wheels at different speed. This is very important when turning, as the inner wheel has to travel less distance than the outer one.
- Malfunctions in the differential are very rare. They're most often detected as under-steer at low speeds, and severe over-steer at high speeds. Remember to always change the oil of your differential as specified by your vehicle's manufacturer.
- Always make sure the tyres of your vehicle are compliant with what the manufacturer specified. The protector (the furrows of the tyre) must be at least 5mm deep.
- Transmission failure is even more dangerous than a complete brake dysfunction!
- Never use a vehicle with transmission dysfunction!
- Never attempt to repair the transmission yourself unless you're a skilled mechanic with the access to the proper equipment, tools, and safety gear.
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