Clutch System Basics and Clutch Failure Symptoms

If you drive a vehicle with a manual transmission, it will be equipped with a “stick shift” and a clutch pedal so you can operate the clutch and shift gears. A clutch transfers the rotational power of the engine to the transmission, and allows the engine to be temporarily disengaged from the transmission so you can shift. Over time, parts of the clutch can wear out, suffer damage, or break, and this can reduce its ability to engage or disengage the engine and the transmission. Understanding the components of a clutch system and the signs and symptoms of a failing clutch can help prevent you from being stranded and in need of a tow. Read on to find out what makes up most clutch system designs and what symptoms to look for when a clutch starts to fail.

The clutch is a complex system of components which have to work together for the clutch assembly to operate properly. Your clutch is actuated by a clutch pedal inside the cabin that is connected to the clutch using either a cable or a hydraulic line and a master and slave cylinder. The clutch can fail if just one of the components in the system breaks or wears excessively. Knowing how the individual parts of the clutch work may help you diagnose any symptoms that could indicate that the clutch is failing, and can allow you to more easily narrow down the cause of the problem. Below is a list of the major clutch components, moving from the engine side to the transmission side of the clutch, to help you understand what makes up the clutch assembly and how it operates.

Flywheel and Friction Surface

The flywheel is permanently mounted to the rear of the engine crankshaft, and it turns when the engine is running. A clutch flywheel is very similar to the flexplate on vehicles equipped with an automatic transmission, and features teeth around the outside of the flywheel that the starter engages when you start the engine. The main difference between flywheels and flexplates is that a clutch flywheel also provides a friction surface for the clutch plate to contact. The friction surface is machined smooth from the factory, but a slipping clutch plate can cause grooves in the flywheel friction surface that may require flywheel resurfacing or replacement. If you need to have your flywheel resurfaced, see if there is an O’Reilly Machine Shop near you, or shop replacement flywheels.

Clutch Plate and Springs

The clutch plate is installed between the flywheel and pressure plate, and is compressed between these parts when the clutch is engaged. The clutch plate is composed of a disc with friction material around the outside perimeter of both sides of the disc. The friction disc allows the flywheel and pressure plate to hold the clutch plate in place so it can transfer engine power to the transmission. It also prevents catastrophic damage by allowing limited slipping in cases where the engine RPMs are too high during shifts or if too much power is being transferred through the engine to the clutch during acceleration. However, too much clutch slippage can damage or break the clutch plate and create a situation where the clutch plate can no longer be properly held in place by the flywheel and pressure plate.

The center of the clutch plate also includes springs that work as vibration dampeners and provide a cushion as the clutch is engaged to absorb any engine RPM fluctuations. This allows for smoother power delivery as the clutch plate flexes and makes contact with the flywheel and pressure plate. If a clutch spring breaks or comes out of the clutch disc, you are likely to notice a vibration or shudder while shifting gears. In most cases, the springs cannot be serviced independently, and the entire clutch plate will need to be replaced.

Pressure Plate and Diaphragm Springs

The clutch pressure plate is bolted to the flywheel, and is rotating with the flywheel any time the engine is running. When the clutch is engaged, the pressure plate holds the clutch plate against the flywheel so it can transfer the rotational power from the engine to the transmission. The pressure plate includes diaphragm springs that hold the pressure plate in place and help disengage the pressure plate from the clutch plate as the clutch pedal is depressed. This engages the clutch release bearing and fork which presses against the springs of the pressure plate to decouple the clutch. The pressure plate also has a friction surface like the flywheel that is susceptible to wear when the clutch plate slips.

Clutch Release Bearing and Clutch Fork

The clutch release bearing, also known as a throwout bearing, connects the clutch fork to the pressure plate. This bearing allows for the rotational movement of the pressure plate and linear movement of the clutch fork to occur at the same time. When the clutch pedal is depressed, the clutch fork and release bearing put pressure on the diaphragm springs to disengage the pressure plate from the clutch plate, allowing the clutch plate to spin freely so the transmission can be shifted. As with most bearings, the throwout bearing can wear out over time and need to be replaced. If the clutch release bearing or clutch fork wears out or is damaged, you may hear a grinding noise when you press the clutch pedal. In more severe cases, the clutch pedal may even become stuck in the down position, preventing you from driving your vehicle at all.

Clutch Master Cylinder and Slave Cylinder

Some clutches use hydraulic pressure to engage and disengage the clutch, and these systems operate in the same way as another hydraulic system with which you may already be familiar – your brakes. When the clutch pedal is pushed down, the hydraulic pressure generated in the clutch master cylinder is sent to the clutch slave cylinder, which actuates the release bearing to disengage the pressure plate and the clutch plate. Both the master and slave cylinders in your clutch hydraulic systems are simply hydraulic valves at their most basic level. If there is a leak or failure in either cylinder, the clutch will not disengage when the clutch pedal is depressed. This could leave you stuck in gear – or out of gear – and unable to move your vehicle.

Clutch Cable

If your vehicle is not equipped with a hydraulic clutch system, it will use a clutch cable to transfer the motion of the clutch pedal to the clutch fork. The cable or pedal will be spring-loaded to release the fork and help re-engage the clutch when the pedal is released. Over time, a clutch cable can stretch, fray, or possibly even break. A broken or separated clutch cable will need to be replaced since there will no longer be a connection between the clutch fork and the pedal. However, many clutch cables can be adjusted as they stretch out to allow them to continue to operate the clutch appropriately. If you notice that you have to press the clutch pedal down farther than usual before the clutch disengages, the cable may have stretched and you might need to adjust the cable or have other adjustments or service performed on the clutch system.

Because the clutch is a key component to the drivetrain of a vehicle, it can cause significant transmission issues if it suffers wear or damage that can lead to complete clutch failure. There are a variety of signs that can help you recognize when the clutch is wearing out or failing, and knowing what to look, listen, or feel for can help you catch the issue early and prevent more expensive transmission repairs. If the clutch problem is not inspected or addressed, any of these symptoms can lead to more wear or damage to the clutch or transmission that could leave you stranded. Read on for some of the common signs and symptoms which occur as a clutch wears out or fails.

Clutch Slipping

A slipping clutch is usually the first sign that a clutch is wearing out or failing. A failing clutch normally starts slipping during acceleration, and the slipping can worsen as the clutch plate wears out. If you frequently tow with your vehicle, you may first notice that the clutch slips while driving uphill or pulling a heavy load. It can also slip if the engine is over-revved during shifts or if the engine has too much power for the clutch. If the clutch is slipping on your vehicle, you may notice that the engine RPM increases, but the vehicle speed will not increase proportionally. The slipping of the clutch against the metal flywheel and pressure plate can also cause a burning smell, and in some extreme cases, smoke from under the hood or vehicle. A slipping clutch can overheat all of the clutch components, but the clutch plate may experience the worst damage. This excessive friction and slipping can cause premature wear or damage to the friction material on the clutch plate that will eventually cause the clutch to no longer engage properly.

Hard or Soft Clutch

Under normal operating conditions, a clutch pedal should still require a reasonable amount of force to depress due to the resistance in the hydraulic system or cable, fork, bearing, and more that must be moved to disengage the clutch. However, if the clutch pedal feels noticeably softer or harder than normal to press, it can point to a problem with the pressure plate, clutch release bearing, or clutch fork. In vehicles with a hydraulic clutch, a stiff clutch pedal can also point to an issue with the clutch master cylinder or slave cylinder, and in non-hydraulic clutch systems, a bound cable. In any case, it is recommended you have the clutch assembly diagnosed to determine the cause of an especially hard or soft clutch pedal before taking on any repairs.

Noise When Pressing the Clutch Pedal

If you hear a grinding noise when you press the clutch pedal without shifting gears, the problem may be in the clutch, not in the transmission. The grinding noise may indicate that the clutch has a worn release bearing or damaged pressure plate. In severe cases, the noise may be coming from a broken or damaged clutch disc. Since there are several possible causes of clutch noise, you should have the clutch diagnosed as soon as possible to prevent any catastrophic damage that could leave you stranded or lead to more severe or expensive problems.

Clutch Pedal is Stuck Down

If the clutch pedal goes all the way to the floor but will not come back up, or if it sticks part of the way down, there is likely an issue with the hydraulics or linkage going to the clutch release bearing. This may be caused by a broken or stretched clutch cable or an issue with the hydraulic clutch master cylinder and slave valve, depending on the design of your clutch system. There may also be an issue with the release bearing, as a worn throwout bearing might not return to its original position and can cause the pedal to stick in the depressed position. If the clutch pedal is stuck in the down position, it does not mean that the clutch is disengaged, and this can cause a safety issue that can prevent you from starting the vehicle without lurching or disengaging the clutch to stop safely. If you notice your clutch is sticking, you should diagnose the problem as soon as possible and make the necessary repairs to ensure you can continue driving safely.

Transmission Won’t Shift into Gear

If you push in on the clutch pedal but the clutch doesn’t disengage or allow you to change gears, there may be a clutch issue. When your transmission won’t shift into gear, the cause may be a little more difficult to diagnose than other failure symptoms because there are several clutch parts that can fail and cause this issue. There may be problems with the hydraulic system that operates the clutch fork, the pressure plate or clutch disc may be worn or damaged, the clutch release disc may not be engaging the diaphragm springs on the pressure plate, or other issues or damage may have occurred. Since there are a variety of parts that can fail in the clutch system, as with most other clutch failure symptoms, it may be best to have the clutch diagnosed before deciding whether or not you need to replace the entire clutch assembly or simply make minor repairs or adjustments.

The transmission may also need to be evaluated while you are having the clutch diagnosed. There may be an issue with the gears, linkage, or synchronizers inside the transmission that are preventing you from shifting gears. In some cases, low oil levels in the transmission can also cause noise, poor shifting, or accelerated wear, and transmission maintenance or repair may be necessary to ensure your clutch repair solves the problem. Not being able to shift into gear may require more extensive diagnosis and component evaluation than most other symptoms, so it is important to make sure you correctly identify the problem before beginning any repair.

We hope this article has helped you better understand the manual transmission clutch and its failure symptoms. You can shop our line of clutch assemblies and other clutch parts to find what you need to perform a complete clutch repair or clutch replacement. If you have any questions, you can visit any O’Reilly Auto Parts location and our parts professionals will be happy to assist you. If you are not comfortable completing a clutch replacement or repair yourself, we would also be happy to recommend a trusted local repair shop to help you.

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