CLUTCH SLIPS WHEN RELEASED
by Mark Davidson

SYMPTOM SUMMARY
The clutch slips when released. The clutch may slip during shifting gears, resulting in an increase in engine RPM and little increase in engine torque that is transferred to the wheels.

ANALYSIS
The usual cause of this symptom is a worn clutch disc. The clutch disc is located between the flywheel, which is attached to the engines crankshaft. The clutch is connected to the input shaft of the transmission. When the clutch pedal is depressed, the pressure plate contracts and seperates the transmission from the engines torque. This is done so that the manual transmission can be shifted from gear to gear. At the point when the clutch pedal is released, the engine is rotating much faster than the transmission. As the pressure plate expands, it places great pressure on the clutch disc while the transmission begins to match the speed of the engine. As this occurs, heat is generated between the surfaces between the pressure plate and clutch disc and the flywheel and clutch disc. For this reason, the clutch disc surface uses a friction material similar to that used on brake pads. During normal operation, the clutch disc wears a slight amount each clutch application. At some point the friction material will completely wear resulting in clutch slippage. Since there is only wear during application of the clutch, which may last only a few seconds at the most, the clutch can last the life of the vehicle, depending on operating conditions. A vehicle driven primarily on the freeway where clutch engagement is infrequent, will last longer than the same vehicle driven primarily in stop and go traffic.

DIAGNOSIS
If the clutch slips in all gears or is worse in first gear, the most likely cause is a worn clutch disc. If the vehicle has low mileage, less than 75,000 miles, the clutch may have worn prematurely due to driving habits or fluid contamination as the result of engine or transmission leaks. A clutch that is worn must be replaced and on almost all vehicles requires the removal of the transmission.

PRECAUTIONS, TIPS, and NOTES
When replacing the clutch, the pressure plate, throw-out, release bearing and pilot bearing should also be replaced. The flywheel should be resurfaced by a trained machinist using the proper surfacing technique. Replacing the clutch can be an involved job that may require the use of special tools. Prior to undertaking this job you should consult a vehicle specific service manual or consult the advice of a professional.

(Mark gave up on sports when the Browns left Cleveland and now spends his Sundays working under a shade tree in the back yard tuning his son’s soap box derby car.)