Clutch Slip Diagnosis

When the clutch disc becomes worn, the clutch will slip because of insufficient friction.

A slipping clutch not only robs your car of performance, it can also leave you stranded if any part of the mechanism fails entirely. But replacing the clutch is a difficult and pricey repair, requiring removal of the transmission to access the inner components. You should diagnose the problem causing slipping issues before attempting repairs. Since different parts of the clutch system exhibit unique symptoms and performance problems when malfunctioning, diagnosis can be performed through a few simple driving tests.

Instructions

1. Park your car on a flat surface with plenty of clear room in front of and behind to allow for safe diagnosis. A long driveway or empty parking lot is ideal.

2. Start the engine with the shifter set in "neutral." Press the clutch pedal in and out several times, noting any inconsistency in the pedal pressure. Inconsistent pressure may indicate a faulty clutch hydraulic system, which can lead to improper clutch activation while driving. If you experience inconsistent pedal pressure, inspect the clutch master and slave cylinders for signs of fluid leaks. If there are no leaks, try flushing and replacing the fluid; air bubbles in the system can also lead to hydraulic issues.

3. Engage the parking brake. Push in the clutch pedal and move the shifter into the highest gear setting. Slowly release the clutch pedal while observing the engine rpm. A severely slipping clutch will allow the engine to remain running when the pedal is fully released. If the clutch is only slipping moderately, the engine rpm will slow as the pedal is fully released. A healthy clutch will grab and stall the engine before the pedal is released all the way.

4. Release the parking brake and drive from a stop as you would in a normal driving scenario, in first gear. If the clutch is slipping, the engine will rev excessively as you release the pedal and increase the throttle. Also note the presence of clutch chatter, which causes the vehicle to sporadically jerk forward when the pedal is released. Clutch chatter can be caused by numerous malfunctions, including excessive heat generated by friction that can lead to a warped clutch disc or flywheel.

5. Find a long, empty roadway on which to test the clutch in driving situations. While driving, shift into the highest gear at low speed, with the engine is running at approximately 1,500 rpm. Then press the throttle pedal all the way to the floor. A slipping clutch will allow the engine to quickly rev up while the vehicle speed remains the same. With a healthy clutch, the engine will slowly accelerate with the vehicle speed.

6. Note any burning smells after normal driving. When a clutch slips, the friction generated will continually burn away the clutch disc material, creating a strong odor similar to burnt plastic. If no smell is present in the cabin, stop the vehicle and open the hood after driving. Because the clutch connects to the rear of the engine, the smell will be much stronger in the engine bay.

Tips & Warnings

• Once a clutch starts to slip, the problem will only get worse as more of the clutch plate is worn away. Replace your slipping clutch as soon as possible to ensure the continued drivability of your vehicle.

References

• Clark's Garage: Troubleshooting -- Clutch
• AA1 Car: Clutch Repair
• Summit Racing: How to Diagnose Common Clutch Problems