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What Is a Clutch Sensor?

by John Cagney Nash

Clutch sensors, also known as clutch switches, are a safeguard built into many modern cars and motorcycles. They exist to prevent an engine paired to a manual transmission from being started in gear. If that should happen, the starter motor is prevented by the drive-train from spinning at speeds sufficient to start the engine; rather, it attempts to locomote the vehicle. This is dangerous and almost certain to result in premature starter motor failure.

Circuitry

With the clutch depressed, the sensor is closed by a mechanical contact somewhere on the clutch lever or pedal; the circuit can be made between the ignition key to the starter motor. When the clutch is not depressed the sensor is open, and the circuit cannot be completed.

Neutral Safety Switches

Clutch sensors are similar in function to the neutral safety switches on vehicles fitted with automatic transmissions. Neutral safety switches prevent the starter motor from being engaged if the transmission is in any position other than “Park” or “Neutral.”

Symptoms of Failure

When a clutch sensor fails, the usual symptom is that all the onboard electrical and electronic components function, but the starter motor does not engage when the key is turned to the “Ign” position. This symptom could equally well mean that the starter motor or its solenoid has failed. Have an assistant tap the key into the “Ign” position while you listen at the solenoid; if it makes a loud clicking noise, the problem is the starter motor. If the solenoid does not click, the issue is with the solenoid itself or upstream; if the solenoid tests as working, the next option to consider is a faulty clutch sensor.

Clutch sensors are simple mechanical switches, typically held in place by two screws and attached to only two wires. Consult your workshop manual to learn the location of the switch. Test by bridging the contacts with a multimeter. Clutch sensors seldom have serviceable parts; a failed unit must be replaced.

On motorcycles, there is another component that may have failed. Some machines are fitted with side-stand cut-out switches, which stop the engine if the transmission is shifted into gear with the side-stand deployed; if a side-stand cut-out switch fails, the symptoms can mimic a failure of the clutch sensor. Test both before replacing either.

Failsafe

The normal state of a clutch sensor is open. It is only closed, and therefore only allows the starter circuit to be made, when it is mechanically closed by the depression of the clutch. This being the case, the switch will almost invariably fail in the open position. While this can be an annoyance in that it prevents the vehicle from starting, it is far less of a hazard than the results of trying to propel the entire vehicle with the starter motor.

As a “get-home” measure, the clutch safety switch can be bypassed by disconnecting its two wires and joining them together. This measure will allow the starter motor to engage, but it can then do so in any of the drive gears: This is dangerous, and should only be done to move the vehicle to a place where a proper repair can be made. Tape a note to the steering wheel clearly stating what has been done in case any other drivers should try to use the vehicle.

About the Author

John Cagney Nash began composing press releases and event reviews for British nightclubs in 1982. His material was first published in the "Eastern Daily Press." Nash's work focuses on American life, travel and the music industry. In 1998 he earned an OxBridge doctorate in philosophy and immediately emigrated to America.

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