How Do Electric Clutches Work on Riding Lawn Mowers?

Clutch Activation

• A 12-volt rotor coil in the rotating PTO side of the clutch generates a powerful magnetic field when the PTO “engage” switch is activated. This interacts with an armature built into the pulley on the drive side of the clutch. The magnetic force supplied by the rotor coils draws in a spring-loaded sliding keyed hub connected to the pulley armature. The keyed hub then mates with a keyway on the rotating PTO shaft and transmits torque from the PTO drive shaft to the armature assembly built into the pulley. This causes the pulley to rotate and transfer power to the cutting deck via the drive belt.

Clutch Deactivation

• As soon as the PTO switch is set to the “disengage” position, power to the 12-volt rotor coil is cut and the magnetic field collapses. This allows the spring-loaded sliding keyed hub to withdraw from the keyway on the PTO shaft, thereby cutting off transmitted torque. In addition, while withdrawing, the keyed hub activates a brake mechanism that stops the deck drive belt pulley immediately.

Clutch Stops and Starts

• After extended use, wear causes the air gap between the rotor coil and the armature to widen. This generates a dangerous amount of heat that could damage the rotor coil. When the temperature rises beyond the safety limit, the bi-metal strip inside a built-in heat override switch flexes and cuts power to the rotor coil. As soon as the clutch cools down, power is restored and the clutch starts working again. This can be corrected by removing the clutch and tightening the three spring-loaded nuts around the clutch perimeter evenly. Do this while sliding a .012-inch feeler gauge into the slots midway between the nuts. When the feeler gauge starts to bind, the air gap is set correctly. However, there are three rivets on the face of the armature side of the clutch that tend to distort the housing. Before making any adjustments, rotate the pulley until the rivets are positioned midway between the feeler gauge slots.

Clutch Stops Working

• This is usually caused by low voltage or a non-existent electrical supply to the clutch. The battery should be checked and charged, as needed. If the battery is dead, replace it before continuing. If the battery is sound, turn the engine off and disconnect the power cord leading to the clutch. Set a mulltimeter to read a 10 amp scale. Insert a probe into one of the holes in the clutch connector and connect the other probe to the corresponding wire in the mating connector. Connect a separate wire to the two remaining connecting points between both connectors to complete the circuit. Turn the PTO switch to the "engage" position. If the meter reads below 4.0 amps, there’s a fault in the tractor's wiring system. This could be a faulty relay, a poor battery connection or a bad PTO switch. Have the system checked out by a qualified technician. If the power supply is above .04 amps, set the multimeter to the ohms setting. Separate the clutch connector and insert a probe into both recesses in the clutch side of the connector. If the multimeter readout is below 2.4 ohms, or if it rises above 2.9 ohms and/or jumps to infinity, the rotor coils have burned out and the clutch must be replaced.