To bypass a vehicle's starter solenoid in order to test its function, create a short across the solenoid's starter and generator wires' contacts, and try to turn the ignition. If the starter solenoid is defective, the starter motor turns on consistently, though the engine doesn't start. If the starter motor doesn't run at all, it's likely the solenoid is functioning but the starter motor is broken.
A car may fail to start due to a defective solenoid. If the engine makes clicking sounds but does not engage when the ignition key is turned, the solenoid may not be transmitting sufficient power to engage the starter motor's gears and needs to be replaced.
To locate the starter solenoid to test it, find the vehicle's starter motor, a tubular device approximately 8 inches long, most commonly on the driver's side of the engine and bolted to the transmission housing. Attached to the starter motor is the solenoid (connected to two wires), the starter wire (which runs from the car's ignition switch), and the generator or jumper wire leading to the starter motor. By placing an insulated screwdriver head or a length of wire between the contacts for these wires, the solenoid is bypassed so that power runs directly from the ignition switch to the starter motor.