What is the reason why the motorcycle cannot start and only the relay makes a sound?

[IsabelaMarie]

It may be due to insufficient battery power, oxidized and corroded battery terminals causing poor contact, insufficient power, or failure/damage of the starter motor preventing rotation. Below is relevant information about relays: Introduction: A relay (Relay), also known as an electrical relay, is an electronic control device that consists of a control system (also called the input circuit) and a controlled system (also called the output circuit). It is commonly used in automatic control circuits, essentially serving as an "automatic switch" that uses a smaller current to control a larger current. Therefore, it plays roles in automatic regulation, safety protection, and circuit switching within a circuit. Main Function: Relays are automatic switching components with isolation capabilities, widely used in remote control, telemetry, communication, automatic control, mechatronics, and power electronic equipment, making them one of the most important control components.

[StBeau]

I had the same issue with my old motorcycle before. When starting it, I heard a clicking sound, but the engine didn't respond at all. It's most likely due to a depleted battery, where insufficient voltage causes the relay to engage and disengage repeatedly. Another possibility is that the carbon brushes in the starter motor are worn out, causing it to spin freely without turning the crankshaft. Last time, I took it apart myself and found the battery terminals covered in green corrosion. After sanding them down and reconnecting, it started right up. If the battery is new, check if the grounding wire from the chassis to the engine is corroded—poor contact can prevent current from flowing. Also, oxidized contacts in the starter switch might block the signal from reaching the motor, so don’t just focus on the relay.

[DiCarter]

If the relay clicks, it means the control circuit is working, but the power circuit isn't engaging. I've handled several cars with this issue: use a multimeter to check the battery's starting voltage—if it's below 9.6 volts, it definitely can't power the starter. Directly short-circuit the relay's two large copper terminals with a wrench (be careful not to touch the frame). If the starter spins wildly, it means the relay contacts are burnt. If the starter remains dead, the rotor might be stuck, requiring disassembly to clean out the carbon dust. Pay special attention during the rainy season to prevent water ingress and short circuits in the starter harness connectors. Last year, my neighbor's Kawasaki had poor contact due to water accumulation in the connector.

[SanReid]

From your description, it sounds like insufficient solenoid engagement. When I encounter this, I first tap the starter motor housing (don't laugh, it really works) - sometimes this can shake rusty carbon brushes back into position. Immediately take out your phone and use the flashlight to check the battery inspection window - if it appears black, it indicates severe power loss. Without a voltmeter, try honking the horn - a weak sound basically confirms battery issues. For emergency starts, you can push-start in second gear, but remember to hold the clutch and coast to about 20 mph before suddenly releasing it. In this weather, be especially careful about battery electrolyte freezing - charging a frozen battery will cause it to swell.

[LaJanelle]

Just helped my brother fix his scooter with the same issue last week. Focus on three key areas: Check the battery terminals—if they wobble when you shake them, tighten the screws. Sand the relay contacts to remove black spots. Unplug and replug the motor power cable three times to clear oxidation. If the battery is over two years old, replace it directly—old batteries may seem fine in summer but fail in cold weather. Also, don’t overlook the kick-start lever; a few kicks can sometimes activate rusty bearings. For those who’ve upgraded to HID lights, be extra cautious—cheap rectifiers can damage the battery.

[Queiroz]

The most common cause of repeated relay engagement is a weak battery, but a seized motor bearing cannot be ruled out. I usually start by removing the motor and testing it directly with a battery. If it doesn't turn, disassemble and clean it—carbon buildup on the armature is a frequent culprit. If the motor spins freely, check for engine seizure—remove the spark plugs and crank; lower cylinder pressure makes diagnosis easier. For electrical issues, use a multimeter to measure voltage drop across the wiring. If the voltage at the relay output drops below 6V during cranking, the harness is definitely aged and needs replacement. Vehicles with upgraded high-power audio systems often burn the copper strips behind the fuse box.