Home mechanic (basic tools) ยท Time: 20 to 40 min (estimate)
Safety switches (interlocks) stop the bike starting or running in a dangerous state. Typical rules: the starter works only in neutral or with the clutch pulled; the engine stops if you select a gear with the side stand down. Scooters usually need a brake lever pulled to start. On the K-Light 125, the starter relay coil reaches earth through the clutch switch or, through a diode, the neutral switch, and the ECU also reads the clutch, neutral and side stand switches (PDF pages 90 and 254). When a switch or its wire fails, the bike will not start, or it cuts out when you engage a gear.
Safety
- Never cut out or permanently bypass a safety switch. A bike that starts in gear can jump forward, and a stand left down can catch in a corner.
- Key OFF and battery negative disconnected before unplugging switches.
- Put the bike on the centre stand or have a helper hold it while you move the side stand.
You need
- A multimeter on continuity.
- The wiring diagram showing each switch's wire colours and which way each one works (closed when the stand is up, or closed when it is down; this differs between bikes).
Steps
- Check you are using it right. Neutral light on, clutch lever pulled fully, side stand up, brake lever pulled on a scooter. Press start. If it now works, the bike is normal.
- Clutch switch. Find the small switch under the clutch lever. Unplug it. Measure across its two terminals: the reading must change between lever pulled and lever released. If not, clean it or replace it. Check the lever's free play too; too much play can stop the lever reaching the switch. See Adjust clutch lever free play.
- Neutral switch. It sits on the gearbox, often under the front sprocket cover or on the crankcase. In neutral it should connect its wire to earth; in gear it should not. If the neutral light does not work, see Test the neutral switch.
- Side stand switch. It is on the stand pivot. Clean off mud. Unplug its connector. Measure across the terminals with the stand up and down: the reading must change. A broken or bent stand bracket can stop the switch working. See Check the side stand.
- Wiring. Look along each switch wire for chafing on the frame or chain, and for wet, green connectors. Clean them: Clean and seal electrical connectors.
- Diode (if fitted). Some bikes have a small diode in the starter circuit, sometimes plugged into the fuse box. Test it with the meter's diode setting: it should conduct one way only.
- Reconnect, fit any covers, and test again with the key on.
Check your work
- The starter works in neutral, and in gear only with the clutch pulled (if your bike is made that way).
- With the engine running in neutral and the stand down, selecting first gear stops the engine (if your bike has a side stand cut-out).
- With the stand up, the engine keeps running in gear.
If it did not fix the problem
- Switches all work but the relay still does not click: Test the starter relay.
- Bike cuts out over bumps: a loose side stand switch connector is a common cause. See Cuts out when riding over bumps.