Diagnose an ABS warning light

An anti-lock braking system (ABS) watches each wheel's speed with a sensor that reads a toothed ring (tone ring) on the hub. If a wheel starts to lock, a control unit (the modulator) eases the brake pressure for a moment, then reapplies it. When the ABS finds a fault, it switches itself off and lights the ABS warning lamp. The normal brakes still work, but without anti-lock. Reading the fault code and working on the modulator needs the maker's diagnostic tool, so this is a workshop job. The K-Light 125 reference bike has CBS, not ABS, so no ABS values are given here.

Safety

• With the ABS light on, ride as if the bike has no ABS: brake gently and early, especially in rain and on gravel.
• If the brake lever or pedal also feels different (soft, sinking, hard), do not ride. Go to Brake lever or pedal feels spongy.
• Never bleed an ABS system with a simple bleed if the modulator has been opened or has run dry. Many need a tool to cycle the valves.

What you can check safely

1. Learn the normal pattern from your owner's manual. On many bikes the ABS light comes on at key on and goes out only after the bike moves off and passes a low speed. That is normal, not a fault.
2. Check the light goes out after a short ride in a straight line. If it stays on, or comes on while riding, there is a stored fault.
3. Look at each wheel sensor and tone ring. Clean off mud, metal chips or chain grease. Check the ring is not bent, cracked or missing teeth, and the sensor is tight in its bracket.
4. Follow the sensor wire from the wheel. Look for cuts, crushed sections near the fork, swingarm or axle, and loose plugs.
5. Check the ABS fuses with Check fuses and relays.
6. Check battery condition: low voltage can set ABS faults. See Test and charge a 12 V battery.
7. Check tyre sizes and pressures. Wrong size tyres or a wrong sprocket change wheel speeds and can confuse the ABS (Check tyre pressure).
8. Spinning the rear wheel fast on a stand, or a wheelie, can also set a fault on some bikes. Switch off and on, then ride: if the light clears, note it and watch.

What the workshop will do

• Read the stored fault code with the maker's tool and follow the maker's chart.
• Measure sensor gap and signal, check wiring and connectors, and replace the sensor or tone ring if faulty.
• Test and, if needed, replace or bleed the modulator with the maker's procedure.
• Clear the code and road test.

What to ask for

• The fault code and what it means, in writing.
• A check of sensors, tone rings and wiring before any modulator replacement (sensors and wiring are much more common and cheaper).

If it did not fix the problem

• Light stays on after cleaning and checks: the bike needs the maker's diagnostic tool. Ride carefully to the workshop as a non-ABS bike.
• See ABS warning light is on.