The Yellow ABS Lamp and the Wet Ardennes Descent
You are driving a 3-axle tractor unit with a semi-trailer, gross combination weight 38 tonnes, on a wet motorway in the Ardennes. The yellow ABS warning lamp has been on since you left the depot; the workshop cannot see you until tomorrow. The road descends for 6 km at 6%. Your retarder is working, the trailer has disc brakes, and the tractor has an engine brake. Traffic is light. The outside temperature is 4 °C and there are patches of fog. Your load is 22 tonnes of bagged cement, evenly distributed but not strapped at the top.
1. Explain the risk of the ABS fault on this descent and how you adapt your driving.
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With the ABS lamp on, the system is disabled or degraded: under braking, one or more wheels can lock, which on a wet 6% descent means loss of steering control and possible jack-knifing or trailer swing. The vehicle is not roadworthy for normal service in this condition; the correct decision is to have it assessed before a long descent, or to use a route avoiding steep gradients and reduce speed substantially if you must move it.
If you continue: select a low gear before the descent, use the engine brake and retarder as the primary means of speed control so the service brakes stay cool, keep a speed well below the limit, increase the following distance, and avoid harsh braking. Do not rely on the ABS. Watch for fog patches and reduce speed further where visibility drops.
2. What is the risk from the unstrapped bagged cement, and how does it interact with braking on this descent?
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Bagged cement is dense and settles; without top restraint, bags can shift rearwards or sideways during braking and cornering, moving the centre of gravity and altering axle loads. On a descent, repeated braking and any wheel lock make a shift more likely, and a load shift can push the trailer sideways or overload the rear axle.
The load must be secured before the journey: straps over the top, or positive blocking, with the bags tight against each other. On the road, treat the vehicle as having a higher risk of instability, brake progressively, and avoid sudden steering. The combination of a degraded ABS and an insecure dense load is a reason to stop and have the load secured and the fault assessed rather than to continue the descent.