The 6 Percent Grade: ABS Light and a Pulsing Pedal With 55 Aboard
Route 8 drops 1,200 feet in two miles at 6 percent. The posted limit is 45 and the pavement is wet with crushed leaves. Bus 8 is an air-brake coach with ABS, carrying 55 students at close to gross weight. At the top you are at 40 mph in the gear that holds 35 on this grade. A mile down, the pedal pulses hard and you hear a rapid growl as the bus slows; at that same moment the amber ABS light on the dash comes on and stays on. The bus is now at 50 mph and the next curve is posted 30. Traffic is close behind you and the wind is gusting across the lanes.
1. What are the pulse and the growl, and what does the light mean? What do your hands and feet do right now?
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The pulse and growl are ABS working: the controller is releasing and reapplying pressure at a wheel that was beginning to lock on the wet leaves. That is normal, and the correct response is to keep firm, steady pressure on the pedal, keep both hands on the wheel, and steer where you want the bus to go. Do not pump, do not stab, and above all do not release the pedal because it feels strange.
The light is the important part. It means the controller has detected a fault and has disabled ABS, at least on the affected wheel - the pulsing may stop and the base brakes still work, but the wheels can now lock. So you brake earlier, lighter, and in a straight line, with more room ahead, because your margin against a skid just shrank with 55 students aboard.
Your immediate job is to get back to the gear's holding speed without locking a wheel.
2. How does the light change your plan for the rest of the descent?
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Slowly and deliberately, before the curve rather than in it. Shed speed with controlled, firm applications to get back to 35 or below, then downshift to the gear that holds the bus at or under 30 - the gear you should have been in at the top - so the engine does the holding and the brakes stay cool. On a 6 percent grade at gross weight, riding the brakes heats them and the next thing you lose is stopping power, which is the one thing you cannot lose with a failed ABS.
If the speed will not come down to the gear's holding speed, use the escape ramp or the next pull-off rather than gambling on the curve at 50. Do not use the parking brake to slow a moving bus. Keep the space ahead open, and if traffic behind is tailgating, let it be tailgating - their impatience is not a reason to enter a 30 mph curve at 50.
3. The bus finishes the route safely. What has to happen before it goes out again?
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The ABS light has to be written up as a defect on the DVIR in specific terms - 'ABS warning lamp illuminated and stayed on; pedal pulsed and released at approximately 50 mph on wet pavement' - and the bus goes to the shop for a wheel-speed sensor and wiring check. Do not clear it because the light went out at the bus lot; intermittent ABS faults are exactly what a sensor or connector problem looks like.
Tell the next driver directly, and tell dispatch, so the bus is not quietly released for an afternoon run. A DVIR that says 'ABS light on' is the difference between a fleet with a maintenance history and a fleet where every driver assumes somebody else reported it.
Note the conditions too: wet leaves on a 6 percent grade at gross weight is a real-service test, not an abuse of the equipment.