Six Percent Grade, Hot Brakes, and an ABS Light
You are driving a 40-foot motorcoach with 38 passengers southbound on a long 6% downgrade. The posted speed for heavy vehicles is 35 mph, but you are at 48 mph in 6th gear. The air pressure gauge reads 105 psi. The ABS warning light has been on since the top of the grade. You smell hot brake lining and see light smoke at the right rear wheel. Traffic ahead is slowing, and a tractor-trailer is about 3 seconds behind you. The next runaway truck ramp is signed 1 mile ahead. The coach has an engine retarder and a Jake brake. Most passengers are asleep. It is 2:15 p.m. and the outside temperature is 98°F.
1. What should you do in the next 30 seconds?
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Reduce speed now, before the smoke becomes a fire. Release the throttle, select a lower gear, and bring in the retarder and Jake brake. Apply the service brakes firmly enough to slow the coach to the posted 35 mph, then release them rather than riding them continuously. Continuous light brake pressure on a long grade builds heat; firm, shorter applications give the brakes time to cool.
Turn on hazard lights, check the mirrors, and assess the right rear wheel for increasing smoke. The tractor-trailer 3 seconds back needs to see your speed change. If the smoke worsens or braking feels weak, take the runaway ramp rather than gambling on the remaining grade. Wake or brief the passengers only after you have the bus under control; panic in the cabin makes a bad situation worse.
2. The low-air warning sounds and the gauge falls toward 60 psi. What do you do?
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Treat this as an emergency. At about 60 psi the warning device should activate, and further loss of pressure can cause the spring brakes to apply automatically. Do not continue down the grade hoping to reach the bottom. Downshift, brake firmly while pressure remains, and get the coach off the road as soon as possible-preferably at a wide shoulder or pullout. Stop, set the parking brake, shut down the engine, and chock the wheels if possible.
Do not fan the brakes or pump them; that wastes air. Call dispatch and report the low-air condition and the hot wheel. If the spring brakes apply on the road, keep the bus under control and stop in the safest available place. Passengers should stay aboard unless there is fire or another immediate danger, but they should be ready to evacuate.
3. Does the ABS warning light mean the coach has no brakes, and how does it change your decisions?
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No. An ABS warning light means the anti-lock function may be disabled, but normal service brakes remain. The risk is that the wheels can lock during a hard stop, especially on a hot, wet, or slick surface, and you may lose steering control.
Given the light, the smoke, and the grade, increase your following distance, reduce speed below the posted limit, and avoid any situation that requires maximum braking. Test the brakes gently at a safe speed when traffic allows. Because you already smell hot lining, the ABS light is not the main problem-the heat is. Stop and inspect the right rear wheel once you are safely off the grade. Continuing with smoke and a possible dragging brake can lead to a wheel fire.