Retarder and Brakes on a Wet Mountain Pass
You are driving a 5-axle tractor-trailer at 72,000 lb down a 7 percent grade in light rain. The road is wet and the surface is slick in shaded curves. Your tractor has an engine brake (retarder) with three stages and a dash switch. You are in 8th gear at 40 mph. The speed limit is 45 mph and the next curve is posted 35 mph. Your air gauges read 110 psi primary and 105 psi secondary. A sign warns of a brake check area 2 miles ahead.
1. How should you use the retarder and the service brakes together on this grade?
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Use the retarder as the primary speed control and the service brakes for trimming speed, not for continuous dragging. On a wet, slick surface, a high retarder stage can cause the drive wheels to lose traction and skid, so select a lower stage and increase it only if the wheels stay planted. Keep the engine in a gear that holds speed near 35-40 mph without over-revving, and let the retarder do the work on the grade.
Apply the service brakes in short, firm applications to bring speed down before the curve, then release. Avoid light, continuous brake pressure, which builds heat without much slowing. Do not use the retarder alone to make a sharp slow-down; combine a brief brake application with a downshift if needed. Watch the gauges: repeated applications consume air, and the compressor must keep up. If pressure begins to fall steadily, stop and investigate.
2. What are the risks of using the retarder on a wet curve, and how do you manage them?
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The main risk is drive-wheel skid. A retarder slows the drive wheels through the driveline; on a slick surface, that can break traction and cause a jackknife or loss of control, especially in a curve or on a bridge deck. The risk rises with higher retarder stages and with sudden activation.
Management: reduce the retarder to a low stage or turn it off before entering a shaded, wet curve. Slow down before the curve using a brief, firm brake application while the wheels are straight. Keep steering inputs smooth and avoid braking hard mid-curve. If the drive wheels begin to slip, release the retarder immediately and ease off the throttle; do not stab the brakes. Re-engage the retarder only after the curve and only if traction is good. On a long wet grade, accept a lower speed rather than risk a skid.
3. The brake check area is 2 miles ahead. What should you do there, and what would make you stop sooner?
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At the brake check area, stop in a safe spot, set the parking brakes, and do a quick walk-around: feel the hubs and drums for excessive heat, look for smoke or a burning smell, check the gauges for a stable reading, and listen for air leaks. Look at the trailer wheels as well as the tractor. If a drum is much hotter than the others, that wheel may be dragging or the brake may be out of adjustment. Do not touch hot metal; use a heat gauge or a brief hand-near approach.
Stop sooner if: either gauge falls steadily or drops below about 100 psi, the low-air warning sounds, you smell burning brake lining, you see smoke, the truck pulls to one side under braking, or you need an unusually long distance to slow down. Any of these means pull over at the first safe place, not at the scheduled check area. A brake check area is a convenience, not a limit you must reach.