Michigan CDL Combination common mistakes
Michigan CDL Combination · Understand the traps and how to avoid them.
During the vehicle inspection, the candidate walks past the tractor protection valve and trailer spring brakes without identifying or testing them, treating the inspection as a check of lights and tires only.
Candidates memorize the general pre-trip sequence for a single vehicle and skip combination-specific components because they do not appear on a basic truck checklist. They also rush because the inspection is timed, so they focus on obvious items.
What to do instead
Build a fixed combination-specific inspection route: after checking the tractor, trace the air lines from the tractor protection valve to the trailer glad hands, then locate and test the trailer spring brake chambers and slack adjusters. Practice saying each component name aloud and its pass/fail criterion so the sequence becomes automatic.
The candidate releases the parking brake and pulls forward before the trailer supply valve is fully charged, causing the tractor protection valve to pop and the trailer brakes to stay set, then stalls or jerks the vehicle during the basic control test.
They do not understand that the trailer air supply must build to a minimum pressure before the tractor protection valve will stay open. They treat the valve like a simple on/off switch and do not watch the gauge.
What to do instead
Before moving, confirm the tractor and trailer air pressure is at normal operating range, push in the trailer supply valve, and wait until the trailer brakes release (listen for the air exhaust and check the gauge). Practice the sequence in a yard until it is consistent, and state aloud what you are waiting for.
When coupling, the candidate backs under the trailer at an angle or too fast, fails to hear the fifth-wheel jaws lock, and then does a pull test with the trailer brakes released, so the tractor pulls out from under the trailer.
They are nervous about alignment and rush the approach, and they skip the mandatory tug test because they assume the handle click means it is locked. They also forget to set the trailer brakes before the pull test.
What to do instead
Approach straight and slowly, stop when the fifth wheel contacts the trailer, and listen for the click. Set the trailer parking brakes, then do a firm pull test in low gear against the locked trailer brakes. Get out and visually confirm the jaws are closed around the kingpin and the safety latch is engaged before raising the landing gear.
During uncoupling, the candidate lowers the landing gear only after pulling the tractor out from under the trailer, or forgets to chock the trailer wheels, and the trailer drops or rolls.
They follow a memorized order from a single-unit vehicle and do not picture the trailer becoming unsupported once the tractor leaves. They also underestimate how much the trailer can move on a slight slope.
What to do instead
Use a fixed uncoupling sequence: park on level ground, set the trailer parking brakes, chock the trailer wheels, lower the landing gear until it touches the ground and bears weight, disconnect the air lines and electrical cord, then release the fifth-wheel jaws and pull out slowly. Rehearse the sequence in the same order every time.
The candidate makes a right turn too wide or too tight because they steer from the rear of the tractor instead of the trailer, and the trailer tires off-track over the curb or into the adjacent lane.
They have not internalized that the trailer follows a shorter path than the tractor, so they either cut the corner or swing out excessively. They also forget to check the right mirror during the turn.
What to do instead
Before the turn, move to the left side of your lane, then pull forward until your tractor cab is past the curb line before turning. Watch the right mirror continuously and track the trailer tires. Practice in an empty lot with cones set at curb height until you can judge the off-tracking distance.
When backing a tractor-trailer, the candidate turns the steering wheel the wrong way, loses the trailer in the mirror, and then overcorrects until the rig jackknifes.
They confuse the direction the trailer moves with the direction the steering wheel turns, and they panic when the trailer disappears from view. They also back too fast to make small corrections.
What to do instead
Remember the rule: turn the wheel toward the mirror that shows the trailer. Back slowly, using the lowest reverse gear or idle, and make small steering inputs. If the trailer disappears from the mirror, stop, pull forward to straighten, and restart. Practice backing in a straight line and then a gentle curve in a large lot.
During the air brake check, the candidate does not perform the static leakage test on the combination vehicle or ignores the tractor protection valve pop-off test, and cannot explain what a pressure loss means.
They memorize only the single-vehicle air brake test and do not know the additional combination-vehicle checks. They also do not link gauge readings to the system components.
What to do instead
Study and rehearse the full combination air brake test: build pressure to governor cut-out, shut off the engine, and check for static leakage on both the tractor and trailer; then perform the tractor protection valve test by fanning off the trailer air supply and confirming the valve closes at the specified pressure. Say the expected readings aloud as you test.
The candidate fails to recognize and correct a skid because they slam on the brakes or steer too aggressively, turning a simple trailer sway into a jackknife.
They have not practiced skid recovery and react with instinct rather than technique. They also do not know the difference between a tractor skid and a trailer skid.
What to do instead
Learn the recovery sequence: for a trailer skid, release the brakes and steer in the direction you want the trailer to go; for a tractor skid, release the brakes and steer the tractor to follow the trailer. Practice in an empty, wet or slippery lot at low speed until releasing the brakes becomes automatic. Always look and steer where you want to go.
The candidate loads cargo without checking the gross and axle weight limits, places heavy items at the rear of the trailer, and then cannot explain how weight distribution affects steering and braking.
They focus only on the total weight and ignore axle ratings and the effect of the trailer's center of gravity. They also assume that any securement is adequate without checking working load limits.
What to do instead
Before loading, calculate the gross vehicle weight and check each axle against its rating. Place the heaviest cargo over or slightly ahead of the trailer axles, never at the extreme rear, and secure it with rated tiedowns at the required number and angle. Practice explaining how a rear-heavy load lightens the steering axle and lengthens stopping distance.
The candidate follows too closely and misjudges the stopping distance of a loaded combination vehicle, then brakes late and hard, causing the trailer to push the tractor or lock up.
They carry over following distances from a passenger car and do not account for the much longer stopping distance of a heavy combination vehicle. They also fail to look far enough ahead to anticipate hazards.
What to do instead
Use the one-second-per-ten-feet rule as a minimum and add more space in bad weather or when loaded. Look at least 12 to 15 seconds ahead, and brake early and smoothly with light, steady pressure to keep the trailer from pushing. Practice stopping from highway speed in a safe area to calibrate your judgment.