310T Trailers — Questions and Answers
Question 1: During a pre-trip inspection, a truck driver checks the fifth-wheel locking mechanism. Which condition would require the vehicle to be placed out of service before moving?
- Grease is visible on the fifth-wheel plate
- The locking jaw does not fully engage around the kingpin and the release handle moves freely (Correct answer)
- The fifth wheel has a 5 mm side-to-side play when the trailer is coupled
- The safety latch is resting against the release handle
Correct answer: The locking jaw does not fully engage around the kingpin and the release handle moves freely
If the locking jaw does not fully close around the kingpin and the release handle still moves freely, the trailer is not secured and could separate from the tractor — an immediately dangerous condition that requires the vehicle to be taken out of service. Grease on the plate is normal and required lubrication. A small amount of lateral play at the fifth wheel is acceptable and expected. The safety latch resting against the handle is its normal locked position.
Question 2: A trailer's tail lights work but the brake lights do not activate. The tractor's brake lights work correctly. Which 7-way trailer connector pin should a technician test first?
- Pin 1 (Ground)
- Pin 4 (Electric Brakes / Battery charge)
- Pin 3 (Left turn / Stop) (Correct answer)
- Pin 7 (Reverse / Aux)
Correct answer: Pin 3 (Left turn / Stop)
On the standard SAE J560 7-way connector, Pin 3 carries the left turn/stop signal and Pin 4 carries the right turn/stop signal. Since tail lights (Pin 5 and 6) work but stop lights do not, the brake light signal circuit (Pins 3 and 4) is suspect. Pin 1 is ground — if it were faulty, tail lights would also fail. Pin 7 is reverse/auxiliary. Starting with Pins 3 and/or 4 targets the failed function directly.
Question 3: A technician is sliding a trailer's tandem axle group rearward. What is the PRIMARY reason a driver would want the tandem positioned fully rearward?
- To maximize the load on the trailer's rear axles for better traction
- To shift weight rearward off the fifth wheel and reduce the drive axle load (Correct answer)
- To improve trailer stability at highway speeds
- To comply with maximum kingpin-to-rear-axle distance regulations
Correct answer: To shift weight rearward off the fifth wheel and reduce the drive axle load
Sliding the tandem rearward shifts load off the fifth wheel (and consequently off the tractor's drive axles) and places more weight on the trailer axles. Drivers do this to reduce drive-axle overloading. Sliding forward concentrates weight over the fifth wheel onto the drives. Stability is not improved by rearward tandem position (it can actually increase trailer sway). Kingpin-to-rear-axle distance regulations exist, but the primary operational reason for sliding rearward is axle-weight management.
Question 4: When inspecting a trailer kingpin, which measurement determines whether it must be replaced?
- The height of the kingpin above the trailer apron plate
- The diameter of the kingpin shank measured at its wear zone (Correct answer)
- The length of the kingpin below the apron plate
- The kingpin-to-apron plate bolt torque
Correct answer: The diameter of the kingpin shank measured at its wear zone
Kingpin shank diameter is the critical wear measurement. Standards (such as CVSA) specify that a kingpin shank worn below a minimum diameter (typically 2 inches / 50.8 mm) must be replaced because excessive wear allows the fifth-wheel jaw to rock, creating coupling slop that can cause trailer separation. Height above and length below the plate are not wear measurements. Bolt torque ensures the kingpin is retained in the apron but does not measure kingpin condition.
Question 5: A trailer's spring-applied, air-released parking brakes release when the trailer supply line is pressurized, but one axle's brakes do not release fully. What is the MOST likely cause on that axle?
- A leaking trailer brake valve
- A seized or corroded spring brake chamber push rod (Correct answer)
- Low air pressure in the trailer reservoir
- A failed glad-hand seal causing air loss
Correct answer: A seized or corroded spring brake chamber push rod
If the air supply is adequate (evidenced by other axles releasing fully), a spring brake chamber that does not release fully indicates a mechanical problem within that chamber — most commonly a seized push rod or corroded brake mechanism that prevents full spring compression. A leaking trailer brake valve would affect all trailer brakes. Low reservoir pressure would affect all axles equally. A failed glad-hand seal would reduce overall supply pressure, again affecting all brakes.
Question 6: Landing gear on a semi-trailer is designed to support the trailer when uncoupled. What type of lubrication is typically specified for landing gear gearbox maintenance?
- Engine oil SAE 15W-40
- Chassis grease (NLGI #2)
- Gear oil such as SAE 90 or equivalent (Correct answer)
- Automatic transmission fluid (ATF)
Correct answer: Gear oil such as SAE 90 or equivalent
Landing gear gearboxes are small enclosed gear sets that require gear oil (typically SAE 90 or a manufacturer-specified GL-4/GL-5 equivalent) to lubricate the worm gear and bevel gears under high load. Engine oil is not formulated for gear mesh pressures. NLGI #2 grease is used for external fittings such as the leg tube, not the enclosed gearbox. ATF is designed for automatic transmissions and is not specified for landing gear gearboxes.
310T Trailers
During a pre-trip inspection, a truck driver checks the fifth-wheel locking mechanism.
Which condition would require the vehicle to be placed out of service before moving?