MITO Heavy Auto L4 key terms

The maximum total weight a vehicle is engineered and certified to carry, including the chassis, cab, body, fuel, coolant, driver, passengers, and payload. It is set by the manufacturer and must not be exceeded in service.

The weight of cargo, passengers, and any added equipment a vehicle may carry, calculated as GVWR minus the empty (curb) weight of the completed vehicle. Exceeding it overloads axles, brakes, and tires.

The weight of a vehicle as it sits ready for the road but empty of cargo and passengers, including fuel and fluids. It is subtracted from scale weight to determine actual payload at a weigh station.

The maximum load a single axle or axle group is designed to carry, as specified by the axle or vehicle manufacturer. Each axle must stay within its rating even when total vehicle weight is legal.

A federal rule (23 CFR 658) that limits the weight carried on any group of consecutive axles based on the number of axles and the distance between the first and last axle. It protects bridges from concentrated loads.

The way total vehicle weight is spread across individual axles and tires, determined by fifth-wheel position, body layout, and load placement. Poor distribution can overload one axle while total weight remains legal.

The horseshoe-shaped coupling plate on a tractor that locks onto the trailer kingpin to transmit pulling and vertical load. Its fore-aft position sets how weight is shared between steer and drive axles.

The hardened vertical pin under a trailer's upper coupler that engages the tractor's fifth-wheel jaws. Its diameter and wear limits are inspected because a worn or cracked pin can cause trailer separation.

The structural assembly on the underside of a trailer's nose that contains the kingpin and bears on the fifth wheel. It transfers vertical load and pulling force from trailer to tractor and is inspected for cracks and wear.

A braking system that uses compressed air to apply service brakes and spring-applied parking brakes. Air pressure is stored in reservoirs and controlled through valves, with fail-safe parking springs that apply when pressure is lost.

A brake chamber combining a service chamber and a spring-applied parking section. Loss of air pressure releases a heavy spring that applies the brake, providing fail-safe parking and emergency braking on heavy vehicles.

The lever and linkage that converts brake chamber pushrod travel into rotation of the brake camshaft (or applies the wedge). It is adjusted so pushrod stroke stays within limits; excessive stroke means brakes are out of adjustment.

The distance the brake chamber pushrod travels when the brakes are applied. It must fall within the manufacturer's limits; too much stroke means the brake is out of adjustment and stopping power is reduced.

A quick-coupling fitting at the front of a trailer or rear of a tractor that connects the service and emergency air lines. Color coding (blue service, red emergency) helps prevent cross-connection and brake failure.

A valve on the tractor that closes the supply line to the trailer when pressure drops or the line separates, preserving enough air to stop the tractor. It also prevents trailer air loss from draining the tractor system.

An electronic system that monitors wheel speed and modulates brake pressure to prevent wheel lockup during hard braking. On heavy vehicles it must be functional, and an illuminated ABS lamp is an out-of-service condition.

A system that detects loss of control (yaw, understeer, or oversteer) and automatically applies individual brakes and reduces engine torque to help the driver regain control. It is required on most new heavy trucks.

A system that senses lateral acceleration and wheel lift and intervenes by reducing engine torque and applying brakes to lower rollover risk in curves. It works alongside ABS and ESC on heavy commercial vehicles.

An exhaust aftertreatment device that traps soot particles from diesel combustion. It regenerates by burning off accumulated soot, either passively at high exhaust temperature or actively with injected fuel and elevated temperature.

A urea-water solution (typically 32.5% urea) injected into the exhaust stream to enable selective catalytic reduction of nitrogen oxides. It is stored in a separate tank and consumed continuously during operation.

An aftertreatment system that injects diesel exhaust fluid into the exhaust ahead of a catalyst, converting nitrogen oxides into nitrogen and water. It is central to meeting modern NOx emission limits.

The process of burning accumulated soot out of a diesel particulate filter to restore flow. It may be passive, active, or forced by a service tool, and a parked regeneration requires the vehicle to be stationary and safe.

A deliberate reduction of engine power or vehicle speed by the engine control module to protect components or enforce emissions compliance, such as when DEF runs low or exhaust temperature limits are exceeded.

The vehicle's self-monitoring system that detects faults in engine and emissions components and stores diagnostic trouble codes. Technicians read these codes with a scan tool to guide fault isolation and repair.

A standardized alphanumeric code stored by a control module when it detects an abnormal condition. The code identifies the circuit or system affected and is the starting point for systematic diagnosis.

The controller area network protocol used on heavy vehicles for communication among engine, transmission, brake, and body controllers. It carries parameters such as engine speed, coolant temperature, and fault messages between modules.

A gauge that accumulates engine running time, used for maintenance scheduling on equipment that idles or operates for long periods. Service intervals are often set by hours rather than miles.

A scheduled inspection and service program covering fluids, belts, hoses, brakes, tires, lights, and safety systems at defined intervals. It aims to catch wear before it causes breakdowns or out-of-service violations.

A written or electronic report the driver completes after each day's work, listing defects found during the pre-trip or post-trip inspection. Carriers must review it and certify that repairs were made before dispatch.

The systematic check of a commercial vehicle's critical systems before driving, covering brakes, tires, lights, coupling devices, and emergency equipment. It is required by regulation and is a core skill on the road test.

The standards used by inspectors to place a vehicle or driver out of service for critical safety defects, such as brake adjustment limits or air loss rates. Understanding them helps drivers and techs prioritize repairs.

Federal rules limiting driving time and requiring rest periods for commercial drivers, including the 11-hour driving and 14-hour on-duty limits and the 30-minute break. Compliance is tracked with electronic logging devices.

A device that automatically records driving time by capturing engine data and driver status. It must be registered and compliant, and drivers must be able to present records during a roadside inspection.

A license required to operate certain heavy vehicles, classified by vehicle weight and configuration (Class A, B, or C) with endorsements for hazmat, tankers, or passengers. It requires knowledge and skills testing.

An addition to a CDL that permits driving vehicles carrying regulated quantities of hazardous materials. It requires a background check and knowledge test and imposes special placarding, routing, and parking rules.

A diamond-shaped warning sign affixed to a vehicle or container that identifies the hazard class of the material being carried. Placards alert emergency responders and determine routing and parking restrictions.