Module 3: Supply Circuit +8Alberta · CDL · English

Compressor performance should build air pressure from 50 to 90 PSI (345 to 621 kPa) within 3 minutes with the engine running at 1200 RPM (vehicles normally reach this in a minute to a minute and a half). Reasons for slow air pressure build-up include a plugged intake, excessive system leakage, and a faulty or worn compressor.

• ✓Compressor must build air pressure from 50 to 90 PSI (345 to 621 kPa) within 3 minutes at 1200 RPM
• ✓Slow air pressure build-up can be caused by plugged intake, excessive system leakage, or a faulty/worn compressor

The governor controls the compressor as needed to maintain air pressure. It cuts out at 120–135 PSI (828–931 kPa) and cuts in approximately 20–25 PSI (138–172 kPa) less than cut-out. It is generally mounted on the top or side of the compressor head, but can be remote or incorporated in an AD-IS (Air Dryer Integrated Solution) unit.

• ✓The governor cuts out at 120–135 PSI (828–931 kPa)
• ✓The governor cuts in approximately 20–25 PSI (138–172 kPa) less than cut-out
• ✓The governor controls the compressor to maintain air pressure between cut-in and cut-out

The air compressor pressurizes air and pumps it into storage tanks. It can be a single or multiple piston pump, is often installed on the left side of the engine, and is driven directly by the internal gearing of the engine.

• ✓The air compressor pressurizes air and pumps it into storage tanks
• ✓The compressor is driven directly by the internal gearing of the engine
• ✓Compressors can be single or multiple piston pumps

The supply circuit compresses and cleans air, maintains consistent air pressure, and protects from over-pressure. Components include the air compressor, governor, supply tank, drain valve, safety valve, and air dryer (moisture and contamination controls). It is colour-coded blue.

• ✓Supply circuit components: air compressor, governor, supply tank, drain valve, safety valve, air dryer
• ✓The supply circuit compresses and cleans air, maintains consistent pressure, and protects from over-pressure

Air pressure must build from 50 to 90 PSI (345 to 621 kPa) within 3 minutes with the engine running at 1200 RPM — a generous allowance, as vehicles normally reach this in about a minute to a minute and a half. Slow build-up reasons include plugged intake, excessive system leakage, or a faulty/worn compressor.

• ✓Build air pressure from 50 to 90 PSI within 3 minutes at 1200 RPM
• ✓Slow build-up causes: plugged intake, excessive system leakage, or a faulty/worn compressor

The governor controls the compressor as needed to maintain air pressure. It cuts out at 120-135 PSI (828-931 kPa) and cuts in approximately 20-25 PSI (138-172 kPa) less than cut-out. It is generally mounted on the compressor head or incorporated into an AD-IS (Air Dryer Integrated Solution).

• ✓Governor cuts out at 120-135 PSI and cuts in about 20-25 PSI below cut-out
• ✓The governor controls the compressor to maintain air pressure

The air compressor pressurizes air and pumps it into storage tanks. It is a single or multiple piston pump, often installed on the left side of the engine, and is driven directly by the internal gearing of the engine.

• ✓The air compressor is driven directly by the engine and pumps air into storage tanks
• ✓Compressors can be single or multiple piston pumps

A vehicle should build from 50 to 90 PSI (345 to 620 kPa) within 3 minutes with the engine at 1200 RPM; this is a generous allowance and vehicles normally reach it in a minute to a minute and a half.

• ✓Air pressure should build from 50 to 90 PSI within 3 minutes at 1200 RPM
• ✓Vehicles normally build this pressure in about one to one and a half minutes

The air compressor is directly driven from the internal gearing of the engine and can be either single or multiple piston pumps. It creates the compressed air for the system.

• ✓The air compressor is directly driven from the engine's internal gearing
• ✓Compressors can be single or multiple piston pumps

The governor controls compressor loading and is generally mounted on the top or side of the compressor head but can be mounted remotely. If the vehicle has an Air Dryer Integrated Solution (AD-IS), the governor is incorporated there. It uses a control line from the supply tank.

• ✓The governor is generally mounted on or near the compressor head or may be remote/integrated in an AD-IS
• ✓The governor uses a control line from the supply tank to control the compressor

Compressor performance should build air pressure from 50 to 90 PSI (345 to 621 kPa) within 3 minutes with the engine at 1200 RPM; vehicles normally reach this in a minute to a minute and a half. Reasons for slow air pressure build-up include a plugged intake, excessive system leakage, and a faulty or worn compressor.

• ✓Air pressure must build from 50 to 90 PSI within 3 minutes at 1200 RPM
• ✓Slow build-up may be caused by plugged intake, excessive leakage, or a faulty compressor

The governor controls the compressor as needed to maintain air pressure. It cuts out at 120-135 PSI (828-931 kPa) and cuts in approximately 20-25 PSI (138-172 kPa) less than cut-out. It is generally mounted on the top or side of the compressor head, or incorporated into an AD-IS (Air Dryer Integrated Solution) unit.

• ✓The governor cuts out at 120-135 PSI (828-931 kPa)
• ✓The governor cuts in approximately 20-25 PSI below cut-out pressure
• ✓The governor controls the compressor to maintain air pressure

A supply circuit failure can be anything from a plugged/collapsed line to a ruptured supply tank. It is recognized by the air pressure gauges not replenishing when pressure drops below the governor cut-in level. All brakes work properly but are limited by available service tank air.

• ✓A supply failure is recognized by gauges not replenishing below governor cut-in level
• ✓Brakes still work but are limited by air remaining in the service tanks

In a primary circuit failure, spring park brakes remain released using secondary circuit air. A service application applies steering axle and trailer brakes. An inversion valve releases controlled park brake air to apply the drive axle foundation brakes gradually. Use a steady brake application to stop.

• ✓In a primary failure, park brakes stay released using secondary circuit air
• ✓The inversion valve modulates park springs to brake the drive axle in a controlled manner
• ✓Use a steady brake application when a gauge drops or the low air alarm activates

In a secondary circuit failure, park brakes remain released using primary circuit air. This is the only tractor air system failure causing a service circuit's brakes not to work during application—the steering axle brakes will not apply.

• ✓In a secondary failure, park brakes stay released using primary circuit air
• ✓The secondary failure is the only tractor failure where a service application won't work — steering axle brakes will not apply

A park circuit failure causes both service circuits to lose air pressure. Spring park brakes are only half as strong as a full service application; lockup depends on vehicle weight, application sharpness, and traction. ABS only operates on service circuits and won't help. Apply a steady service application and expect springs to apply around 55-60 PSI.

• ✓A park circuit failure causes both service circuits to lose air pressure
• ✓Spring park brakes are only half as strong as a full service application
• ✓ABS only operates on service circuits and will not assist in a park circuit failure

A trailer service line failure is very dangerous—a Super-B at 63,500 kg has braking reduced to that of a 22,500 kg straight truck, dramatically increasing braking distance. Use a steady service application, release around 70 PSI (483 kPa) to rebuild air, and continually downshift. If the trailer must stop quickly, pull the trailer air supply valve (red button).

• ✓A trailer service line failure dramatically increases braking distance
• ✓Release the service application around 70 PSI (483 kPa) to allow air to rebuild
• ✓If trailer service brakes fail and a quick stop is needed, pull the trailer air supply valve

When the emergency line ruptures, all trailer control is lost. The tractor's protection system activates and trailer emergency brakes apply. Trailer axle lockup depends on weight, application rate, and traction. If the trailer loses traction it swings—typically to the right due to pavement crown, or outside in a turn from centrifugal force.

• ✓An emergency line rupture triggers the tractor protection system and applies trailer emergency brakes
• ✓A trailer losing traction typically swings right due to pavement crown
• ✓Proper air line security during coupling and inspections is critical to prevent this failure

Polarized glad hand couplers in good shape cannot be reversed. Reversing lines on spring brake trailers won't release the park brakes. On older air park brake trailers, reversed lines with an empty tank means no service or park brakes, and repeated applications cause park brakes to drag.

• ✓Park brakes will not release on spring brake trailers if lines are reversed
• ✓Reversed lines on older air park brake trailers can cause brakes to drag as air accumulates

After coupling, the driver monitors air gauges for expected pressure loss when pushing in the red trailer air supply valve. If gauges do not drop, the trailer tanks are already full or the lines are reversed. Charge the trailer system during pre-trip and en-route inspections to catch leaks.

• ✓Air pressure loss when charging an empty trailer is expected and normal
• ✓If gauges do not drop after coupling, tanks are already full or lines are reversed
• ✓Charge the trailer system during inspections to detect air leaks early

When the secondary circuit fails, park brakes stay released from primary circuit air. This is the only tractor air system failure that causes a service circuit's brakes not to work during a service application—the steering axle brakes will not apply.

• ✓Park brakes remain released from primary circuit air during secondary failure
• ✓This is the only failure that causes a service circuit's brakes not to work in a service application
• ✓The steering axle brakes will not apply during a secondary circuit failure

A park circuit failure causes both service circuits to lose air pressure. Apply a steady service application and expect spring park brakes to gradually apply as pressure drops. ABS only operates on service circuits.

• ✓A park circuit failure causes both service circuits to lose air pressure
• ✓Apply a steady service application; spring park brakes apply gradually near 55-60 PSI (380-414 kPa)
• ✓ABS only operates on service circuits and will not assist a spring park brake stop

A trailer service line failure is very dangerous, drastically reducing braking capacity. If road ahead permits, use a steady service application, monitor gauges, and release near 70 PSI to rebuild air. Down shift to help rebuild pressure.

• ✓Release the service application when air pressure reaches approximately 70 PSI (483 kPa) to rebuild
• ✓Down shift continually to help the compressor rebuild air faster
• ✓If trailer service brakes fail and a quick stop is needed, pull the trailer air supply (red) button

When the emergency line ruptures, all control of the trailer is lost. The tractor's protection system activates and trailer emergency brakes apply. If the trailer loses traction it will swing, typically to the right due to pavement crown.

• ✓All control of the trailer is lost when the emergency line ruptures
• ✓The tractor protection system activates and trailer emergency brakes apply
• ✓A swinging trailer typically swings right due to the pavement crown; centrifugal force swings it outside in a turn

A supply circuit failure ranges from a plugged line to a ruptured supply tank. It is recognized by air gauges not replenishing when pressure drops below governor cut-in. All brakes work but are limited by available service tank air.

• ✓Supply circuit failure is recognized by gauges not replenishing below governor cut-in level
• ✓All brakes work but are limited by the air pressure available in the service tanks
• ✓Failure could be a plugged/collapsed line or a ruptured supply tank

When the primary circuit fails, spring park brakes stay released from secondary circuit air. A service application still applies steering axle and trailer brakes. Many trucks have an inversion valve that modulates park spring air to apply drive axle brakes.

• ✓Spring park brakes remain released from secondary circuit air during a primary failure
• ✓A service application applies steering axle and trailer brakes
• ✓The inversion valve releases controlled park spring air to apply drive axle brakes

Polarized glad hand couplers in good shape cannot be reversed. Spring brake trailers will not release park brakes if lines are reversed. Older air park brake trailers have problems with reversed lines including dragging brakes.

• ✓Spring brake trailers will not release park brakes if trailer lines are reversed
• ✓On older air park trailers with reversed lines and empty tank, there will be no service or park brakes
• ✓If gauges do not drop when charging, tanks are already full or lines are reversed

A trailer air tank failure is recognized by an unusually low reading on both gauges. The response is the same as a trailer service line failure. Charging the trailer during inspections reveals leaks early.

• ✓Trailer air tank failure is recognized by unusually low readings on both air pressure gauges
• ✓Response is the same as a trailer service line failure
• ✓Charge the trailer during pre-trip and en-route inspections to detect leaks before service

To take the knowledge test at a Registry Agent office, the student must present the original Notice of Air Brake Completion form received after passing the practical test - this original form authorizes the test and cannot be a copy. The knowledge test consists of 30 random multiple choice questions. A passing score is 83 per cent, allowing five incorrect answers; when the sixth incorrect answer is submitted, the test discontinues and the student must retake it another day.

• ✓The knowledge test has 30 random multiple choice questions
• ✓83 per cent is required to pass, allowing a maximum of five incorrect answers
• ✓On the sixth incorrect answer the test stops and must be retaken another day
• ✓The original Notice of Air Brake Completion form is required to authorize the test at a Registry Agent

Students are practised and tested on three categories of tasks. For practice and testing, only the brakes on one axle need to be checked, but during an actual vehicle trip inspection all brakes must be inspected. During the practical test, students may be prompted for the next group of tasks but must not be coached through individual procedures.

• ✓Only one axle's brakes need be checked for the practical test, but ALL brakes must be inspected in a real trip inspection
• ✓The practical test covers three categories of tasks
• ✓Examiners may prompt for the next task group but must not coach individual procedures

The Air Brake Course requires a minimum of 6.5 hours (excluding breaks) of theory instruction using an approved Government of Alberta air brake board including trailer functions, two personally performed practice brake adjustments on a foundation brake display stand, an instructor demonstration of the air brake pre-trip and confirming/maintaining brake adjustment on a tractor-trailer, two student practice pre-trips and two practices of maintaining/confirming adjustment, and a practical test. The classroom session is a prerequisite for the practical training session, which is a prerequisite for the test.

• ✓Minimum 6.5 hours (excluding breaks) of theory instruction is required
• ✓Classroom session is a prerequisite for the practical training session, which is a prerequisite for the practical test
• ✓Students must personally perform two practice brake adjustments and two practice pre-trip inspections

To test for air system leaks, push both park control valves and rebuild air pressure. Turn off the engine. Apply and firmly hold a full service brake application for 2 minutes. The maximum allowable loss is 4 PSI (28 kPa) for power units, plus an additional 2 PSI (14 kPa) per trailer, measured after the system stabilizes.

• ✓Apply and firmly hold a full service brake application for 2 minutes
• ✓Maximum 4 PSI (28 kPa) loss for power units
• ✓Add an additional 2 PSI (14 kPa) allowance per trailer
• ✓Measure leakage after the system stabilizes and with the engine off

Some vehicles must by law stop at all uncontrolled railway crossings: school buses, vehicles carrying explosives as cargo, and vehicles designated for carrying flammable liquids or gas (whether loaded or empty). Obey police or railway flag person directions and be aware municipal bylaws may also apply.

• ✓School buses, explosives carriers, and flammable liquid/gas vehicles must stop at all uncontrolled crossings
• ✓Flammable liquid/gas vehicles must stop whether loaded or empty
• ✓Obey police or railway flag person directions; municipal bylaws may also apply

A controlled crossing has a flag person, stop sign, crossing gate, or an electric/mechanical signalling device. All vehicles are required to stop at controlled railway crossings if signalled to do so.

• ✓A controlled crossing has a flag person, stop sign, gate, or electric/mechanical signal
• ✓All vehicles must stop at controlled crossings when signalled

A person or company operating a commercial truck or bus is a 'motor carrier'. Both provincial and federal NSC legislation may require a Safety Fitness Certificate (SFC), but only one applies at a time. Federal law applies to carriers operating outside Alberta; provincial law applies to carriers operating solely within Alberta.

• ✓Federal SFC required for trucks/trailers over 4,500 kg operating outside Alberta
• ✓Provincial SFC required for trucks/trailers registered at 11,794 kg or greater within Alberta
• ✓SFC required for commercial passenger vehicles seating 11 or more including driver
• ✓Farmers are exempt from provincial SFC but NOT from federal regulations

In Alberta a carrier must establish, maintain and follow a written safety program covering safe use and operation of vehicles. Failure may result in charges, administrative penalties, or SFC cancellation. All drivers must know and follow their company's safety plan policies.

• ✓Carriers must maintain a written safety program
• ✓Safety plan must address speed, seat belts, drug/alcohol use, load security and fuelling
• ✓Drivers must report violations, convictions and collisions to the carrier
• ✓Drivers must be medically fit and not fatigued or impaired

Carriers must maintain a written maintenance and inspection program covering all Alberta-registered vehicles including leased vehicles. Every driver must understand the plan, carry out required inspections, report defects and never operate a vehicle with a safety-jeopardizing defect.

• ✓Carriers must maintain a written maintenance and inspection program including leased vehicles
• ✓Drivers must carry out legally required trip inspections
• ✓Drivers must notify the carrier of any defects found during inspection
• ✓Drivers must not operate a vehicle with a defect jeopardizing safety

The Notice of Air Brake Program Completion is issued after a student successfully completes the classroom session, practical training session and practical test. It authorizes Registry Agents to provide the air brake knowledge test and is valid for 12 months from passing the knowledge test. The original is presented to a Registry Agent with government photo ID. Forms that appear tampered with will not be accepted.

• ✓The Notice of Program Completion authorizes the Registry Agent to provide the knowledge test
• ✓The form is valid for 12 months from the date of passing the knowledge test
• ✓The original must be presented with government issued photo identification
• ✓Tampered forms will not be accepted

All student records must be retained for a minimum of two years. The Air Brake Student Record, Practical Test forms, and the Training School/Agency copy of the Notice of Program Completion must be filed together in the student file. All documents must be available for inspection by Alberta Transportation.

• ✓Student records must be retained a minimum of two years
• ✓Records must be made available for inspection by Alberta Transportation personnel

An A train uses an A-shaped drawbar (A-dolly) that joins a single pintle hitch on the lead trailer, providing two points of articulation.

• ✓An A-dolly has an A-shaped drawbar joining a single pintle hitch point
• ✓An A train provides two points of articulation, one at the pintle and one at the fifth wheel

In a B train the converter is part of the lead trailer, with the fifth wheel assembly on the rear axle. No converter dolly is required as the second unit connects directly to the extended frame.

• ✓In a B train the fifth wheel assembly sits on the rear axle of the lead trailer
• ✓No converter dolly is required because the second unit connects directly to the extended frame of the lead unit

A C train uses an independent converter with two drawbars and two pintle hitches, giving only one articulation point and reducing sway. Double drawbar converters have a self-steering axle requiring correct air pressure.

• ✓A C train has two drawbars and two pintle hitches with only one articulation point
• ✓Check that air pressure on the self-steering axle is within manufacturer standards; low pressure causes instability and skid steering

Adding a second trailer requires a converter mechanism, each with its own fifth wheel. Three converter types exist: A train, B train and C train.

• ✓A train uses an A-shaped drawbar (A-dolly) joining a single pintle hitch, providing two points of articulation
• ✓B train converter is part of the lead trailer with a fifth wheel on the rear axle; no converter dolly required
• ✓C train uses an independent converter with two drawbars and two pintle hitches giving only one articulation point
• ✓C train double drawbar converters have a self-steering axle for improved performance

When linking multiple trailers, always place the heaviest trailer closest to the tractor and the lightest furthest away.

• ✓Hook the heaviest trailer directly to the tractor
• ✓Place the lightest trailer furthest from the towing unit
• ✓This rule applies regardless of trailer length; improper joining makes the unit unstable and the rear trailer will sway

When driving a C train, the self-steering axle air pressure must be maintained within manufacturer standards to prevent instability.

• ✓Check that self-steering axle air pressure is within manufacturer's standards
• ✓If air pressure falls too low, wheels steer too much and the unit becomes unstable, leading to skid steering that can damage the converter and hitch