Frame and Chassis Flashcards
(112 cards)
How would you check the air parking brake?
Start the engine and wait until the air system reaches system pressure
Puh in the yellow-colored diamond-shaped dash control valve knob to release the park brakes.
It should stay in; if it doesn’t, check for a pressure loss or a defective control valve.
Test the park brakes by putting the truck in low gear, moving it forward at full speed, and applying the park brakes by pulling out on the knobs.
The truck should stop promptly.
What affects the park brake performance?
Park brake performance is affected by brake free stroke adjustment. An excessive free stroke will reduce both park and service brake performance.
What does the governor manage?
Compressor loaded and unloaded cycles. It defines the system pressure.
What values is system pressure in most trucks set at?
System pressure in most trucks is set at values between
120 and 135 pSI, with 125 psi being most typical.
What is cut out pressure?
Cur out pressure is the pressure at which the governor outputs the unloader signal to the compressor.
The unloader signal is maintained until the pressure in the supply tank drops to the cut-in valve.
What is one way to easily check the governor’s operation?
One method is to drop the aiW top of the governor, release the lock-nut and turn the adjusting screw either clockwise to lower or counterclockwise to raise the cut-out pressure.
What happens if the unloader signal not delivered to the compressor unloader assembly?
Will result in high system pressures.
What is usually an indication of governor or compressor unloader malfunction?
If the safety pop-off valve on the supply tank trips (this ours at 150 psi)/
What happens when the compressor unloader cycles?
It triggers the purge valve in the air dryer sump, ejecting moisture and any accumulated sludge.
The purge valve may become clogged and need to be cleaned.
What temperature should the air drier heater operate?
The heater should operate when the temp falls below about 40 degrees F.
How should you test the purge valve?
By cycling the compressor through several cut-in/cut-out cycles, and observing the operation of the purge valve. Each time the unloader cycles, there should be a discharge of air and moisture from the bottom of the air dryer.
How do you check the air system for leaks brakes applied?
- First block the wheels on the vehicle.
- Build air pressure to governor cut-out, release the parking brakes and shut off the engine.
- Walk around the unit, listening for audible air leaks.
- Then check dash gauges for system pressure loss.,
- Investigate any out-of-specification loss.
According to the Commercial Vehicle Safety Alliance, under what conditions shall a vehicle be declared out of service?
A vehicle with more than 20% of its air brakes defective (measured with the engine off, reservoir pressure at 80-90 psi, and the brakes fully applied.
How do you check the air system for leaks brakes released?
- Begin by starting the engine and bringing the air supply to system pressure.
2, With the engine idling, make and hold a full brake application until pressure stabilizes.
If the compressor is unable to maintain reservoir pressure of 89-90 psi with the engine at idle and the brakes fully applied, it is recommended that the truck be deadlined until repairs are completed.
How do you examine the truck for audile air leaks?
Shut off the engine, and have an assistant hold the brakes applied while you examine the truck for audible leaks.
What part of the brake system does the supply tank feed?
The primary and secondary reservoirs of the brake system.
Each is pressure protected by means of a one-way check valve.
How do you verify one-way check valve operation?
Drain the supply tank and watch for pressure loss in the other reservoirs.
How are one-way check valves used throughout an air-brake system?
To pressure-protect and isolate portions of the circuit.
What does two-way (double) check valves do?
Two-way check valves provide a means of providing the primary circuit with secondary circuit air and vice versa, in case of a circuit failure.
Is a T with two inlets and a single outlet.
It outputs the larger of the two-source pressures to the outlet ports and blocks the lower pressure source; the valve will shuttle in case of a change in the source pressure value.
What should the technician do before performing a pressure drop test on a heavy truck?
Ensure the engine is stopped and the system is at full pressure.
How do you perform a pressure drop test on a heavy truck?
Make and maintain a brake application.
Allow pressure to stabilize for one minute and then begin timing for two minutes while watching the dash gauge.
What is the pressure drop for single vehicles?
4 psi within two minutes while watching the dash gauge.
What is the pressure-drop for singles’ combination vehicles?
6 PSI within two minutes.
What federal legislation defines system air buildup times?
FMVSS 121