Governor

the governor is comparable to a railway switch – it either moves the train off the main line or allows it to remain on the main line. And like a train that never stops when it’s on the main line, the compressor pumps air the entire time the engine is running.

Technician A says that in a tractor/trailer B-train combination, the crack pressures of the relay valves over the pup axles would be the highest on the rig. Technician B says that quick-release valves on the service gladhands can speed up service brake release times on multitrailer combinations. Who is correct?
Technician A says that air disc brakes usually produce higher mechanical braking efficiencies than equivalent S-cam brakes. Technician B says that the reason wedge brakes produce higher mechanical braking efficiencies than equivalent S-cam brakes is that they are usually double actuated. Who is correct?

When the governor routes air to the compressor unloader circuit, what happens?

a.
the compressor drive disconnects
b. 
the compressor intake valves are held open
c.
system air pressure is at its lowest
d.
all of the above

What pressure is a pop-off valve set to trip at in most highway truck applications?

a.
125 psi
b.
governor cut-out pressure
c.
governor cut-in pressure
d. 
150 psi

How often should truck air tanks be drained during normal operation?

a.
weekly
b.
at each PM service
c. 
daily
d.
monthly

When the differential between governor cut-out and cut-in pressures is less than 20 psi, which of the following will more probably result?

a.
unloader valve failure
b.
lower than specified system pressures
c.
extended intervals between compressor effective cycles
d. 
increased compressor cycling frequency

Technician A says that if the electrical supply to an air dryer is interrupted, the dryer purge valve will immediately cease to function. Technician B says that the heater used in many air dryers is cycled by a thermostat. Who is correct?

a.
Technician A only
b.
Technician B only
c.
Both A and B
d.
Neither A nor B

Where does the governor sense air pressure?

a.
compressor discharge
b.
supply tank
c.
primary tank
d.
air dryer

If a truck air compressor cutout pressure is specified at 125 psi, what must the governor cut-in pressure range be?

a.
85 – 100 psi (5.8 – 7 bar)
b.
95 – 115 psi (6.6 – 7.9 bar)
c.
110 – 120 psi (7.6 – 8.3 bar)
d.
100 – 105 psi (7 – 7.2 bar)

Which of the following pressure values are FMVSS-121 compliant for governor cut-out and cut-in pressures?

a.
cut-out at 115 psi and cut-in at 85 psi
b.
cut-out at 135 psi and cut-in at 90 psi
c.
cut-out at 110 psi and cut-in at 80 psi
d.
cut-out at 135 psi and cut-in at 115 psi

What is primarily used to cool air compressors on most truck air brake systems?

a.
alcohol injection
b.
ram air
c.
engine coolant
d.
water

If you have to make an adjustment to air system cut-out pressure, where should the adjustment be made?

a.
dryer purge valve
b.
governor
c.
pressure protection valve
d.
unloader valve

In what year was FMVSS 121 introduced?

a.
1957
b.
1997
c.
1975
d.
1998

Which of the following best describes the role of an air compressor?

a.
an engine
b.
an air pump
c.
an air cleaner
d.
a motor

Technician A says that most truck compressors today are seldom rebuilt in the field today. Technician B says that it most cases it is cost effective to rebuild compressors in the shop rather than stock rebuilt/exchange units. Who is correct?

a.
Technician A only
b.
Technician B only
c.
Both A and B
d.
Neither A nor B

When must a low pressure warning switch activate an alert?

a.
when system pressure drops to 60 psi
b.
as system cut-out pressure
c.
when actuation pressure is 5 psi
d.
at system cut-in pressure

Which of the following pressure value ranges are FMVSS-121 compliant governor cut-out pressures?

a.
60 – 90 psi
b.
115 – 135 psi
c.
90 – 115 psi
d.
120 – 150 psi

Which of the following terms is often used to describe a truck safety pressure relief valve?

a.
emergency relay valve
b.
pop-off valve
c.
quick release valve
d.
inversion valve

A quick release valve with a 2-way check valve provides anti compounding protection preventing park brake and service brake from working at the same time. An Inversion valve is used to protect system from a Primary system pressure failure.

What is the function of the compressor on a truck air brake system?

a.
compress ambient air
b.
clean ambient air
c.
dry ambient air
d.
all of the above

Which of the following components functions to control the compressor effective cycles?

a.
safety pop-off valve
b.
inversion valve
c.
treadle valve
d.
governor

How could a driver determine the air pressure in the secondary circuit?
Selected Answer:
a.
check the secondary pressure gauge needle
b.
check the primary circuit gauge needle and subtract from system pressure
c.
low air pressure warning switch
d.
check the actuation pressure gauge needle

What are S-cam type foundation brakes mounted to?

a.
spider
b.
wheel hub
c.
brake drum
d.
axle spindle

What are the compounded forces that an anti-compounding valve limits?

a.
service and hold-off air pressure
b.
mechanical spring and pneumatic service application forces
c.
primary and secondary air pressure
d.
mechanical and hydraulic force

Technician A says that the steering column mounted, hand control valve actuates the tractor service brakes without applying the trailer brakes. Technician B says that the hand control valve sources its air from the tractor secondary circuit when functioning normally. Who is correct?

a.
Technician A only
b.
Technician B only
c.
Both A and B
d.
Neither A nor B

What service braking is actuated by the primary circuit on a typical tandem drive axle, air brake equipped tractor?

a.
all tractor axles
b.
front brakes
c.
drive axle brakes
d.
emergency braking only

How is the relay (secondary) section of an FMVSS-121 compliant dual-circuit foot valve actuated when the air brake circuit is functioning properly?

a.
by foot pressure alone
b.
pneumatically
c.
both mechanically and pneumatically
d.
mechanically

Air continuously leaks from the exhaust port of a tractor relay valve when the park brakes are released. What should be done?

a.
replace the relay valve
b.
replace the park/emergency quick-release valve
c.
repair the exhaust valve
d.
check the related spring brake chambers for hold-off pressure back-flow

What service braking is actuated by the secondary circuit on a typical tandem drive axle, air brake equipped tractor?

a.
all tractor axles
b.
emergency braking only
c.
front axle brakes
d.
drive axle brakes

Assuming a major air leak in tractor-trailer combination that will result in complete system air pressure loss, which of the following sequence of events will occur FIRST?

a.
tractor inversion valve actuates
b.
spring brakes start to apply when hold-off air drops below 60 psi (4 bar)
c.
trailer supply button pops out
d.
tractor protection valve trips to OFF

Technician A says that when a supply reservoir is completely drained, air pressure in the primary circuit should drop to zero. Technician B says that when the secondary reservoir is completely drained, pressure in the primary reservoir should drop to zero. Who is correct?

a.
Technician A only
b.
Technician B only
c.
Both A and B
d.
Neither A nor B

If a truck is FMVSS 121 compliant, which of the following air reservoirs must be incorporated into the air brake circuit?

a.
front brake and rear brake tanks
b.
wet tank, supply tank, and air start tanks
c.
supply, primary, and secondary tanks
d.
wet tank and purge tanks

An air brake equipped highway tractor features single dash air pressure gauge with two needles, one green and the other red. What does the red needle display?

a.
hold-off pressure
b.
service brake application pressure
c.
secondary circuit pressure
d.
primary circuit pressure

In an S-cam foundation brake, where would you find brake shoe rollers?

a.
on the shoe anchor
b.
S-cam bushings
c.
riding the S-cam profiles
d.
return spring clips

What is the function of a quick release valve when used in a front service brake circuit?

a.
speeds up application time
b.
speeds up release time
c.
intensifies service pressure
d.
all of the above