Free ASE School Bus Brakes (S4) Examination Practice Questions

Sample Practice Questions

Try these 10 sample questions from the ASE School Bus Brakes (S4) Examination practice bank. Each includes the answer and explanation. Continue with the interactive practice above to review the full question set.

1. 1A school bus has a Bendix D-2 governor. What does the governor operate to start and stop air compression while the engine continues running?

   1. A.The compressor unloading mechanismCorrect answer: A
   2. B.The compressor crankshaft drive clutch
   3. C.The service relay valve inlet mechanism
   4. D.The reservoir drain valve mechanism

   Explanation: The description identifies the governor as "actuating the compressor unloading mechanism". Thus compression changes through the unloaders even though the compressor continues to run; the other mechanisms do not provide this D-2 control.

   Reference: Bendix D-2 Governor — Service Data SD-01-503 — p. 1, Description

2. 2The HPB service manual permits DOT 3 or DOT 4 brake fluid depending on vehicle specifications. How should a technician select fluid for a particular bus?

   1. A.Use DOT 4 whenever the bus has ABS, regardless of its specifications
   2. B.Use the fluid specified for that vehicle from a new sealed containerCorrect answer: B
   3. C.Use either grade mixed with the drained fluid to match its color
   4. D.Use steering-system fluid when the hydraulic booster is electric

   Explanation: The manual directs the technician to the "vehicle specification sheet" to determine which fluid to use and requires new specified fluid from a sealed container. The general DOT 3/DOT 4 listing does not authorize arbitrary substitution, mixing, or steering fluid.

   Reference: WABCO Hydraulic Power Brake (HPB) System — Maintenance Manual MM0401 — pp. 129–130, Brake Bleeding Procedures, fluid selection and cleanliness instructions

3. 3What electrical signal does a Bendix WS-24 wheel-speed sensor generate as exciter-ring teeth pass it?

   1. A.A fixed DC reference voltage with no speed-dependent frequency
   2. B.An AC signal whose voltage and frequency vary with wheel speedCorrect answer: B
   3. C.A powered digital DC pulse train at a fixed reference amplitude
   4. D.A changing static resistance interpreted as the wheel-speed signal

   Explanation: The sensor produces an "AC signal" as the ring teeth pass, and both voltage and frequency vary with speed. Static resistance is useful for circuit checking but is not the generated speed signal.

   Reference: Bendix ESP EC-80 Electronic Controller — Service Data SD-13-4986 — p. 5, Bendix WS-24 Wheel Speed Sensors

4. 4The dash gauge suggests that a school bus D-2 governor cuts out at the wrong pressure. How should the technician verify the suspected setting error before condemning or adjusting the governor?

   1. A.Accept the dash reading if repeated readings are consistent
   2. B.Check the setting with a known accurate test gaugeCorrect answer: B
   3. C.Average the dash readings over several compressor cycles
   4. D.Adjust the governor until the dash shows its rated pressure

   Explanation: Bendix requires an "accurate test gauge" before governor condemnation or adjustment. Consistency and averaging do not establish gauge accuracy, and adjusting to an unverified dash indication can create an incorrect setting.

   Reference: Bendix D-2 Governor — Service Data SD-01-503 — p. 3, Operating Tests, paragraph before adjustment procedure

5. 5While pressure-bleeding an HPB circuit, the master-cylinder reservoir approaches MIN. Which fluid-level management follows MM0401?

   1. A.Keep bleeding until the reservoir empties to remove all old fluid
   2. B.Return collected bleed-bottle fluid to hold the reservoir at MIN
   3. C.Fill above MAX to avoid needing another level check
   4. D.Replenish with new specified fluid and keep the level above MINCorrect answer: D

   Explanation: During bleeding, fluid must "not be allowed to fall below the MIN mark". Replenish with new specified fluid, without exceeding MAX. Letting it empty admits air; reuse of collected fluid and overfilling conflict with the same service instructions.

   Reference: WABCO Hydraulic Power Brake (HPB) System — Maintenance Manual MM0401 — pp. 129–131, Brake Bleeding Procedures, minimum-level, new-fluid and overfill instructions

6. 6To include both the WS-24 sensor and its vehicle wiring in the EC-80 wheel-speed resistance checks, where does the repair procedure specify taking measurements?

   1. A.Across sensor terminals with the vehicle harness disconnected
   2. B.Across the ECU power and ground terminals only
   3. C.Across two sensor circuits connected in series at the axle
   4. D.Across the relevant sensor pins at the ECU harness connectorCorrect answer: D

   Explanation: The repair test begins "Take all measurements at the Electronic Control Unit" harness connector pins to check the harness and sensor together. Isolating the sensor omits its harness; supply-pin and series-sensor readings do not test the individual sensor loop.

   Reference: Bendix ESP EC-80 Electronic Controller — Service Data SD-13-4986 — p. 28, Speed Sensor Repair Tests, step 1

7. 7An adjustable D-2 governor needs an approximately 4 psi increase, confirmed with an accurate gauge. Which screw movement matches the specified approximate adjustment rate and direction?

   1. A.One-quarter turn clockwise
   2. B.One-half turn clockwise
   3. C.One-quarter turn counterclockwiseCorrect answer: C
   4. D.One-half turn counterclockwise

   Explanation: The procedure specifies "counter-clockwise" to raise the setting and says, "Each 1/4 turn of the adjusting screw raises or lowers the pressure setting approximately 4 psi." Thus a quarter-turn counterclockwise matches the approximate increase; a half-turn changes it about 8 psi, and clockwise lowers it.

   Reference: Bendix D-2 Governor — Service Data SD-01-503 — p. 3, Operating Tests, adjustment step C

8. 8Which approach conforms to MM0401 for bleeding an HPB master-cylinder circuit after replacement?

   1. A.Use the specified pressure-assisted bleeding procedureCorrect answer: A
   2. B.Use ordinary pedal-only manual bleeding as an equivalent substitute
   3. C.Let the system gravity-bleed without following the pressure procedure
   4. D.Run the pumps with the bleeders closed instead of bleeding the circuit

   Explanation: Section 7.2 specifies "pressure-assisted brake bleeding" and warns against manual bleeding procedures. Neither gravity drainage nor pump operation with closed bleeders substitutes for the prescribed circuit bleed.

   Reference: WABCO Hydraulic Power Brake (HPB) System — Maintenance Manual MM0401 — p. 131, section 7.2 — Brake Bleeding Procedures

9. 9An EC-80 wheel-speed DTC identifies excessive WS-24 air gap. What is the first adjustment specified before rotating the wheel and checking sensor output?

   1. A.Move the sensor inward until it contacts the exciter ringCorrect answer: A
   2. B.Position the sensor with a one-millimeter clearance from the ring
   3. C.Move the sensor outward to the widest gap that still produces a signal
   4. D.Leave the gap unchanged if the sensor's static resistance is in range

   Explanation: The excessive-air-gap repair specifies "contact the exciter ring" before wheel rotation and output checks. A selected stand-off distance or maximum signal-producing gap is not that initial adjustment. Normal static resistance does not establish a correct mechanical air gap.

   Reference: Bendix ESP EC-80 Electronic Controller — Service Data SD-13-4986 — p. 27, Wheel Speed Sensors, Excessive Air Gap — Repair Information

10. 10A D-2 governor fails its operating test. Its cover is marked non-adjustable and its adjusting stem has been sheared off. What repair does Bendix specify?

    1. A.Fit a longer screw and reset the pressures
    2. B.Reform the stem and retain the valve body
    3. C.Shim the spring to restore the pressures
    4. D.Replace it with a new or remanufactured unitCorrect answer: D

    Explanation: For this identified non-serviceable governor, the manual states it "must be replaced". The stated combination of cover marking and sheared stem calls for replacement rather than an improvised adjustment or internal repair.

    Reference: Bendix D-2 Governor — Service Data SD-01-503 — p. 3, Operating Tests, non-serviceable governor note