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ASE H6 Electrical/Electronic Systems (Transit Bus) study materials
ASE H6 Electrical/Electronic Systems (Transit Bus) study resources

ASE H6 Electrical/Electronic Systems (Transit Bus) practice questions

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Question 1Battery and Starting Systems

The cold cranking ampere (CCA) rating of a bus battery describes its ability to:

  1. Deliver 30 seconds of cranking current at 0°F while staying above 7.2 volts Correct answer.
  2. Withstand a specified number of hours of vibration on a shake table without physical damage
  3. Accept recharge current from the alternator at a controlled taper rate
  4. Sustain a 25-ampere load at 80°F until terminal voltage falls to 10.5 volts

Explanation

Cold cranking amperes are measured at 0°F (-18°C): the battery must deliver the rated current for 30 seconds while terminal voltage stays at or above 7.2 volts on a 12-volt battery. The rating describes short-duration cranking power in cold weather, and nothing else.

Question 2Battery and Starting Systems

A 12-volt battery that has rested for several hours reads 12.2 volts open-circuit. This indicates a state of charge of approximately:

  1. About 25 percent
  2. About 100 percent
  3. About 50 percent Correct answer.
  4. About 75 percent

Explanation

Open-circuit voltage tracks state of charge on a rested battery: about 12.6 volts is fully charged, 12.4 is roughly 75 percent, 12.2 is roughly 50 percent, and 12.0 is about 25 percent. A 12.2-volt reading therefore means the battery is about half charged and should be recharged before testing.

Question 3Battery and Starting Systems

A gear-reduction starter differs from a direct-drive starter mainly in that it:

  1. It mounts the shift solenoid inside the motor housing instead of on top
  2. It uses a planetary gear set between the armature and pinion Correct answer.
  3. It uses two field coils instead of the four used on direct-drive units
  4. It eliminates the need for a drive pinion and an overrunning clutch assembly

Explanation

A gear-reduction starter places a planetary gear set between the armature and the pinion, letting a smaller motor spin fast and produce high cranking torque at lower current. Direct-drive starters transmit armature torque straight to the pinion without that reduction.

Question 4Battery and Starting Systems

The primary function of the solenoid on a heavy-duty starter is to:

  1. Regulate charging voltage supplied to the battery by the alternator
  2. Engage the drive pinion and close the motor circuit Correct answer.
  3. Limit the current that flows through the field windings during cranking
  4. Convert alternating current from the alternator into direct current

Explanation

On most heavy-duty starters, energizing the solenoid pulls the drive pinion into mesh with the flywheel ring gear and closes the heavy contacts that feed the motor. One unit therefore controls both engagement and motor current.

Question 5Battery and Starting Systems

A bus with an automatic transmission cranks in both Park and Drive. The most likely cause is:

  1. A shorted contact disc inside the starter solenoid
  2. A misadjusted or faulty neutral start switch Correct answer.
  3. An open circuit in the starter solenoid hold-in winding
  4. A defective overrunning clutch inside the starter drive assembly

Explanation

The neutral start switch is wired into the starter control circuit so cranking is possible only in Park or Neutral. If the engine cranks in a drive range, the switch is out of adjustment, bypassed, or internally shorted.

Question 6Battery and Starting Systems

With the engine at fast idle and all accessories off, a healthy 12-volt charging system should read approximately:

  1. 12.0 to 12.4 volts
  2. 11.0 to 11.8 volts
  3. 15.5 to 16.5 volts
  4. 13.5 to 14.5 volts Correct answer.

Explanation

A healthy 12-volt charging system holds roughly 13.5 to 14.5 volts at the battery with the engine above idle and no loads. Readings near 12 volts mean the alternator is not charging, while readings above about 15 volts indicate overcharging.

Question 7Battery and Starting Systems

Battery hold-down brackets and trays are important because they:

  1. Reduce the battery's internal resistance when the engine is cold
  2. Increase the cold cranking ampere rating printed on the label
  3. Prevent vibration damage to plates and internal connections Correct answer.
  4. Keep the electrolyte from freezing during cold weather operation

Explanation

Hold-downs and trays keep the battery from moving, which prevents plate shedding, internal connection fatigue, and case cracking. A loose battery may pass a capacity test when new but fail early from vibration damage.

Question 8Battery and Starting Systems

Which condition increases voltage drop in a battery cable?

  1. Using a larger gauge cable for the same run
  2. Cleaning and tightening the terminal connections at both ends
  3. Shortening the cable run between battery and starter
  4. Increasing the length of the cable run Correct answer.

Explanation

Cable voltage drop rises with length and current and falls with larger conductor size. Long or undersized cables, or corroded connections, add resistance that starves the starter and slows cranking.

Question 9Battery and Starting Systems

Hydrometer readings on a 12-volt battery show five cells at 1.250 and one cell at 1.140. The technician should:

  1. Replace the battery; the low cell indicates internal damage Correct answer.
  2. Add electrolyte to the low cell and return the bus to service
  3. Test the alternator AC ripple output with a digital multimeter
  4. Perform an equalizing charge and return the battery to service

Explanation

A spread of 0.050 or more in specific gravity between cells indicates a failed cell that will not respond to charging. Because the cells are in series, the weak cell limits the entire battery, so replacement is required.

Question 10Battery and Starting Systems

Two 12-volt, 100 Ah batteries are connected in series. The resulting bank is:

  1. 24 volts at 100 Ah Correct answer.
  2. 24 volts at 200 Ah
  3. 12 volts at 200 Ah
  4. 12 volts at 100 Ah

Explanation

Series connections add voltage while amp-hour capacity stays the same because the same current flows through both batteries. Two 12-volt, 100 Ah batteries in series therefore make a 24-volt, 100 Ah bank.

Question 11Battery and Starting Systems

A conductance tester determines battery condition by:

  1. Measuring the battery's response to a small AC signal Correct answer.
  2. Measuring only the open-circuit voltage after a rest period
  3. Applying a 15-second load equal to half the CCA rating
  4. Measuring the specific gravity of the electrolyte in each cell

Explanation

A conductance tester injects a small AC signal and measures how the battery responds, which correlates with plate surface area and internal resistance. It produces a fast pass/fail decision without the heat and time of a full load test.

Question 12Battery and Starting Systems

During a capacity test, a 12-volt battery loaded to half its CCA rating should:

  1. stay above 12.0 volts for 15 seconds at room temperature
  2. stay above 9.6 volts for a full 60 seconds at 70°F
  3. stay above 9.6 volts for 15 seconds at 70°F Correct answer.
  4. stay above 10.5 volts for a full 60 seconds at 70°F

Explanation

The capacity test loads the battery to half its CCA rating at 70°F and requires terminal voltage to stay at or above 9.6 volts for 15 seconds. A battery that fails this test cannot support cranking and should be replaced.

Question 13Battery and Starting Systems

A bus shows 250 mA of parasitic draw after all modules have gone to sleep; the specification is 50 mA maximum. The best next step is to:

  1. Add a second ground strap between the engine block and the frame rail
  2. Measure voltage drop across fuses to isolate the offending circuit Correct answer.
  3. Install a higher-output alternator to cover the additional parasitic load
  4. Replace the battery with a unit that has a higher cold cranking rating

Explanation

Once the parasitic draw exceeds specification, the technician narrows it down by measuring the millivolt drop across fuses, or by removing fuses one at a time, to find which circuit holds the draw. This identifies the offending circuit without replacing parts by guesswork.

Question 14Battery and Starting Systems

With the key in START, the starter relay has battery voltage at the coil feed but no ground at the coil's other terminal. This indicates:

  1. The relay contact disc has welded itself closed inside the housing
  2. An open in the control circuit ground side Correct answer.
  3. The battery is discharged and cannot pull the relay in
  4. The hold-in winding inside the starter solenoid has failed open

Explanation

The relay coil needs both a feed and a ground to close. Battery voltage on one side with no ground on the other points to an open in the control side, such as the neutral start switch, the key switch, or a ground connection.

Question 15Battery and Starting Systems

A battery stored for months at partial charge now refuses to accept charge and shows high internal resistance. It most likely has:

  1. A shorted diode in the alternator rectifier bridge
  2. Loose hold-down hardware allowing the case to vibrate
  3. Hard sulfation on the plates Correct answer.
  4. An electrolyte level that was maintained too high

Explanation

A battery held at partial state of charge forms hard lead sulfate on the plates that ordinary charging cannot reverse. The result is high internal resistance, low capacity, and refusal to accept charge.

Question 16Battery and Starting Systems

When selecting a master battery disconnect switch, the rating that matters most is its:

  1. Resistance to electrolyte spillage and to continuous road vibration
  2. Continuous and intermittent current ratings versus cranking current Correct answer.
  3. Voltage rating alone, since the battery cables already limit the current
  4. Current capacity measured at one ampere of relay coil control current

Explanation

The disconnect switch carries all cranking current and the alternator's recharge current, so its continuous and intermittent ratings must exceed those values. An underrated switch will overheat and develop high resistance.

Question 17Battery and Starting Systems

During cranking, voltage drop from the battery negative post to the starter case measures 0.9 volt; the specification is 0.2 volt maximum. This indicates:

  1. A normal reading on a 24-volt series battery system
  2. Excessive resistance in the positive battery cable
  3. Excessive resistance in the ground circuit Correct answer.
  4. A defective contact disc inside the starter solenoid

Explanation

Voltage drop testing measures resistance in the path being tested, so a drop from the battery negative post to the starter case measures the ground side. Anything above the manufacturer's limit, typically 0.2 volt, indicates a bad ground strap, cable, or connection.

Question 18Battery and Starting Systems

A starter draw test shows 550 amperes against a specification of 350 to 450 amperes; the battery passes a load test and circuit voltage drops are within spec, but cranking speed is low. The most likely cause is:

  1. A battery that is discharged below 50 percent state of charge
  2. High resistance in the battery ground cable or strap
  3. An open field winding inside the starter motor
  4. Excessive internal drag in the starter motor Correct answer.

Explanation

Higher-than-spec current draw with slow cranking, a good battery, and acceptable cable drops points to a problem inside the starter, such as a dragging bearing, a bent armature, or shorted windings. Low draw with slow cranking would suggest the opposite, an open winding or added circuit resistance.

Question 19Battery and Starting Systems

The reserve capacity rating of a battery is the time it can:

  1. Supply 25 amperes at 80°F down to 10.5 volts Correct answer.
  2. Accept recharge current at a controlled 14.4 volts
  3. Crank at 0°F while staying above 7.2 volts for 30 seconds
  4. Withstand vibration on a shake table before the plates fail

Explanation

Reserve capacity is the number of minutes a fully charged battery can supply 25 amperes at 80°F before terminal voltage drops to 10.5 volts. It predicts how long the bus can keep running its electrical loads if the alternator fails.

Question 20Battery and Starting Systems

Heavy white-green corrosion is present on a battery terminal. The correct service procedure is to:

  1. Rinse the corroded terminal with plain water while the cable is still connected, then apply grease over the corrosion
  2. Loosen the clamp and pour baking soda solution over the battery case and terminals to neutralize them
  3. Disconnect, neutralize and brush the post and clamp, then coat the reassembled joint Correct answer.
  4. Coat the corroded terminal with petroleum jelly without removing the cable clamp from the post

Explanation

Corrosion is removed by disconnecting the cables, neutralizing the acid salts with a baking soda solution, brushing the post and clamp, rinsing and drying, then coating the reassembled joint. Leaving corrosion in place raises resistance and lets the connection degrade further.

Question 21Battery and Starting Systems

A starter motor spins at normal speed but the engine does not crank, and no grinding is heard. The most likely cause is:

  1. A badly worn ring gear on the engine flywheel
  2. A slipping overrunning clutch in the drive assembly Correct answer.
  3. A battery discharged below 11 volts
  4. An open hold-in winding inside the starter solenoid

Explanation

When the motor spins but no torque reaches the flywheel and there is no grinding, the pinion is meshing but the overrunning clutch is slipping. Grinding would instead point to a misadjusted pinion or damaged ring gear teeth.

Question 22Battery and Starting Systems

A battery in a 24-volt bank is hot to the touch, gassing heavily, and its case is swelling while on charge. The correct action is to:

  1. Stop charging, isolate the battery, and let it cool Correct answer.
  2. Increase the charge rate to break up the sulfation on the plates
  3. Add distilled water to bring the electrolyte above the plate tops
  4. Check specific gravity immediately to confirm the state of charge

Explanation

Heat, heavy gassing, and a swelling case during charging are signs of thermal runaway, where internal heat generation accelerates out of control. The charger must be shut off and the battery isolated and allowed to cool before any further testing.

Question 23Battery and Starting Systems

When jump-starting a 24-volt series battery system from another 24-volt vehicle, connect:

  1. The jumper cables across any one 12-volt battery in the bank
  2. Positive to positive and negative to negative at the 24-volt terminals Correct answer.
  3. Disconnect the battery equalizer before attaching the jumper cables
  4. Positive to negative at the battery interconnecting cable

Explanation

A series bank must be jumped as a bank: positive cable to the 24-volt positive terminal and negative cable to the 24-volt negative terminal, or to the approved jump-start posts. Connecting to a single 12-volt battery applies 24 volts across one battery and can make it explode.

Question 24Battery and Starting Systems

With the engine at 1500 rpm and 14.2 volts DC charging, a DMM set to AC volts reads 0.9 volt across the battery. This most likely indicates:

  1. A shorted rotor field winding inside the alternator
  2. A loose, glazed, or slipping alternator drive belt
  3. One or more open or shorted rectifier diodes Correct answer.
  4. Normal AC ripple for a healthy rectifier bridge

Explanation

Some AC ripple is normal, but more than about 0.5 volt AC at the battery under charge indicates the rectifier bridge is not converting all of the alternator's AC output. A failed diode also cuts charging current and can cause dim or flickering lights.

Question 25Battery and Starting Systems

A bus battery rests at 11.0 volts open-circuit and needs recharging. The correct approach is to:

  1. Apply 24 volts briefly to break up the sulfation on the plates
  2. Add electrolyte to the cells before connecting the battery charger
  3. Charge at a low, tapered rate and monitor battery temperature Correct answer.
  4. Charge at the highest available rate to shorten the charging time

Explanation

A deeply discharged battery has high internal resistance, so a high charge rate can overheat it and buckle the plates. Charging at a low tapered rate while watching battery temperature lets the battery recover safely.

Question 26Battery and Starting Systems

The group size number printed on a transit bus battery specifies:

  1. The reserve capacity in minutes at a 25-ampere load
  2. The case dimensions and terminal arrangement Correct answer.
  3. The cold cranking current available at zero degrees Fahrenheit
  4. The number of plates installed in each battery cell

Explanation

Group size is a physical standard covering case length, width, height, and terminal location or type. It allows a replacement battery to fit the tray and connect to the existing cables. It does not describe electrical performance such as cold cranking amps or reserve capacity.

Question 27Battery and Starting Systems

The cold cranking amp (CCA) rating of a 12-volt bus battery is the current it can deliver for 30 seconds at 0°F while keeping terminal voltage at or above:

  1. 10.5 volts
  2. 7.2 volts Correct answer.
  3. 9.6 volts
  4. 12.0 volts

Explanation

The CCA rating is defined by a 30-second discharge at 0°F with the battery terminal voltage held at or above 7.2 volts. This low-voltage cutoff reflects the heavy voltage drop expected from cold electrolyte and high current. A battery that cannot maintain 7.2 volts for the full test does not meet its CCA rating.

Question 28Battery and Starting Systems

On a heavy-duty transit bus starter, the solenoid's primary job is to:

  1. Close the high-current motor circuit and engage the drive Correct answer.
  2. Reduce battery voltage for the ignition control module
  3. Convert alternating current from the alternator into direct current for the starter motor
  4. Limit the starter motor speed after the engine has started

Explanation

The solenoid combines a heavy-duty relay with a movable plunger. When its coil is energized, the plunger closes the contacts that feed the starter motor and simultaneously shifts the drive into mesh with the flywheel. This provides both the high-current switching and the mechanical engagement needed for cranking.

Question 29Battery and Starting Systems

A starter drive is fitted with an overrunning clutch mainly to:

  1. Disconnect the solenoid from the battery after cranking ends
  2. Prevent the engine from driving the starter after start Correct answer.
  3. Hold the drive gear engaged after the engine has started
  4. Increase cranking torque by reducing the armature rotational speed

Explanation

Once the engine starts, the flywheel spins faster than the starter drive. The overrunning clutch lets the drive gear freewheel in that direction so the engine cannot overspeed and damage the starter armature. It also allows the drive to disengage smoothly as the solenoid retracts.

Question 30Battery and Starting Systems

A transit bus uses a starter relay between the ignition switch and the starter solenoid. This relay is used mainly because it:

  1. Provides a ground return path for the starter motor field coils
  2. Increases current to the solenoid during cold cranking
  3. Reduces voltage drop across the starter motor brushes
  4. Lets a low-current switch control the solenoid's high current Correct answer.

Explanation

The ignition switch and its wiring are designed for low control current. The relay coil draws little current, while its contacts can carry the larger solenoid current with short, heavy wiring. This protects the switch and reduces voltage loss in the control circuit.

Question 31Battery and Starting Systems

A bus with an automatic transmission will not crank in any gear except neutral. The component that provides this interlock is the:

  1. Starter magnetic switch
  2. Neutral start switch Correct answer.
  3. Starter overrunning clutch
  4. Battery disconnect switch

Explanation

The neutral start switch is wired into the starter control circuit and closes only when the transmission selector is in neutral or park, depending on the design. This prevents cranking while the bus is in a drive gear. If the switch is misadjusted or faulty, the starter may not receive a control signal.

Question 32Battery and Starting Systems

A flooded battery in a transit bus has electrolyte just above the plates. The recommended service is to:

  1. Add electrolyte until each cell is full
  2. Drain and refill the battery with fresh electrolyte
  3. Add distilled water after charging to the specified level Correct answer.
  4. Add tap water until the level reaches the vent cap

Explanation

During charging, water is lost and the electrolyte level drops. The plates must remain covered, but water should be added after charging to the manufacturer's fill level so expansion does not cause overflow. Distilled water avoids introducing minerals that can contaminate the cells.

Question 33Battery and Starting Systems

During cranking, a starter solenoid's pull-in winding is energized together with the hold-in winding. Once the solenoid contacts close, the pull-in winding is bypassed because it:

  1. produces too little magnetic force to keep holding the plunger
  2. reverses polarity and opens the hold-in circuit
  3. supplies the armature with field current after engagement
  4. would overheat if it stayed energized through the contacts Correct answer.

Explanation

The pull-in winding is a low-resistance winding that creates a strong force to move the plunger and close the contacts. Once the contacts close, the pull-in winding is shorted out so it cannot continue drawing heavy current and overheating. The higher-resistance hold-in winding then keeps the plunger seated.

Question 34Battery and Starting Systems

A partially discharged battery is more likely to freeze in cold weather than a fully charged one because:

  1. its plates expand and press against the cell separators
  2. its electrolyte becomes more acidic and freezes sooner
  3. its dilute electrolyte freezes at a higher temperature Correct answer.
  4. its case material becomes brittle at lower terminal voltages

Explanation

As a battery discharges, sulfuric acid is converted to water and lead sulfate, so the electrolyte becomes more dilute. Dilute electrolyte has a freezing point much closer to that of water, while fully charged electrolyte remains concentrated and resists freezing to a much lower temperature. This is why batteries should be kept charged in cold weather.

Question 35Charging Systems

In a typical alternator, which component converts the stator's AC output into DC for the vehicle's electrical system?

  1. The rotor winding
  2. The voltage regulator
  3. The slip rings
  4. The rectifier bridge Correct answer.

Explanation

The stator produces three-phase AC, and the rectifier bridge uses six diodes to convert that AC into usable DC. The regulator and slip rings control field current but do not perform rectification.

Question 36Charging Systems

A transit bus has the ignition on and the engine not running. The charging system warning lamp is illuminated. What does this condition indicate?

  1. The regulator is limiting current
  2. The lamp circuit is being proved Correct answer.
  3. The alternator is producing full output
  4. The battery is fully charged

Explanation

With the engine off, the warning lamp should light as a bulb and circuit check when the ignition is turned on. Once the alternator begins charging, the lamp should go out.

Question 37Charging Systems

For a 12-volt nominal transit bus charging system at normal temperature, which regulated voltage range is typical?

  1. 12.0 to 13.0 volts
  2. 15.5 to 16.5 volts
  3. 11.0 to 12.0 volts
  4. 13.5 to 14.5 volts Correct answer.

Explanation

A healthy 12-volt charging system typically regulates between about 13.5 and 14.5 volts at normal temperature. This range charges the battery without excessive gassing.

Question 38Charging Systems

A charging system complaint includes a loud squeal from the engine accessory drive at start-up, and the warning lamp stays on briefly. Which is the most likely cause?

  1. A shorted rectifier diode
  2. An open sense wire
  3. A failed voltage regulator
  4. A slipping alternator drive belt Correct answer.

Explanation

A squeal at start-up that accompanies a charging warning lamp usually indicates the belt is slipping and the alternator is not spinning fast enough. Belt tension, condition, and pulley alignment should be checked first.

Question 39Charging Systems

When performing a parasitic draw test on a transit bus, how should an ammeter be connected?

  1. In series between the negative battery cable and the battery post Correct answer.
  2. In parallel across the battery terminals
  3. In parallel with the alternator field circuit
  4. In series with the alternator output cable

Explanation

An ammeter must be connected in series so all current flowing out of the battery passes through the meter. All accessories and lamps must be off and modules allowed to sleep to avoid false readings.

Question 40Charging Systems

What is the primary purpose of a fusible link installed in the alternator output circuit?

  1. To open and protect the circuit if a short develops Correct answer.
  2. To step down system voltage for the battery
  3. To limit alternator output current during normal charging
  4. To reduce radio frequency interference

Explanation

A fusible link is a calibrated wire designed to open when current exceeds its rating, protecting the alternator output circuit from a short. It does not regulate normal charging current.

Question 41Charging Systems

A battery load test is being performed. What does this test primarily evaluate?

  1. The alternator's maximum current output
  2. The amount of AC ripple in the charging circuit
  3. The voltage regulator's set point
  4. The battery's ability to deliver current under load Correct answer.

Explanation

A load test applies a high-current load and observes voltage to determine whether the battery can deliver cranking current. It does not test the alternator or voltage regulator.

Question 42Charging Systems

What is the purpose of field current in an alternator?

  1. To drive the accessory belt
  2. To create the magnetic field in the rotor Correct answer.
  3. To regulate the battery voltage
  4. To rectify stator AC output

Explanation

Field current flows through the rotor winding and creates the magnetic field that induces voltage in the stator. Controlling field current is how the regulator controls output.

Question 43Charging Systems

A voltage drop test is made from the alternator B+ output terminal to the battery positive post while the charging system is under load. The meter reads 0.65 volt. What does this indicate?

  1. An open voltage regulator sense wire
  2. A defective rectifier diode
  3. Excessive resistance in the B+ cable or connections Correct answer.
  4. Normal voltage drop for a charging circuit

Explanation

Voltage drop in the B+ cable should typically be 0.5 volt or less under load. A reading of 0.65 volt indicates excessive resistance, which can cause undercharging and heat.

Question 44Charging Systems

An alternator's remote voltage sense wire has high resistance. The B+ cable is good. What is the most likely result?

  1. The battery will be undercharged at low engine speed
  2. The warning lamp will flash rapidly
  3. The regulator will raise charging voltage above normal Correct answer.
  4. The alternator will not charge at all

Explanation

The regulator uses the sense wire to monitor battery voltage. If resistance in that wire causes a low reading, the regulator increases field current and raises charging voltage, risking overcharge.

Question 45Charging Systems

A charging system has excessive AC ripple voltage at the battery with the engine running. Which component is most likely at fault?

  1. The voltage regulator field winding
  2. The starter solenoid
  3. The battery temperature sensor
  4. One or more rectifier diodes Correct answer.

Explanation

Excessive AC ripple indicates one or more rectifier diodes are open or shorted, allowing unfiltered AC into the DC system. A diode test or oscilloscope pattern check confirms the fault.

Question 46Charging Systems

A bus alternator charges normally at highway speed, but the warning lamp glows at idle. Belt tension is correct. Which is the most likely cause?

  1. Low alternator output at idle Correct answer.
  2. A failed battery equalizer
  3. A shorted stator winding
  4. An open fusible link

Explanation

Alternator output rises with rotor speed. If output is normal at high RPM but insufficient at idle, the alternator is likely worn or the pulley ratio is wrong, even when the belt is tight.

Question 47Charging Systems

A transit bus uses a battery isolator to separate the starting battery from the house battery bank. What is the main operating advantage?

  1. The alternator can charge only the starting battery
  2. The isolator converts AC shore power to DC
  3. The house battery can crank the engine when the starting battery is low
  4. House loads cannot drain the starting battery Correct answer.

Explanation

A battery isolator allows the alternator to charge both battery banks while preventing house loads from discharging the starting battery. This preserves cranking ability.

Question 48Charging Systems

A 24-volt transit bus uses two 12-volt batteries in series and a battery equalizer. What is the equalizer's function?

  1. It disconnects the batteries during engine cranking
  2. It rectifies the alternator's AC output
  3. It converts 24 volts to 12 volts for the alternator field
  4. It keeps the two 12-volt batteries at equal charge Correct answer.

Explanation

In a 24-volt bus with two 12-volt batteries in series, the equalizer maintains equal voltage across each battery as loads are drawn from the center tap. This prevents one battery from being overcharged and the other undercharged.

Question 49Charging Systems

A battery temperature sensor reports a high battery temperature to the charging control module. How should the module respond?

  1. Reduce charging voltage to limit battery heating Correct answer.
  2. Disable the alternator completely
  3. Increase charging voltage to cool the battery
  4. Increase field current to the maximum

Explanation

High battery temperature increases the risk of thermal runaway and gassing, so the control module reduces charging voltage. This lowers charging current and heat generation.

Question 50Charging Systems

A transit bus with an overrunning alternator decoupler pulley has a belt tensioner that oscillates and a chirping noise at idle. What is the most likely cause?

  1. A defective battery equalizer
  2. A shorted rectifier diode
  3. An open voltage sense wire
  4. A seized decoupler pulley Correct answer.

Explanation

A decoupler pulley allows the alternator to overrun the belt during rapid engine deceleration. When it seizes, it can cause belt chirp and tensioner oscillation because the alternator cannot freewheel.