CSLB General Building (B) — All Questions
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Using Ohm's law, a 12-volt circuit with 4 ohms of resistance will carry a current of:
- a.48 amps
- b.0.33 amps
- c.3 amps✓
- d.16 amps
Ohm's law states current equals voltage divided by resistance, so 12 volts divided by 4 ohms equals 3 amps. Understanding this relationship is essential for diagnosing circuit faults. Higher resistance in a circuit reduces current flow for a given voltage.
Technician A says a voltage-drop test across a battery cable can reveal unwanted resistance. Technician B says a voltage-drop test is more accurate than a visual inspection for finding poor connections. Who is correct?
- a.Technician A only
- b.Technician B only
- c.Both A and B✓
- d.Neither A nor B
A voltage-drop test measures the voltage lost across a connection under load, so high drop reveals corrosion or loose connections that a visual check may miss. Both technicians are correct. Acceptable drop is typically a few tenths of a volt or less.
A fully charged 12-volt lead-acid battery at rest should read approximately:
- a.10.5 volts
- b.11.8 volts
- c.13.8 volts
- d.12.6 volts✓
A healthy, fully charged 12-volt battery reads about 12.6 volts open-circuit at rest. Around 12.4 is roughly 75 percent, and 12.0 is about 25 percent charged. A surface charge should be removed before testing for accuracy.
Technician A says the charging system voltage at the battery with the engine running should typically be around 13.5 to 14.5 volts. Technician B says it should read exactly 12.6 volts while running. Who is correct?
- a.Technician A only✓
- b.Both A and B
- c.Technician B only
- d.Neither A nor B
With the engine running, the alternator should raise system voltage to roughly 13.5 to 14.5 volts to charge the battery and run loads. A reading of 12.6 running would indicate the charging system is not working. Technician B describes a resting, not charging, voltage.
Two 6-ohm resistors connected in parallel produce a total resistance of:
- a.12 ohms
- b.6 ohms
- c.3 ohms✓
- d.0.5 ohms
For two equal resistors in parallel, total resistance is half of one resistor, so two 6-ohm resistors equal 3 ohms. Parallel resistance is always lower than the smallest branch. This principle explains why adding parallel loads increases total current draw.
An open in a series circuit will cause:
- a.Increased current flow
- b.No current flow in the circuit✓
- c.Higher voltage at the load
- d.Reduced resistance
A series circuit has only one path, so an open anywhere stops all current and the load will not operate. Voltage will be present up to the open point when tested. Locating the open often uses a test light or voltmeter along the circuit.
A starter that cranks slowly, with the battery known good and fully charged, most likely has:
- a.Too little engine oil
- b.A blown headlight fuse
- c.A weak alternator
- d.High resistance in the starter circuit or a failing starter✓
Slow cranking with a good battery points to high resistance in the cables/connections or a worn starter, both of which limit current to the motor. A starter-circuit voltage drop test isolates the problem. Clean, tight connections are essential for full cranking current.
Power (watts) in an electrical circuit is calculated by:
- a.Voltage multiplied by current✓
- b.Voltage divided by resistance
- c.Current divided by voltage
- d.Resistance multiplied by voltage
Electrical power equals voltage times current (P = E x I). A 12-volt circuit drawing 5 amps consumes 60 watts. This relationship helps size fuses, wiring, and components for their load.
A parasitic (key-off) battery drain is measured by connecting an ammeter:
- a.In parallel across the battery terminals
- b.Across a spark plug
- c.In series between a battery terminal and the cable✓
- d.Between two ground points
Parasitic draw is measured by placing an ammeter in series in the battery circuit after modules go to sleep. Excessive draw discharges the battery overnight. Pulling fuses one at a time while watching the meter isolates the offending circuit.
In a wiring diagram, a component shown connected to ground provides:
- a.The source of voltage
- b.The return path for current back to the battery negative✓
- c.Fuse protection
- d.Signal amplification
Ground is the return path that completes the circuit back to the battery's negative terminal. A poor ground raises resistance and causes dim lights, erratic operation, or feedback. Diagrams use ground symbols to trace current return paths.
Technician A says a fuse that blows again immediately after replacement indicates a short to ground or an overload. Technician B says repeatedly replacing the fuse will fix the circuit. Who is correct?
- a.Technician A only✓
- b.Both A and B
- c.Technician B only
- d.Neither A nor B
A fuse that blows again immediately is protecting the circuit from a short to ground or an overload drawing excess current. Technician B is wrong because the underlying fault, not the fuse, must be repaired. Inspect the wiring for chafing and test the component.
Technician A says a diode allows current to flow in only one direction. Technician B says alternators use diodes to rectify AC into DC. Who is correct?
- a.Technician A only
- b.Technician B only
- c.Neither A nor B
- d.Both A and B✓
A diode conducts in one direction only, and the alternator's diode (rectifier) bridge converts the stator's AC into the DC the vehicle uses. Both statements are correct. A shorted or open diode causes low output and AC ripple on the charging system.
Excessive AC ripple voltage measured at the battery with the engine running usually indicates:
- a.One or more failed alternator diodes✓
- b.A weak starter
- c.A blown headlight bulb
- d.A stuck thermostat
Failed rectifier diodes let AC leak through, producing measurable ripple and reduced DC output. A meter set to AC volts or an oscilloscope reveals it. This often causes charging complaints and can disturb electronic modules.
Technician A says a relay lets a small control current switch a larger load. Technician B says relays protect switches and allow smaller control wiring. Who is correct?
- a.Neither A nor B
- b.Technician A only
- c.Technician B only
- d.Both A and B✓
A relay uses a low-current coil to close contacts that carry a high-current load, which protects switches and permits smaller control wiring. Both technicians are correct. Common uses include headlights, fuel pumps, and cooling fans.
Dim headlights that brighten when engine rpm increases most likely indicate:
- a.A blown fuse
- b.A shorted headlight switch
- c.A weak battery or undercharging condition✓
- d.An open ground on one lamp
Lights that brighten with rpm suggest the charging system is barely keeping up, so a weak battery or low alternator output is likely. A charging system test measures output under load. A high-resistance connection can also dim lights but usually affects specific circuits.
A voltmeter is connected how, relative to the component being tested?
- a.In series with the load
- b.Between the two battery posts only
- c.After removing the fuse
- d.In parallel (across) the component✓
A voltmeter is connected in parallel across the component or connection to measure the voltage difference, since it has very high internal resistance. An ammeter, by contrast, connects in series. Connecting a voltmeter in series would give an incorrect reading.
The gauge or module that displays engine coolant temperature relies on a sensor whose resistance:
- a.Changes with temperature (a thermistor)✓
- b.Stays constant regardless of temperature
- c.Only works above 200 degrees
- d.Produces AC voltage
Most coolant temperature sensors are thermistors whose resistance falls as temperature rises (negative temperature coefficient). The computer or gauge interprets this changing resistance as temperature. A faulty sensor can cause inaccurate readings and driveability or fan-control problems.
A circuit has 12 volts applied and draws 2 amps. Its total resistance is:
- a.24 ohms
- b.6 ohms✓
- c.2 ohms
- d.0.17 ohms
By Ohm's law, resistance equals voltage divided by current, so 12 volts divided by 2 amps equals 6 ohms. Rearranging Ohm's law lets a technician solve for any unknown value. Verifying calculated resistance against measured values helps confirm circuit condition.