A technician is diagnosing a vehicle with a no-crank condition. When the ignition switch i… — Armed Services Vocational Aptitude Battery (ASVAB)

A technician is diagnosing a vehicle with a no-crank condition. When the ignition switch is turned to the START position, a single distinct click is heard from the starter solenoid, but the engine does not turn over. A digital multimeter connected directly across the battery posts reads $12.6\text{ V}$ at rest and drops slightly to $12.2\text{ V}$ when the key is held in the START position. However, measuring between the starter motor's main positive battery terminal ( $B$ -terminal) and the starter case ground shows $12.5\text{ V}$ at rest, but drops to $0.4\text{ V}$ during the crank attempt. Which of the following is the most likely root cause of this failure?

A
A discharged 12-volt battery unable to supply high cranking amperage
High resistance in the positive battery cable or its terminal connectionsCevap
C
An internal short circuit across the alternator stator windings
D
A seized starter motor armature drawing locked-rotor current

Cevap

High resistance in the positive battery cable or its terminal connections

The correct answer is high resistance in the positive battery cable or its terminal connections. When the ignition switch activates the starter solenoid, the heavy internal contact disc closes to connect the battery feed to the starter motor windings. Because high resistance exists along the cable or connection (due to corrosion, looseness, or damaged strands), the attempted current draw causes a massive voltage drop across the resistance point, leaving only

0.4\text{ V}

at the starter motor. The battery itself maintains

12.2\text{ V}

, proving the voltage drop is strictly isolated to the feed circuit.

Adım Adım Çözüm

Analyze the battery state of charge and battery post voltage under load.

The battery post voltage is

12.6\text{ V}

open circuit and stays at

12.2\text{ V}

during the crank attempt, confirming the battery is fully charged and not under excessive total load.

A healthy, fully charged automotive lead-acid battery maintains

>9.6\text{ V}

during starting.

Evaluate the voltage drop between the battery and the starter motor main terminal.

Voltage at the starter drops from

12.5\text{ V}

down to

0.4\text{ V}

during the attempt, indicating an unwanted voltage drop of over

11.8\text{ V}

(

12.2\text{ V} - 0.4\text{ V}

) across the supply line.

According to Ohm's law (

V = IR

), when current flows through high unwanted resistance (corrosion, loose terminal, frayed cable), voltage drops significantly across that point instead of reaching the load.

Differentiate between an open supply path, a shorted/locked starter, and a cable resistance fault.

The single click confirms the solenoid pull-in coil activates and closes the internal disc contacts, but the supply cable cannot deliver current due to excessive series resistance.

The failure is isolated to the primary current-carrying feed between the battery positive post and the starter terminal.

Anahtar Kavram

Starting circuit voltage drop diagnosis and series circuit resistance behavior under heavy electrical loads.

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