Starter voltage drop test and A8 general diagnosis
A starter voltage drop test finds resistance where an ohmmeter can’t: in a cable or connection that is carrying cranking current. It is one of the 23 tasks in area A of A8, General Diagnosis, which also covers engine vacuum and compression readings, cam timing, cooling and oil faults, and wiring diagrams.
12scored questions
24%of the test
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How to run a starter voltage drop test
Test the circuit while it carries current. Disable the engine so it cranks without starting (follow the service information, for example by pulling the fuel pump or ignition fuse), set the meter to DC volts, and put the leads across the part of the circuit you are testing, not across the battery:
Positive side: red lead on the battery positive post itself (not the cable clamp), black lead on the starter’s battery terminal. Crank and read.
Ground side: red lead on the starter housing or engine block, black lead on the battery negative post. Crank and read.
Solenoid contacts: across the solenoid’s two large terminals while cranking.
Compare each reading with the maximum in the service information. Over spec means resistance somewhere between your leads: move one lead a connection at a time toward the other until the drop disappears, and the section you just stepped over holds the fault. Don’t test with the circuit off. With no current flowing, even a corroded cable shows almost no drop.
The charging side and current draw
An alternator voltage drop test works the same way with the engine running and a heavy electrical load on: alternator output terminal to battery positive, then alternator case to battery negative. A high drop on the charging side makes the battery see less than the alternator puts out, which can look like a weak alternator.
Starter current draw (task 19) is the other half of the picture. High draw with slow cranking points at the starter or a mechanical drag in the engine. Low draw with slow cranking points at resistance in the cables and connections, which the voltage drop test will find. For voltage drop on other circuits, see A6 general electrical diagnosis.
Trap: battery voltage is not voltage drop
Cranking voltage measured at the battery tells you how the battery holds up under load, not where the loss is. A healthy battery reading with slow cranking sends you to the cables and the starter, one side at a time.
7 general diagnosis questions
0 of 7 answered · 0 correct
Question 1 of 7 · General Diagnosis · 24%Not answered
Parameter
Measurement
Specified Cam/Crank Separation
14 degrees
Actual Cam/Crank Separation
29 degrees
Signal Amplitudes
Normal (5V square wave)
A vehicle has a prolonged crank and sets a camshaft correlation code. A dual-trace oscilloscope is used to measure the relationship between the crankshaft and camshaft position sensor signals. Based on the data table, what is the most likely cause?
Explanation
Correct: Clean signals with a fixed 15-degree offset mean the cam has moved relative to the crank, which is exactly what a chain that skipped teeth does.
Incorrect: A damaged cam reluctor shows missing or misshapen pulses, not a clean pattern shifted by a constant angle.
Incorrect: A failing crankshaft sensor drops pulses or loses amplitude; here both signals are clean 5 V square waves.
Incorrect: A high-resistance ground lifts the signal's low level or cuts amplitude; it cannot shift the timing between two signals.
Incorrect: The scope reads the signals directly, so the offset exists before the ECM ever sees it.
Question 2 of 7 · General Diagnosis · 24%Not answered
A vehicle with variable valve timing (VVT) hesitates and sets a camshaft correlation code. On a scope, the cam/crank relationship stays exactly the same when the PCM commands the cam to advance, even though the oil control valve receives the command signal and clicks. What is the most likely cause?
Explanation
Incorrect: A failed crank sensor shows dropouts or a ragged signal; this pattern is clean and simply does not change.
Incorrect: A shorted ignition coil primary misfires its cylinder; it has no effect on cam phasing.
Correct: The oil control valve gets its command and responds, yet cam/crank timing never moves; a phaser that is stuck or locked keeps the cam from turning relative to the crank.
Incorrect: The command signal is present at the oil control valve, so the ECM output driver is doing its job.
Incorrect: An open injector circuit kills fuel to one cylinder but cannot hold cam timing still.
Question 3 of 7 · General Diagnosis · 24%Not answered
A technician measures a primary ignition coil waveform and the flyback peak reads only 40 volts. The engine runs perfectly with no misfires. Which operator error most likely caused this artificially low reading?
Explanation
Incorrect: A coarse 100 V per division scale makes the trace smaller on screen, yet the scope still reads the true peak voltage.
Incorrect: The primary pattern lives on the control (coil-negative) wire, so back-probing it is correct; the power feed would show only battery voltage.
Correct: A 10:1 probe divides the signal by ten; if the scope still assumes a 1:1 probe, a normal 400 V flyback is drawn as 40 V.
Incorrect: AC coupling removes the DC offset and shifts the trace, but a fast flyback spike keeps nearly its full height.
Incorrect: Battery negative is a sound ground reference for a primary ignition test and does not cut the measured peak.
Question 4 of 7 · General Diagnosis · 24%Not answered
A vehicle fails the EVAP monitor readiness after a long highway drive. Freeze-frame data shows coolant temperature 62 degrees C after 20 minutes of driving on a 10 degrees C day. The engine does not overheat and the heater output is cool. Which of the following is the most likely cause?
Explanation
Correct: A thermostat stuck open sends coolant through the radiator all the time: 62 °C (about 144 °F) after 20 minutes with cool heater output, and the cold engine keeps the EVAP monitor from running.
Incorrect: A dead cooling fan causes overheating in traffic; it cannot hold a highway engine at 62 °C.
Incorrect: A bad coolant temperature sensor could misreport, but the cool heater output confirms the engine really is cold.
Incorrect: Low coolant tends toward overheating and erratic gauge readings, not a steady cold reading backed by a cool heater.
Incorrect: Clogged radiator passages make the engine run hot, not cold.
Question 5 of 7 · General Diagnosis · 24%Not answered
Which tool is required for accurate exhaust gas analysis?
Explanation
Correct: A five-gas analyzer reads HC, CO, CO2, O2 and NOx, which together show combustion quality and catalyst efficiency.
Incorrect: A vacuum gauge reads manifold vacuum to judge engine breathing and sealing, not exhaust gas content.
Incorrect: A timing light checks when the spark fires; it measures nothing in the exhaust.
Incorrect: A battery tester checks battery condition and has no role in exhaust gas analysis.
Question 6 of 7 · General Diagnosis · 24%Not answered
What is the proper sequence for performing a diagnostic procedure?
Explanation
Correct: Confirming the concern first, then checking bulletins, inspecting and reading codes before testing keeps every later step aimed at the real fault.
Incorrect: Going from codes straight to replacing parts skips both verifying the concern and testing the circuit the code points to.
Incorrect: Verifying the complaint last means parts get replaced before anyone knows what the customer actually noticed.
Incorrect: Starting by replacing suspected parts is guesswork; a code names a circuit or system, not a failed part.
Question 7 of 7 · General Diagnosis · 24%Not answered
An engine cranks normally but will not start. Spark and fuel delivery both check good. What should be verified next?
Explanation
Incorrect: Coolant level has no effect on whether the engine fires when it cranks.
Incorrect: The engine already cranks normally, so battery voltage has done its part; it is not the next check after spark and fuel.
Incorrect: Oil level matters for engine life, but a low level does not stop an engine that cranks normally from starting.
Correct: With spark and fuel confirmed, the engine still needs its valves to open at the right moment; a jumped belt or chain throws valve timing off and keeps it from starting.
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What else area A tests
The guide lists 23 tasks for area A. Grouped:
Complaint and research: verify the complaint, road test, check service history, bulletins and recalls (tasks 1–2).
Base engine: noise, vibration, exhaust color and smell; manifold vacuum, power balance, compression and leak-down tests (tasks 3–9). Reading those results: engine tests.
Analyzer work: scope, engine analyzer or scan tool on mechanical, ignition and fuel faults (task 10).
Valves and cam timing: valve adjustment, cam timing, variable valve timing and lift (tasks 11–12).
Oil and cooling: driveability faults from oil pressure, quality, level or type; operating temperature, pressure test, fans, fan clutches and active grille shutters (tasks 13–15).
Electrical: wiring diagrams, battery condition and key-off drain, starter current draw, starter and charging voltage drop, alternator belts, pulleys and clutches (tasks 16–23).
Area A treats cooling as a driveability topic. A thermostat stuck open keeps the engine below its regulating temperature: the coolant temperature reading stays low, the engine runs richer than it should, monitors that need full warm-up may never run, and the engine computer can set a thermostat performance code, as in one of the guide’s own samples. Read the coolant temperature in every freeze frame you are given.
Use the maximum in the service information for that vehicle, and test the positive and ground sides separately while the engine cranks. A reading taken with no current flowing proves nothing.
How do you do an alternator voltage drop test?
Engine running, heavy load on: meter from the alternator output terminal to battery positive, then from the alternator case to battery negative. Compare each side with the service spec.