VIENIDMY Sign in Start practicing

Home › Lessons › Automobile Maintenance (SSW 2)

Fault Diagnosis, Measurement, and Completion Inspection (Practical)

Automobile Maintenance (SSW 2) · 自動車整備分野(2号) · Written by Tokutei Pass based on the official study texts; not copied verbatim.

How to Read a Circuit Tester and Lamp Circuit Voltage Drop Diagnosis (Practical Task 1)

In 'Lamp Circuit Voltage Drop Diagnosis' appearing in Task 1 of the Type 2 practical exam, with the switch turned ON so current flows through the circuit (under load), fix the black lead of the digital circuit tester to the 'negative (−) battery terminal (ground)' and touch the red lead to each measurement point (V1 to V7) to read the voltage.

【How to Read a Digital Circuit Tester】 (1) Verify that the rotary selector is set to the DC voltage 'V' range. (2) Always check not only the numerical value on the LCD screen, but also whether the unit displayed at the right edge is 'V (volts)' or 'mV (millivolts: 1,000 mV = 1 V)' (for example, '0.0 mV' means '0 V').

In a normal 12 V lamp circuit (when the switch is ON), all measurement points from the power supply up to just before the lamp (V1 to V4) read 'approx. 12.00 V', and because all 12 V is consumed (dropped) across the lamp, the downstream ground-side points after the lamp (V5 to V7) all read 'approx. 0 V (0.0 mV)'.

Suppose during a fault where 'the lamp is dim', V1 to V4 before the lamp all read '12.00 V (Good / 良)', but immediately after the lamp on the ground side, 'V5 reads 6.00 V (NG / 否)', while the next points 'V6 and V7 read 0.0 mV (Good / 良)'.

This measurement result means 6.00 V remains at V5 (which should be 0 V) and an abnormal voltage drop of 6.00 V is occurring between V5 (6.00 V) and V6 (0 V); thus, the faulty location can be pinpointed as 'between V5 and V6 (poor contact / excessive contact resistance in the connector or harness)'.

How to Read a Circuit Tester and Lamp Circuit Voltage Drop Diagnosis (Practical Task 1)

Key points


  • When reading a digital circuit tester, always verify the difference between the units 'V' and 'mV (0.0 mV = 0 V)' at the right edge of the display.

  • Normal values with the switch ON are 'approx. 12 V' on the power supply side before the lamp (V1–V4) and 'approx. 0 V (0.0 mV)' on the ground side after the lamp (V5–V7).

  • When V5 on the ground side reads '6.00 V (NG)' and V6 reads '0.0 mV (Good)', the faulty section with abnormal contact resistance is 'between V5 and V6'.

  • Example: When V4 upstream of the lamp is 12.00V, V5 downstream is 6.00V, and V6 is 0.0mV (0V) → Voltage applied to the lamp = 12.00 − 6.00 = 6.00V; abnormal voltage drop between V5 and V6 = 6.00 − 0 = 6.00V
WordMeaning
circuit tester (V range and mV display)
voltage drop
poor contact and contact resistance
pass/fail judgment (Good 良 / NG 否)

Resistance, Continuity, and Current Measurement Procedures Using a Circuit Tester

When measuring component or wiring 'resistance (Ω)' or 'continuity' with a circuit tester, the tester sends a measuring current from its internal battery; if vehicle battery voltage is present on the target circuit, accurate measurement is impossible and the tester may be damaged.

Therefore, before measuring resistance or continuity, it is an absolute rule to 'always turn the circuit switch OFF (or disconnect the negative battery terminal or unplug the connector of the target component)'.

Additionally, when measuring resistance with an 'analog (pointer) circuit tester', before measurement and every time you switch the resistance range (×1, ×10, ×1k, etc.), touch (short) the tips of the red and black test leads together and turn the '0Ω adjustment knob' to align the pointer with '0Ω' at the right end (zero adjustment).

When measuring 'current (A)' flowing in a circuit with a circuit tester, connect the tester in 'series' with the circuit (accidentally connecting it in parallel across the power supply will cause a large current to blow the tester's internal fuse).

Note that when measuring large currents—such as starter current or alternator output current—or dark current without disconnecting wiring, a 'clamp meter (current probe)' that detects the magnetic field around the wire is used.

Key points


  • When measuring resistance (Ω) or continuity, always turn circuit power 'OFF' and unplug the connector to measure the isolated part.

  • Whenever changing the resistance range on an analog tester, short the red and black leads together and perform '0Ω adjustment'.

  • Connect a voltmeter in 'parallel' with the circuit, and connect an ammeter in 'series' with the circuit.
WordMeaning
resistance measurement (Ω range) and continuity check
0Ω (zero-ohm) adjustment
parallel connection (voltage) and series connection (current)
clamp meter and dark current

How to Read a Compression Gauge and Compression Pressure Measurement (Practical Task 2)

In 'Compression Pressure Measurement Using a Compression Gauge' (Task 2 of the Type 2 practical exam), the 'four preparation and measurement conditions' for accurate and safe testing appear most frequently.

【4 Conditions for Compression Measurement】 (1) Thoroughly 'warm up the engine (coolant temp 80–90℃)' and verify the battery is fully charged. (2) Unplug injector connectors, etc. to 'stop fuel injection' during cranking and disable ignition. (3) 'Remove the spark plugs from ALL cylinders' to reduce cranking resistance. (4) Depress the accelerator pedal fully to 'open the throttle valve all the way', crank the engine with the starter at specified RPM, and read the maximum pointer deflection.

【How to Read a Compression Gauge & Judgment Procedure】 (1) Check the dial unit (MPa) and the value of 1 smallest division (e.g., if 0.1 MPa is divided into 5 parts, 1 division = 0.02 MPa; if into 2 parts, 0.05 MPa) and read the pointer. (2) Compare each cylinder's reading against the 'limit value (e.g., 1.05 MPa)'; any cylinder below the limit is judged 'NG (否)'.

Furthermore, (3) calculate 'Highest reading − Lowest reading' among all cylinders to find the 'inter-cylinder difference'; if that difference exceeds the standard (e.g., within 0.10 MPa), the inter-cylinder difference is also judged 'NG (否)'.

Note that if a small amount of engine oil is poured through the spark plug hole of a low-compression cylinder and re-measured, a large rise in pressure diagnoses 'piston ring or cylinder wall wear', whereas little to no change diagnoses 'poor valve seating or a blown head gasket'.

Key points


  • 4 compression measurement conditions: (1) After warm-up, (2) Stop fuel injection, (3) Remove spark plugs from 'ALL cylinders', (4) Crank with the throttle valve 'fully open'.

  • Inter-cylinder difference = 'Highest cylinder compression − Lowest cylinder compression', compared against the limit value.

  • If adding oil through the plug hole raises pressure, it indicates 'ring/cylinder wear'; if unchanged, it indicates 'poor valve contact or head gasket leak'.

  • Example: Cyl #1 = 1.30MPa, Cyl #2 = 0.85MPa, Cyl #3 = 1.25MPa, Cyl #4 = 1.30MPa (limit 1.05MPa, difference ≤ 0.10MPa) → Cyl #2 is 'NG (否)' at 0.85MPa, max inter-cylinder difference = 1.30 − 0.85 = 0.45MPa ('NG / 否')
WordMeaning
compression gauge
throttle valve fully open and fuel injection stop
inter-cylinder difference (highest − lowest)
poor valve contact and piston ring wear

How to Read a Dial Gauge and Shaft Runout / Bend Measurement (Practical Task 3)

In 'Shaft Bend Measurement Using a Dial Gauge' (Task 3 of the Type 2 practical exam), first as preparation, 'thoroughly clean dirt and oil from the shaft and V-blocks', and support both end journals of the shaft on left and right V-blocks placed on a surface plate.

Next, secure the dial gauge with a magnetic stand, place the spindle (measuring contact point) 'perpendicular (90°) to the center of the shaft', pre-depress the spindle slightly so it can move both up and down, and rotate the outer bezel to set the long pointer to '0'.

【How to Read a Dial Gauge (0.01 mm minimum graduation)】 (1) Read '1 mm increments' on the small inner 'short pointer'. (2) Read '0.01 mm increments (1 division = 0.01 mm, 1 full turn of 100 divisions = 1.00 mm)' on the large outer 'long pointer'. (3) Slowly rotate the shaft 1 full revolution by hand; the difference between the maximum and minimum readings of the long pointer (total swing) is the 'runout (振れ)'.

The single most important calculation rule here is that rotating the shaft one full turn measures both the high peak (+ side) and low valley (− side) of the bend, so the shaft 'bend (曲がり)' is 'half of the dial gauge runout (Bend = Runout ÷ 2)'.

Therefore, do not directly compare the 'runout' read from the dial gauge against the limit value; always divide by 2 to calculate the 'bend' first, and then compare it with the 'bend limit' to judge Good (良) or NG (否) (distinguish this from disc rotor runout or flywheel face runout, which are read directly as runout).

How to Read a Dial Gauge and Shaft Runout / Bend Measurement (Practical Task 3)

Key points


  • Clean the shaft and V-blocks before measurement, and set the dial gauge spindle 'perpendicular (90°)' to the object being measured.

  • On a dial gauge with 0.01 mm minimum graduations, the short pointer indicates '1 mm units' and the long pointer indicates '0.01 mm units (1 turn = 1.00 mm)'.

  • Shaft 'Bend = Dial gauge runout ÷ 2'; compare the halved value against the bend limit to judge pass or fail.

  • Example: When rotating a shaft 1 turn moves the dial gauge long pointer from '0' to '8 (0.08mm)' and the bend limit is 0.03mm → Runout = 0.08mm; Shaft bend = 0.08 ÷ 2 = 0.04mm (exceeds the 0.03mm limit, so judged 'NG / 否')

  • Example: When the long pointer swings 2 divisions left (−0.02mm) and 4 divisions right (+0.04mm) across 0 → Runout = 0.02 + 0.04 = 0.06mm; Shaft bend = 0.06 ÷ 2 = 0.03mm
WordMeaning
dial gauge (long pointer 0.01 mm, short pointer 1 mm)
magnetic stand, V-blocks, and surface plate
shaft runout and bend (Bend = Runout ÷ 2)
perpendicular (90°) contact and zero setting

How to Read a Vernier Caliper, Micrometer, Cylinder Gauge, and Plastigage

【How to Read a Vernier Caliper (0.05 mm minimum reading)】 (1) First, read the 'main scale graduation' immediately to the left of (before) the position pointed to by the '0' graduation line of the vernier scale in '1 mm increments'. (2) Next, find where a graduation line on the vernier scale 'aligns in a single straight line' with a graduation line on the main scale, and read the vernier number in '0.05 mm increments'. (3) Finally, add 'Main scale value + Vernier scale value'.

【How to Read an Outside Micrometer (0.01 mm minimum reading)】 Before measuring, adjust the zero point using a standard bar, and during measurement turn the ratchet stop until it clicks to apply constant pressure. (1) Read the '1 mm increments' visible to the left of the thimble edge on the 'upper sleeve scale'. (2) As the most important check, see whether a '0.5 mm graduation line on the lower sleeve scale' is visible! If visible, add '0.50 mm'; if not visible, add '0.00 mm'.

(3) Next, read the 'thimble circumferential scale (1 division = 0.01 mm, 1 turn of 50 divisions = 0.50 mm)' that aligns with the sleeve baseline (horizontal line). (4) Finally, sum 'Upper sleeve (1 mm) + Lower sleeve (0.50 mm if visible) + Thimble (0.01 mm)'.

Additionally, a 'cylinder gauge' measures cylinder bore diameter: the diameter difference along the vertical axis (top, middle, bottom) is called 'taper wear', and the diameter difference between the piston pin axis direction and the thrust (perpendicular) direction is called 'oval (out-of-round) wear'.

'Plastigage' for measuring crankshaft 'oil clearance' is used by placing a thread-like plastic gauge on the journal, tightening the bearing cap to specified torque (never rotate the shaft at this time), removing the cap again, and matching the 'widest part' of the crushed gauge against the paper scale (the wider the crushed strip, the smaller the oil clearance).

How to Read a Vernier Caliper, Micrometer, Cylinder Gauge, and Plastigage

Key points


  • Vernier caliper reading = 'Main scale graduation before the vernier 0 line (1 mm units) + Vernier graduation aligned with a main scale line (0.05 mm units)'.

  • Micrometer reading = 'Upper sleeve (1 mm) + Lower sleeve (always check if 0.50 mm is visible!) + Thimble graduation (0.01 mm)'.

  • Read Plastigage at its 'widest crushed part'; a wider strip means a 'smaller' oil clearance (rotating the shaft during measurement is strictly prohibited).

  • Example: When the vernier '0' of a caliper lies between 24mm and 25mm on the main scale and the vernier '3.5 (0.35mm)' line aligns with the main scale → 24.00 + 0.35 = 24.35mm

  • Example: On a 0–25mm micrometer, when the upper sleeve shows '6mm', the lower '0.5mm line' is visible, and the thimble indicates '28 (0.28mm)' → 6.00 + 0.50 + 0.28 = 6.78mm
WordMeaning
vernier caliper (main scale and vernier scale / 0.05 mm)
micrometer (sleeve 0.5 mm scale and thimble 0.01 mm)
cylinder gauge (taper wear and oval wear)
Plastigage (oil clearance measurement)

Basic Troubleshooting Workflow and Completion Inspection Testers (Exhaust, Headlights, Brakes)

The basic workflow for efficient automotive troubleshooting consists of five steps: (1) 'Customer interview and symptom confirmation (when and under what conditions it occurs)', (2) 'Reading DTCs and Freeze Frame Data with an external scan tool plus basic inspection', (3) 'Narrowing down causes and component testing', (4) 'Repair and part replacement', and (5) 'Clearing DTCs and post-repair verification testing'.

With a 'CO/HC tester (carbon monoxide / hydrocarbon analyzer)' used in completion inspection: after thoroughly warming up the engine, warming up the tester, checking for leaks, and performing 'zero and span calibration' with clean air, insert the sampling probe to the specified depth (approx. 60 cm) into the exhaust pipe and measure at 'idling'.

An 'opacimeter (light-extinction black smoke meter)' that measures black smoke (particulate matter) from diesel vehicles shines light through exhaust gas during no-load rapid acceleration and measures the degree of light obscuration as the 'light absorption coefficient (unit: m⁻¹)'.

When inspecting with a 'headlight tester', adjust tire inflation pressure to the specified value, position the vehicle squarely facing the tester in an unloaded state with 'one driver seated', and measure headlamp 'luminous intensity (cd: candela)' and 'up/down and left/right optical axis (elbow point) deviation'.

A 'brake tester' measures whether the sum of braking forces on each front and rear axle, the left-to-right braking force difference, and parking brake force conform to the Safety Standards.

Key points


  • Troubleshooting begins with 'customer interview and reproducing/confirming the fault symptom', and concludes after repair by clearing DTCs with a scan tool and verifying no recurrence.

  • A CO/HC tester measures at 'idling' after warm-up and clean-air zero calibration, while a diesel opacimeter measures the 'light absorption coefficient (m⁻¹)'.

  • A headlight tester measures luminous intensity (cd) and optical axis with tires at specified pressure and 'one driver seated'.
WordMeaning
customer interview and symptom confirmation
CO/HC tester and zero calibration
opacimeter (light absorption coefficient m⁻¹)
headlight tester and brake tester

Common Exam Points and How to Remember Them

On the Type 2 practical exam (3 tasks, 21 questions) and written inspection/measurement questions, 'how to read measuring instrument scales' and 'calculations & pass/fail judgment' directly determine whether you pass or fail.

For Task 1 (Circuit Tester Voltage Drop), be ready to answer instantly: 'Do not miss the unit V vs. mV (0.0 mV = 0 V) on the right side of the display', 'Before lamp = 12 V and after lamp = 0 V is normal', 'If V5 after the lamp is 6.00 V (NG) and V6 is 0.0 mV (Good), the poor contact is between V5 and V6'.

For Task 2 (Compression Gauge), memorize: '4 conditions = Warm up, Stop fuel injection, Remove ALL spark plugs, Open throttle fully' and 'Inter-cylinder difference = Highest − Lowest'. For Task 3 (Dial Gauge), the most critical rules are: 'Clean & set spindle perpendicular (90°) at shaft center' and 'Shaft bend = Runout ÷ 2 (always halve it!)'.

For Vernier Calipers and Micrometers, master: 'Caliper = Main scale before vernier 0 (1 mm) + Aligned vernier line (0.05 mm)', 'Micrometer = Upper sleeve (1 mm) + Check if lower sleeve 0.50 mm line is visible + Thimble (0.01 mm)', and 'Plastigage = wider crushed strip means smaller clearance'.

Key points


  • Circuit tester: distinguish 'V' from 'mV (0.0 mV = 0 V)', and identify the section between two points where voltage changes abruptly (e.g., between V5 at 6.00 V and V6 at 0.0 mV) as the fault location.

  • Compression measurement: test under the 4 conditions 'Warm up, Stop fuel, Remove ALL plugs, Throttle fully open'; Inter-cylinder difference = Highest − Lowest.

  • Dial gauge: apply the spindle perpendicularly (90°), and calculate 'Shaft bend = Runout ÷ 2' before comparing with the limit value.

  • Caliper & micrometer: read the main scale before the vernier '0' on a caliper, and watch out for the lower sleeve '0.5 mm graduation line' on a micrometer.
WordMeaning
voltage drop diagnosis (distinguishing V and mV)
4 compression measurement conditions and inter-cylinder difference
shaft bend (Runout ÷ 2)
sleeve 0.5 mm graduation and vernier scale

Practical: Diagnosis, Measurement & Final Inspection

Practice this chapter in the app

Official references

💬

Zalo