Slow-crank complaint, battery load-tests good, starter draws about normal amperage.

Voltage-Drop Testing Procedure:

1. Battery Side (Positive):

• Red lead on battery positive post, black lead on starter solenoid's "B+" terminal (large cable).
• Crank the engine. Acceptable drop: ≤0.5V

2. Ground Side:

• Red lead on solenoid ground terminal (or engine block near starter), black lead on battery negative post.
• Crank the engine. Acceptable drop: ≤0.3V

3. Control Circuit (S terminal):

• Red lead on solenoid "S" terminal, black lead on battery negative.
• Crank the key. Acceptable drop: ≤0.3V (indicates good ignition switch/signal)

4. Individual Cable Tests:

• Test each battery cable separately by placing leads at each end while cranking.
• Any single cable drop >0.2V indicates a bad cable/connection.

Key Points:

• Clean all connections first - corrosion causes high resistance
• Use a load (cranking) for accurate readings
• If total circuit drop exceeds 0.8V, the circuit has excessive resistance
• Replace cables showing >0.3V drop or visible damage

Safety: Ensure vehicle is in park/neutral with parking brake set. Battery load-testing good doesn't rule out high resistance under load - voltage drop testing is the definitive method per ASE E1/E2 standards.

Voltage-Drop Test Procedure (Starter Circuit)

Safety First: Disable ignition (pull fuel pump relay or coil wire) so engine cranks but won’t start. Battery must be fully charged (12.6 V+). Use a digital multimeter (DMM) on the DC volts (mV) scale.

1. Positive Side (Battery → Starter)

| Test Point | Red Lead | Black Lead | Max Acceptable Drop (Cranking) | | :--- | :--- | :--- | :--- | | Battery Terminal → Post | Battery Post | Terminal Clamp | 0.1 V (100 mV) | | Main Feed Cable | Battery (+) Terminal | Starter B+ Stud (large lug) | 0.2 V (200 mV) | | Solenoid Contacts (Internal) | Starter B+ Stud | Starter M Terminal (motor feed) | 0.1 V (100 mV) |

If B+ to M drop is high, solenoid contacts are pitted—replace solenoid/starter.

2. Negative Side (Starter → Battery)

| Test Point | Red Lead | Black Lead | Max Acceptable Drop (Cranking) | | :--- | :--- | :--- | :--- | | Starter Case → Battery (-) | Starter Housing (clean spot) | Battery (-) Post | 0.1 V (100 mV) | | Engine Block → Battery (-) | Engine Block (clean) | Battery (-) Post | 0.1 V (100 mV) | | Body/Frame → Battery (-) | Body Ground Point | Battery (-) Post | 0.1 V (100 mV) |

High drop here = bad engine-to-body strap, corroded battery cable ends, or loose block ground.

3. Control Circuit (Key → Solenoid “S” Terminal)

| Test Point | Red Lead | Black Lead | Max Acceptable Drop | | :--- | :--- | :--- | :--- | | Ignition Switch → “S” Term | Battery (+) | Solenoid S Terminal | 0.5 V (500 mV) |

High drop = ignition switch, neutral safety/clutch switch, or wiring resistance.

Key Tips

• Crank engine while reading; voltage drop only exists under load.
• Clean probe points with a wire brush for accurate contact.
• Total circuit drop (Pos + Neg) should be < 0.5 V.
• If all drops are low but cranking is still slow: check for mechanical binding (seized accessory, hydro-lock, wrong oil viscosity) or starter internal wear (brushes, bushings, armature drag).

Reference: SAE J1455 / OEM service manuals (e.g., GM 0-0.3V total, Ford <0.5V total).