Hong Kong Commercial Diesel Vehicle Shows Intermittent Injection Control During Cranking Despite Normal Battery Resting Voltage as Starter Current and Cranking Voltage Drop Are Investigated

The Battery Looked Healthy Until the Starter Was Used

A commercial diesel vehicle in a Hong Kong operating scenario showed intermittent no-start behavior.

Before cranking, battery voltage appeared normal. Diagnostic communication was available and no obvious injector defect had been confirmed.

During starting, however, injector control occasionally disappeared.

The workshop focused on the electrical conditions that existed only while the starter motor was operating.

Resting Voltage and Loaded Voltage Are Different Measurements

A battery can show acceptable open-circuit voltage while still experiencing a large voltage drop under heavy load.

Cranking places substantial demand on:

• battery;
• cables;
• terminals;
• ground paths;
• and starter motor.

If starter current becomes excessive, system voltage may fall below the level required by the ECU or injector driver.

Observe the Starting Event

Technicians recorded:

• battery voltage before cranking;
• minimum voltage during cranking;
• starter current;
• ECU supply voltage;
• engine cranking speed;
• and whether injection enable remained active.

The timing relationship was critical.

Before Starter Operation

ECU behaved normally.

During Heavy Cranking

Voltage fell and injection control became intermittent.

This connected the symptom to the start event.

Why Starter Current Can Become Excessive

Possible causes include:

• internal starter wear;
• mechanical drag;
• starter shorting;
• engine mechanical resistance;
• poor cables;
• or damaged connections.

High current should therefore be diagnosed rather than simply blamed on the battery.

Cranking Speed Also Matters

Diesel injection control may depend on the engine reaching sufficient cranking speed.

Low voltage and low cranking RPM can occur together.

A starting problem may therefore involve both electrical supply quality and mechanical starter performance.

Why Injector Replacement Would Miss the Problem

If ECU voltage collapses, the control system may not generate or maintain valid injector commands.

The injectors themselves can be completely functional.

The sequence becomes:

Battery resting voltage normal → Starter engaged → Current rises excessively → ECU voltage falls → Injection control interrupted → No-start

Case Insight

The Hong Kong vehicle case demonstrates why starting diagnostics must capture transient electrical behavior.

A voltage reading taken before the starter is engaged cannot describe what the control system experiences during cranking.

FAQ

Can a vehicle have normal battery voltage but still lose ECU power while starting?

Yes. Load-related voltage drop can occur during cranking.

Does high starter current automatically mean the starter is faulty?

No. Engine drag, cabling and connection problems can also affect current demand.