ECU, Sensors, and Solenoids (Diagnosing Modern Excavator Electrical Failures Without Guesswork)

Modern excavators don’t fail the way old machines did.

They don’t always leak.They don’t always make noise.They don’t always throw obvious mechanical symptoms.

Instead, they hesitate.They derate.They lose power “sometimes.”They flash fault codes that point everywhere—and nowhere.

Welcome to the era of ECUs, sensors, solenoids, and CAN bus systems, where many perfectly good hydraulic components get replaced because the electrical diagnosis was lazy.

This blog breaks down how modern excavator electrical systems actually work, how they fail, and how to diagnose them properly—especially on Volvo, Hyundai, Doosan, and Komatsu machines.

The Shift From Mechanical to Managed Machines

Older excavators were:

• Mechanically governed

• Hydraulically controlled

• Diagnosed with pressure gauges and experience

Modern excavators are:

• Electronically managed

• Sensor-dependent

• Software-driven

Hydraulics still do the work—but electronics decide how much, when, and whether.

When electronics lie, machines behave badly.

Understanding the ECU: The Brain You Can’t Ignore

The Engine Control Unit (ECU) and Machine Control Unit (MCU) manage:

• Engine RPM

• Fuel delivery

• Hydraulic pump displacement

• Load-sensing logic

• Safety derates

On many machines (especially Volvo and Komatsu), multiple controllers communicate continuously.

Important Truth:

The ECU rarely fails.The inputs feeding it fail all the time.

Replacing ECUs without proving inputs is one of the most expensive diagnostic mistakes in modern equipment repair.

CAN Bus Systems: Where Most Ghost Faults Live

What Is CAN Bus?

CAN (Controller Area Network) is the communication backbone that allows:

• ECUs

• Sensors

• Displays

• Joysticks

• Controllers

…to talk to each other using shared data lines.

Instead of individual wires for each signal, everything rides on two communication wires.

Why CAN Bus Matters

If CAN communication is compromised:

• Sensors appear to fail

• Solenoids don’t respond

• Fault codes multiply

• Systems derate or shut down

But the actual problem may be:

• A damaged wire

• A bad ground

• Electrical noise

• A terminating resistor issue

Common CAN Bus Failure Causes

☐ Chafed wiring looms

☐ Corrosion in connectors

☐ Poor grounding

☐ Water ingress

☐ Aftermarket accessories spliced incorrectly

Key Symptom Pattern

• Multiple unrelated fault codes

• Intermittent problems

• Faults that disappear when restarted

This is rarely a real component failure.

Pressure Sensors: The Most Blamed, Least Understood Component

Modern excavators rely heavily on pressure sensors for:

• Load sensing

• Pump control

• Power management

• Safety derates

Why Pressure Sensors Fail (or Appear To)

Pressure sensors:

• Live in harsh environments

• Are exposed to vibration

• Are sensitive to contamination

But more often, they:

• Drift out of calibration

• Send unstable signals

• Lie intermittently

The Trap

A sensor reading plausible but wrong causes:

• Reduced pump output

• Sluggish hydraulics

• False overload conditions

The ECU believes the sensor—even when it’s wrong.

Diagnosing Pressure Sensor Issues Properly

Don’t just replace the sensor.

Verify:

☐ Reference voltage (usually 5V)

☐ Ground integrity

☐ Signal stability under load

☐ Correlation with mechanical gauge

If the mechanical gauge and sensor disagree, trust the gauge.

RPM Sensors: Small Parts, Big Consequences

RPM sensors inform the ECU about:

• Engine speed

• Pump synchronization

• Fuel mapping

A failing RPM sensor can cause:

• Power loss

• Erratic engine response

• Sudden derates

• Stalling under load

Classic Symptom

Machine works fine at idle or light load, then:

• Loses power suddenly

• Recovers after restart

This is often mistaken for:

• Fuel problems

• Turbo issues

• Hydraulic overload

It’s frequently electrical.

Solenoids: Where Hydraulics Meet Electronics

Solenoids convert electrical commands into hydraulic action.

They control:

• Pump displacement

• Valve spool movement

• Brake release

• Travel logic

When solenoids fail, machines behave unpredictably.

Solenoid Resistance Testing (Done Properly)

Resistance testing is basic—but often done wrong.

Correct Method:

☐ Disconnect solenoid

☐ Measure resistance cold

☐ Compare to OEM spec

☐ Heat solenoid and retest

What Matters:

• Consistency

• Not just absolute value

A solenoid that changes resistance dramatically when hot is failing—even if it passes cold tests.

Voltage Drop: The Silent Solenoid Killer

Many solenoids are blamed when the real issue is voltage drop.

☐ Corroded connectors

☐ Weak grounds

☐ Undersized wiring

Result:

• Solenoid receives command

• But not enough current to actuate fully

Partial actuation causes:

• Slow movements

• Weak functions

• Heat buildup

False Fault Codes: Why Code Readers Lie

Fault codes do not diagnose failures.They report what the ECU thinks is wrong.

A code may indicate:

• A sensor reading out of range

• A solenoid not responding

• A communication fault

But the cause could be:

• Wiring

• Grounding

• Voltage instability

• CAN interference

Replacing parts based only on codes is parts roulette.

Brand-Specific Electrical Failure Tendencies

VOLVO

• Heavy reliance on sensors

• Thermal-related sensor drift

• CAN bus sensitivity

Volvo machines often derate correctly—but for the wrong reason.

KOMATSU

• Complex load-sensing logic

• Multiple controllers

• Fault cascades

One bad input can trigger multiple false alarms.

DOOSAN

• Robust hydraulics

• Wiring and connector vulnerability

• Ground-related issues

Electrical faults often look mechanical.

HYUNDAI

• Cost-effective electronics

• Sensitivity to contamination and vibration

• Early-life sensor failures

Sensor replacement without validation leads to repeat issues.

Wiring: The Least Sexy, Most Important System

Most electrical failures are:

• Not sensors

• Not solenoids

• Not ECUs

They’re wiring issues.

Common Wiring Failures:

☐ Looms rubbing on frames

☐ Oil-soaked insulation

☐ Connector pin fretting

☐ Poor aftermarket repairs

A single damaged wire can mimic:

• Pump failure

• Valve failure

• Engine failure

Diagnostic Order (Non-Negotiable)

Professionals diagnose electrical failures in this order:

1. Power supply

2. Grounds

3. Wiring integrity

4. Signal verification

5. Component testing

6. ECU replacement (last, rarely)

Skipping steps creates expensive mistakes.

Why Electrical Failures Kill Good Hydraulic Parts

A bad signal can:

• Overstroke a pump

• Under-command a valve

• Overheat a motor

Hydraulic components fail because electronics lied.

Replacing hydraulics without fixing electronics guarantees repeat failure.

The Vikfin Position on Electrical Diagnostics

At Vikfin, we see the aftermath:

• New pumps destroyed

• Good valves scrapped

• Travel motors replaced unnecessarily

Because the root cause wasn’t mechanical.

Modern excavators demand electrical literacy, not just spanners.

Final Truth

Modern excavators don’t ask:

“Is the part broken?”

They ask:

“What does the signal say?”

If you don’t verify the signal, you’re guessing.

And guessing is expensive.

Final Takeaway

Electrical systems don’t fail loudly.They fail convincingly.

They convince machines to:

• Derate

• Hesitate

• Protect themselves

Your job is to determine whether they’re right—or lying.

That’s how real diagnostics happen.