“Relay Not Working? Here’s How to Test and Find the Problem”

Complete Guide to Finding Relay Faults in Cranes and Vehicles

A relay is a small but critical electrical component used in cranes, trucks, cars, buses, and all heavy equipment. When a relay fails, many systems stop working—AC compressor, cooling fan, lights, horn, boom functions, hydraulic pumps, sensors, and safety systems.

This guide explains relay faults in a simple, serial sequence so technicians, mechanics, and electricians can diagnose issues quickly.

Pin Relay Basic Explanation

A standard 4-pin automotive relay has:

Pin 30 → Power input (from battery)
Pin 87 → Power output (to load: fan, light, horn, compressor etc.)
Pin 85 → Coil negative (ground)
Pin 86 → Coil positive (signal from switch/ECU)

How it Works

1. When pin 86 gets 12/24V, current flows through coil (85–86).

2. Coil creates magnetic field.

3. Relay internal contact closes between 30 → 87.

4. Power flows to the load. 

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1. Understanding the 4-Pin Relay

A standard 4-pin relay has four terminals:

• Pin 85 – Coil negative (ground)

• Pin 86 – Coil positive (signal)

• Pin 30 – Power input from battery

• Pin 87 – Power output to load

When voltage is applied to pins 85 and 86, the relay clicks and connects pin 30 to pin 87, allowing current to flow to the load.

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2. Common Symptoms of Relay Faults

1. Fan not working

2. AC compressor not engaging

3. Boom functions or safety systems failing

4. Motor or load weak

5. Intermittent electrical operation

6. Burning smell or hot relay

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3. Types of Relay Failures

Relay faults generally occur in six major ways:

1. Coil failure – No click sound

2. Contact failure – Click present but no output

3. Intermittent operation – Works sometimes

4. Overheating – Relay gets hot

5. Weak output – Reduced voltage at output

6. Socket/terminal issues – Corrosion or loose fit

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4. Step-by-Step Relay Fault Diagnosis

Follow these serial steps to identify any relay problem in cranes or vehicles.

4.1 Check for Click Sound

• Turn ON the switch controlling the relay.

• Place your finger on the relay.

• If there is no click, the issue is with:

  • Coil

  • Pin 86 input

  • Pin 85 ground

  • Fuse before relay

4.2 Check Signal Voltage (Pin 86)

• Turn ON the system.

• Measure voltage on pin 86.

• If voltage is missing, the fault is before the relay (switch, sensor, ECU).

• If voltage is present but relay doesn’t click — the relay coil is faulty.

4.3 Check Coil with Multimeter (85–86)

• Set multimeter to Ohms.

• Measure between pins 85 and 86.

• 40–120 Ω = good

• OL = coil open

• 0–10 Ω = coil short

4.4 Check Relay Contacts (30–87)

• In continuity mode, measure between pin 30 and 87.

• Without activation → should be open.

• With activation → should be closed (continuity).

4.5 Voltage Test under Load

• Power ON the relay.

• Measure input voltage at pin 30.

• Measure output voltage at pin 87.

• Voltage drop must be less than 0.3V.

• Drop above 1V = weak or burnt contacts.

4.6 Load Operation Test

• Connect a bulb or motor to pin 87.

• Activate the relay.

• If load flickers or stays dim → relay contacts weak.

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5. Identifying Relay Faults by Symptoms

5.1 Relay Not Clicking

Symptoms: Fan/AC/light not working.

Cause: Coil failure, missing signal, or bad ground.

Fix: Check voltage at pin 86 and ground at pin 85.

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5.2 Relay Clicking But No Output

Symptoms: Click sound but system still dead.

Cause: Contacts burnt or damaged.

Test: Check voltage at pin 87 when relay clicks.

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5.3 Relay Works Sometimes (Intermittent)

Symptoms: Operation ON/OFF randomly.

Causes: Weak coil, dirty contacts, loose socket.

Test: Tap the relay lightly. If system responds, relay is faulty.

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5.4 Relay Overheating

Symptoms: Hot relay body, melted plastic, burnt smell.

Cause: Overload, wrong relay rating, resistance in contacts.

Test: Voltage drop test (30→87).

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5.5 Weak Output

Symptoms: Fan slow, compressor weak.

Cause: High resistance in relay contacts.

Test: Measure voltage on pin 87 when relay is ON.

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5.6 Relay Socket Fault

Symptoms: Works when relay is moved or pushed.

Causes: Loose pins, corrosion, heat damage.

Check: Inspect relay socket for green or black discoloration.

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6. Fast Technician Methods for Relay Diagnosis

Professional technicians use three quick tests:

6.1 Swap Test

Replace relay with another identical working relay.

• If system works → old relay defective.

6.2 Jump Test

Join pin 30 to 87 using a jumper wire.

• If load turns ON → relay is faulty.

6.3 Ground & Power Verification

• Ensure pin 85 has ground.

• Ensure pin 30 has battery power.

If both are OK and relay doesn’t click → relay is dead.

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7. Summary Table

Fault Type	Symptom	Detection Method
Coil failure	No click	Check pin 86 voltage & ohm test
Contacts burnt	Click but no output	Check voltage at pin 87
Intermittent	Works randomly	Tap test
Overheating	Relay hot	Voltage drop test
Weak output	Load weak	Measure voltage at 87
Socket fault	Works when moved	Inspect terminals

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8. Final Technician Tips

• Always check fuses before relays.

• Never use a lower ampere relay than the original.

• Keep relay bases clean and dry.

• Use dielectric grease in cranes (prevents moisture).

• Mark relays to avoid confusion.

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