How to Test Glow Plugs: A Comprehensive Guide for Diesel Engine Owners

Glow plugs are essential components in diesel engines, playing a crucial role in cold starting. Unlike gasoline engines that rely on spark plugs, diesel engines use compression to ignite the air-fuel mixture. However, during cold starts, the compressed air isn’t always hot enough to ignite the fuel effectively. This is where glow plugs come in. They heat up the combustion chamber, providing the necessary temperature for reliable ignition. If your diesel engine is having trouble starting, especially in cold weather, faulty glow plugs are a likely culprit. Knowing how to test glow plugs is a valuable skill for any diesel owner, allowing you to diagnose and address issues efficiently. This comprehensive guide will walk you through the process of testing glow plugs using various methods, from basic visual checks to more detailed electrical tests. Before beginning any work on your vehicle, always ensure your safety. Wear appropriate safety glasses and gloves, and disconnect the battery’s negative terminal to prevent accidental shorts.

Understanding Glow Plug Function and Failure

Before we dive into testing, let’s understand how glow plugs work and what causes them to fail. Glow plugs are essentially small heating elements located in the cylinder head, near the injectors. When the ignition is turned on, the glow plug relay sends electrical current to the plugs, causing them to heat up. This intense heat warms the air in the combustion chamber, facilitating the ignition process when fuel is injected. After the engine starts, the glow plugs may continue to operate for a short time to improve combustion and reduce emissions. Common symptoms of failing glow plugs include:

• Difficult or slow engine starting, particularly in cold weather
• White smoke from the exhaust during startup
• Engine misfires or rough idling after starting
• Reduced fuel efficiency
• Check Engine Light (CEL) illumination (in some vehicles)

Glow plugs can fail due to several reasons:

• Overheating: Excessive voltage or prolonged glow plug operation can lead to burnout.
• Carbon Buildup: Carbon deposits can accumulate on the glow plug tips, hindering their ability to heat effectively.
• Physical Damage: Physical impact, incorrect installation, or engine malfunctions can damage the plugs.
• Electrical Issues: Problems with the glow plug relay, wiring harness, or control module can prevent the plugs from functioning correctly.
• Normal Wear and Tear: Like any other component, glow plugs have a limited lifespan and will eventually need replacement.

Tools and Materials Needed for Testing

Before you start testing, gather the necessary tools and materials:

• Multimeter: A digital multimeter is essential for measuring voltage, resistance, and continuity.
• Jumper Wires (optional): Jumper wires with alligator clips are helpful for making connections when testing outside the engine.
• Wrench Set or Socket Set: Needed to remove glow plug wiring and the plugs themselves if necessary.
• Penetrating Oil (optional): Useful if glow plugs are corroded or difficult to remove.
• Safety Glasses: Always protect your eyes.
• Gloves: Protect your hands from dirt and oil.
• Vehicle Service Manual (recommended): Your vehicle’s service manual will provide specific instructions and torque specifications.

Testing Glow Plugs: Step-by-Step Guide

There are several ways to test glow plugs, ranging from simple visual inspections to more advanced electrical testing. Here’s a breakdown of the methods:

Method 1: Visual Inspection (Quick Check)

Steps:

1. Locate Glow Plugs: Identify the location of the glow plugs on your engine. They are typically located near the injectors on the cylinder head. Consult your vehicle’s service manual if needed.
2. Disconnect Wires: Carefully disconnect the electrical connectors or wires attached to each glow plug.
3. Inspect the Tips: Examine the tips of the glow plugs. Look for signs of excessive carbon buildup, physical damage (cracks, bends, swelling), or corrosion. If the tips look damaged or significantly corroded, the glow plug should be replaced.

Limitations: Visual inspection is a quick way to identify obviously damaged glow plugs, but it does not tell you if the plug’s heating element is still functioning. A physically fine plug may still be electrically defective.

Method 2: Continuity Test Using a Multimeter

A continuity test measures whether electricity can flow through a circuit. This test will determine if the internal heating element of the glow plug is still intact.

Steps:

1. Set Multimeter to Continuity Mode: Turn your multimeter on and set it to the continuity setting (usually indicated by a diode symbol or a speaker icon).
2. Connect the Probes: Touch one multimeter probe to the terminal (top) of the glow plug and the other probe to the body (base) of the glow plug.
3. Check the Multimeter: If the multimeter beeps or the resistance is close to 0 ohms, it indicates good continuity, and the glow plug is likely functional (at least partially). A reading of “OL” or a very high resistance means there is no continuity, indicating a failed heating element.
4. Repeat for Each Glow Plug: Test all glow plugs using the same method.

Interpreting Results:

• Continuity (Beep or Low Resistance): The glow plug is likely functional. Note that continuity alone doesn’t guarantee the glow plug heats effectively, but it’s a good indication that it’s not completely broken.
• No Continuity (No Beep, “OL” or Very High Resistance): The glow plug is defective and needs to be replaced.

Method 3: Resistance Test Using a Multimeter

This method involves measuring the resistance across the glow plug’s terminals. Comparing the measured resistance to the manufacturer’s specifications can indicate the health of the glow plug.

Steps:

1. Refer to Vehicle’s Specifications: Find the recommended resistance range for your vehicle’s glow plugs. This information can be found in the vehicle’s service manual or by searching online with your vehicle’s make, model, and year.
2. Set Multimeter to Resistance Mode: Set your multimeter to measure resistance (Ohms, represented by the Ω symbol). Choose a low resistance range, like 200 ohms, if available.
3. Connect the Probes: Touch one multimeter probe to the terminal (top) of the glow plug and the other probe to the body (base) of the glow plug.
4. Read the Multimeter: Observe the resistance reading. Compare the measured resistance to the manufacturer’s specified range.
5. Repeat for Each Glow Plug: Test all glow plugs using the same method.

Interpreting Results:

• Resistance Within Specified Range: The glow plug is likely functional.
• Resistance Below Specified Range: Indicates a short circuit or internal damage and the glow plug should be replaced.
• Resistance Above Specified Range or “OL” (Open Line): Indicates a broken heating element, and the glow plug needs replacement.

Typical Resistance Ranges: While it varies, a healthy glow plug typically shows a resistance between 0.5 and 2 ohms. Lower resistance (below 0.5 ohms) can indicate an internal short, while higher resistance (over 2 ohms or an “OL” reading) signifies an open circuit. Consult your vehicle’s manual for specific values.

Method 4: Direct Battery Test (Advanced Method)

This method is more advanced and involves directly connecting the glow plug to a 12V battery to see if it heats up. This method helps to visibly confirm if a glow plug heats up as expected. This method should be performed with caution as it can damage the glow plug if not done correctly. Do this ONLY if you understand basic electrical safety.

Steps:

1. Disconnect Glow Plugs: Disconnect all electrical connectors from the glow plugs.
2. Remove a Glow Plug: Carefully remove one glow plug from the cylinder head. It might require some force and possibly penetrating oil to break any corrosion that may be present. Consult your vehicle’s repair manual for proper removal procedures.
3. Connect Jumper Wires: Connect one jumper wire to the positive terminal of a 12V battery. Attach the other end of that jumper wire to the terminal (top) of the removed glow plug.
4. Connect Ground Wire: Connect another jumper wire to the negative terminal of the 12V battery. Attach the other end of that jumper wire to the threaded base (body) of the glow plug. Ensure good metal-to-metal contact for both connections.
5. Observe the Plug: The glow plug tip should start to glow red-hot within a few seconds (typically 3-5 seconds for most standard glow plugs). Do NOT leave it connected for more than 10 seconds to avoid overheating and potentially damaging the plug or yourself.
6. Repeat for Each Glow Plug: Perform this test for each glow plug you removed.

Interpreting Results:

• Plug Glows Red-Hot: The glow plug is working correctly, although still check for proper resistance.
• Plug Does Not Glow or Glows Very Little: The glow plug is defective and needs to be replaced.
• Plug Sparks or Smokes: The plug is internally shorted and should not be used. Disconnect it immediately and replace.

Important Safety Notes:

• Short Duration: Do not connect the glow plug to the battery for more than 10 seconds. Extended exposure can overheat and damage the glow plug.
• Use Gloves: Wear heat-resistant gloves when handling glow plugs, as they become extremely hot.
• Keep Away from Flammable Materials: Ensure there are no flammable materials nearby when conducting this test.

Method 5: Using a Glow Plug Tester

Dedicated glow plug testers are available for purchase. These tools are designed specifically to test glow plugs and provide a quick assessment of their health. They often indicate good, bad, or open circuit conditions. Follow the manufacturer’s instructions for using the specific tester you have acquired. These tools are convenient and generally provide a simple pass/fail result.

Diagnosing Related Issues

If your glow plugs are functioning correctly, but you’re still experiencing starting problems, other components in the glow plug system may be faulty. These include:

• Glow Plug Relay: This relay controls the power supply to the glow plugs. A faulty relay will prevent the glow plugs from heating up. You can test the relay using a multimeter or by swapping it with a known-good relay.
• Glow Plug Control Module: Some vehicles use a dedicated control module to manage glow plug operation. Issues with this module can cause glow plug problems. Diagnosing this may require professional tools or diagnostic knowledge.
• Wiring Harness: Damaged or corroded wiring can prevent the glow plugs from receiving power. Inspect the wiring and connectors carefully for any signs of damage.
• Engine Control Unit (ECU) / Powertrain Control Module (PCM): The Engine Control Unit or Powertrain Control Module may be at fault. If other tests reveal no problems, it may be time to seek advanced diagnosis.

Replacing Faulty Glow Plugs

If you’ve determined that one or more glow plugs are faulty, replacing them is necessary. Here’s a general procedure:

1. Disconnect the Battery: As always, disconnect the negative terminal of your vehicle’s battery to prevent electrical shorts.
2. Locate and Disconnect Glow Plugs: Remove the electrical connectors from the glow plugs.
3. Remove the Glow Plugs: Use the appropriate socket or wrench to carefully unscrew the glow plugs. If they are difficult to remove, apply penetrating oil and let it sit for a while before trying again. Be very careful not to break the glow plugs inside the engine, as they can sometimes seize.
4. Install New Glow Plugs: Install the new glow plugs, ensuring they are tightened to the manufacturer’s specified torque. Over-tightening can damage the glow plugs or the cylinder head.
5. Reconnect Wiring: Reconnect the electrical connectors to the glow plugs.
6. Reconnect Battery: Reconnect the negative terminal of the vehicle’s battery.
7. Start the Engine: Start the engine and verify that the starting issue is resolved.

Important Notes:

• Torque Specifications: Always tighten glow plugs to the manufacturer’s specified torque. Over-tightening can damage the plugs or the cylinder head. Use a torque wrench for accurate tightening.
• Anti-Seize Compound: Some mechanics recommend applying a small amount of anti-seize compound to the threads of the new glow plugs to prevent future seizure. Use sparingly.

Conclusion

Testing glow plugs is a crucial step in diagnosing and resolving cold starting issues in diesel engines. By understanding how glow plugs work and following the testing methods described above, you can efficiently pinpoint problems and take corrective action. Regular maintenance and proper diagnosis of your vehicle’s glow plug system will help ensure reliable starts, optimal engine performance, and a longer lifespan for your diesel engine. Remember, while some diagnostic procedures are suitable for DIY work, complicated electrical problems may require the expertise of a trained mechanic. If you are not comfortable working on your vehicle, seek professional assistance.