Testing Diesel Engine Glow Plugs for Proper Operation

Oct 22, 2020

Glow plugs play a crucial role in the ignition process of diesel engines. They are designed to preheat the combustion chamber, ensuring reliable cold starts and smooth engine performance. If you're experiencing issues with your diesel engine, it's important to test the glow plugs to determine if they are functioning correctly. In this comprehensive guide, Grafco Electric, a leading expert in electrical services, will walk you through the process of testing diesel engine glow plugs for proper operation.

Understanding Glow Plugs

Glow plugs are heating elements that help ignite the air-fuel mixture inside the diesel engine's combustion chamber. Unlike spark plugs used in gasoline engines, glow plugs rely on high resistance to generate heat, which is essential for starting a diesel engine, especially in colder temperatures.

When the ignition key is turned, the glow plug controller sends an electrical current to the glow plugs. This current heats up the tip of each plug to extreme temperatures, causing them to glow red-hot. As a result, the heated plugs vaporize the diesel fuel mist, aiding in combustion and starting the engine smoothly.

Diagnosing Glow Plug Issues

Identifying potential problems with glow plugs requires a systematic approach. Here are some common symptoms of faulty glow plugs:

  • Difficult engine start, especially in cold weather
  • Long cranking time before the engine starts
  • Engine misfires or rough idling after a cold start
  • Excessive smoke from the exhaust during startup
  • Reduced engine power or acceleration

If you notice any of these signs, it's crucial to perform a thorough glow plug inspection to determine the root cause of the issue.

Step-by-Step Guide to Testing Glow Plugs

Step 1: Safety First

Before you start testing the glow plugs, ensure your diesel engine is turned off and cooled down. Working with hot components can be dangerous, so exercise caution and wear protective gloves to prevent any burns.

Step 2: Visual Inspection

Begin by visually inspecting the glow plugs. Look for signs of physical damage, such as cracks or breakages. Additionally, check for carbon buildup or excessive corrosion, as these can impact the performance of the glow plugs.

Note: If you notice any severe damage or signs of wear, it's recommended to replace the glow plugs entirely to ensure optimal engine performance.

Step 3: Electrical Continuity Test

The next step involves testing the electrical continuity of each glow plug. Use a multimeter set to the resistance mode and connect the meter's probes to the glow plug terminals. A resistance within the specified range indicates a functioning glow plug. If the resistance is too high or too low, it's an indication of a faulty plug that requires replacement.

Step 4: Glow Plug Activation Test

After confirming the electrical continuity, you should proceed with conducting a glow plug activation test. This test verifies whether the plugs are heating up properly.

Start by connecting a test wire to an ignition power source, such as the positive terminal of the battery or the glow plug relay terminal. Touch the other end of the wire to each glow plug terminal, one at a time, for a few seconds. If the glow plug heats up and starts to glow red-hot, it is functioning correctly. Repeat this process for all glow plugs.

Additional Tips and Considerations

  • Replace glow plugs as a set: It is recommended to replace all glow plugs simultaneously, even if only one is found to be faulty. This ensures balanced performance and avoids potential issues down the line.
  • Follow manufacturer specifications: Always refer to your diesel engine's manual or consult with a professional technician to determine the specific testing procedures and resistance values recommended by the manufacturer.
  • Regular maintenance: To prevent future glow plug issues, it's essential to adhere to a regular maintenance schedule. This includes cleaning or replacing the glow plugs according to the manufacturer's recommendations.
  • Consult a professional: If you're uncertain about performing the glow plug testing yourself, it is advisable to seek the assistance of a qualified mechanic or electrician. They have the experience and knowledge to diagnose and resolve any complex glow plug issues.


Testing diesel engine glow plugs for proper operation is essential to ensure the smooth performance and reliability of your diesel engine, particularly during cold starts. By following the step-by-step guide provided in this article, you'll gain the knowledge and confidence to diagnose glow plug issues accurately. Remember, safety should always be your top priority, so exercise caution and seek professional help if needed. For more information and expert assistance, contact Grafco Electric, the trusted name in electrical services.