How to test glow plugs

Written by alibaster smith
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How to test glow plugs
The diesel truck doesn't use spark plugs. (semi trucks image by palms from

Glow plugs are used in diesel engines instead of spark plugs. Diesel engines rely on compression alone to ignite their fuel, so a spark plug is not necessary. A glow plug is necessary to initially start the process, however, and to ensure that the diesel fuel begins to burn before the compression cycle can start. Electrical current is sent through a glow plug, and resistance creates heat on the tip of the plug, causing it to glow. This heat is what initially ignites the fuel. Over time, these plugs will fail. Before you begin replacing glow plugs, learn how to test them to ensure they are no longer good.

Skill level:

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Things you need

  • Voltmeter
  • Socket wrench with extension and plug remover
  • Solvent
  • Clean rags

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  1. 1

    Pull the electrical connector wires off to expose access to the glow plugs. Remove one plug at a time by placing the socket over the top of the glow plug and turn it counterclockwise to remove.

  2. 2

    Clean the threads of the glow plug with solvent and a rag.

  3. 3

    Set your voltmeter to Ohms and touch the electrodes of the voltmeter to the glow plug.

  4. 4

    Check to see what the resistance is of the glow plug. If you do not have an auto-ranging meter, try other resistance setting to verify you measurement. If the meter reads an open or infinity measurement (on Fluke meters, it will read OL), then the plug is defective. If the glow plug shows resistance that is approximately less than 5 Ohms, then the plug is not defective.

  5. 5

    Repeat Steps 1 through 4 for each glow plug.

Tips and warnings

  • The removal procedure and the resistance specs for glow plugs varies depending on the vehicle and glow plug used; for specific information, consult your particular vehicle's manual (see Resources).

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