Single glow plug (using low-current clamp)

The purpose of this test is to check the glow plug voltage and current to ensure correct operation.

How to perform the test

Channel A

Plug the current clamp into Channel A of the scope. Set the clamp to the 60 amp setting (if applicable) and auto-zero it. Place the clamp around the supply wire of the glow plug to be tested.

Channel B

Plug the BNC to 4 mm test lead into Channel B of the scope. Connect a Back-pinning Probe to the positive (colored) connection of the test lead. Place a black clip on the negative connection (black) and clip to a suitable earth connection within the engine bay. Probe the supply wire of the glow plug to be tested.

The connections are illustrated in Figure 1.

Example waveform

Waveform notes

Channel A

This shows the glow plug current. The three sections of the waveform are:

  • Pre-heat
  • Engine cranking - during this period the power supply to the glow plug is removed
  • Post-heat

Channel B

This shows the glow plug voltage. Zoom in and you will see that the supply is modulated in order to control the operating temperature of the glow plug.

glow plugs diesel engines

Figure 2

Technical information

For generic glow plug information please see the "Glowplug" topic.

Various types of modern glow plugs are used in diesel engines. Figure 2 shows some examples. The main properties of these are that they heat up very quickly, so they are sometimes referred to as 'rapid glow' or 'quick glow' systems.

We can see in the zoomed section of the example waveform that constant voltage is supplied to the glow plug for only about 2.8 seconds - this is the rapid heat-up period. The resistance of this type of glow plug is very low - the unit tested here had a resistance of 0.6 ohms. Refer to manufacturer's technical data for specific resistance readings.

Important note: Do not test this type of glowplug by supplying a constant battery voltage to the glow plug using a test lead - the glow plug will overheat and burn out.

Often glow plugs that use 'pre-heat' (continued operation after the engine has started) are of the ceramic type. Pre-heat reduces combustion noise and enables smooth idling on a cold engine.

The waveform shows that the supply is removed from the glow plugs during engine cranking - this is to ensure maximum power delivery to the starter motor. Once the engine has started the modulation of the glow plug continues.

The modulation ensures that the glow plug maintains a constant temperature. The duration of the post-heat period depends on ambient and coolant temperature and is controlled by the engine control module (ECM). Depending on the manufacturer, the ambient temperature must be below 9 °C for the glow plug to operate and post-heat may last for up to four minutes. Post-heat may cut off if the vehicle is driven and the rpm increases above 2,500 rpm.


This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.

Suitable accessories

  • 30 A (low amps) current clamp


  • 20 A / 60 A DC (low amps) current clamp


  • Back-pinning Probe Set


  • Flexible Back-pinning Probe


  • PicoScope Battery Clip


  • Large Dolphin/Gator Clips


  • Premium Test Lead: BNC to 4 mm, 3 m


  • Premium Test Leads: Set of four leads 3 m (TA125 - TA128)


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Guided test: Single Glow Plug