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

The engine will need to be below normal operating temperature to capture the pre-heat waveform.

Note; select the current clamp 60 Amp scale switch on and zero the current clamp. Observe the current flow directional arrow, incorrect direction will invert your waveform.

  1. Connect PicoSope Channel A current clamp and Channel B voltage probe to the suspect glow plug.
  2. Minimise the help screen and with the Example Waveform displayed on your screen PicoScope has already selected suitable scales for you to capture a waveform. Select GO or press the space bar to see live data.
  3. Switch on the ignition and wait a few seconds before starting the engine.
  4. Allow the engine to idle and observe your waveform.

Example waveform

Waveform notes

Channel A  shows the glow plug current. 

The waveform can best be interpreted in 3 sections from left to right;

This initial section shows the switch on current spike. The current falls as the glow plug heats up. This lasts 2.5 seconds, then the pre-heat stage enters a modulation phase in order to prevent overheating damage to the glow plug.
Engine cranking,
The engine cranks for just over 3 seconds. During this period the control unit switches the glow plug current off.
Post Heat,
The glow plug is switched on again and settles down to a modulated current/temperature whilst the engine is running. This helps to reduce engine cold running noise and emissions. In this case the glow plugs are switched off after 1.5 mins engine running time. The run time with glow plugs on is dependant on many factors monitored by the Engine Control Module (ECM) and therefore will vary.

Channel B  shows the glow plug voltage, again in 3 sections;

As expected the voltage is below 12V during the switch on and pre-heat phase but increases slightly as the current falls.
Engine cranking,
The glow plug is switched off 0V.
Post Heat,
The modulated control in this example allows just over 13V to be maintained.

Expanded Waveform.

The Expanded  Waveform shows the on-off modulation switching controlled by the ECM.

Waveform Library

The Waveform Library has 27 examples of glow plug current and voltage.

Go to the drop-down menu bar in the lower left corner of the Waveform Library window and filter for glow plug current or voltage.

glow plugs diesel engines

Technical information

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 had a resistance of 0.6 ohms. Refer to manufacturer 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 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