Diesel glow plugs

The purpose of this test is to evaluate the condition of the glow plugs based on the total current draw, while also confirming their operation time.

Connection Guidance

How to perform the test

  1. Connect the high amp current clamp into Channel A of your PicoScope. Switch on and zero the clamp.
  2. Place the current clamp onto one of the battery cables (+) or (-), or directly onto the glow plug supply cable from the timer relay access permitting. Observe the current directional arrow, if incorrect the waveform will be inverted.
  3. Minimise the help page and with the example waveform on the screen PicoScope has already selected suitable scales for you to capture a waveform.
  4. Select GO or press the space bar to see live data.
  5. Switch on the ignition to activate the glow plugs. Wait a few seconds before starting the engine.
  6. Allow the engine to idle and observe your waveform.

Example waveform

Waveform notes

The Example Waveform shows the current characteristics and the on time for all 4 glow plugs.

There is a current peak as the glow plugs switch on, the downward current curve is due to the glow plugs heating up.

The on time in this example is around 11 seconds.

Waveform Library

There are 21 examples in the Waveform Library. Use the drop down menu in the lower left corner of the Waveform Library window, filter for Glow Plug Current.

glow plug heater

Figure 2

Further Guidance

Technical Information

Glow plugs are used to aid diesel engine cold starting they are normally connected in parallel so that each plug receives battery voltage. They can be controlled in several ways, the simplest being to switch them on with the ignition supply or during cranking. The glow plugs timer relay switching the circuit off after a set time.

Some systems may keep them on until the engine coolant reaches a predetermined temperature. More systems are now using Pulse Width Modulation (PWM) to prevent the plugs from overheating during extended use.

NOTE, glow plugs which are controlled by PWM, used on many engines as an aid to emission control as well as cold starting must NOT be tested outside of the engine. This type of glow plug will be destroyed by direct uncontrolled battery connection.

Glow plugs can be tested either by leaving them in place and monitoring the combined current draw, as in the waveform above, or by removing each one in turn to check visually that it heats up and measuring its current draw at the same time.

The plugs heat to their operating temperature in a few seconds and glow white hot when tested out of the engine. The plug should heat from the tip backwards as shown in Figure 2. Failure to do this means that the plug needs replacing.

The operating current can be calculated using the wattage rating of the glow plug. This rating is available in the appropriate diesel data books.

Once you know the wattage, multiply it by the number of cylinders and then divide by the voltage to calculate the expected final current.


One glow plug = 150 watts
4 glow plugs = 600 watts
Current = watts divided by volts
600 watts divided by 12 volts = 50 amps.


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

  • 200 A / 2000 A (high amps) DC current clamp


  • 600 A DC (high amps) current clamp


Help us improve our tests

We know that our PicoScope users are clever and creative and we’d love to receive your ideas for improvement on this test. Click the Add comment button to leave your feedback.

2 comments | Add comment

Mike Valentine
January 26 2017

Thanks. Checked and adjusted.

Mike Clarke
December 25 2016

Glow plugs are normally wired in parallel, not in series.

Your email address will not be published. Required fields are marked *

Guided test: Diesel glow plugs