Diesel glow plugs

You will require a PicoScope to perform this test. A list of suitable accessories can be found at the bottom of this page.

How to perform the test

Channel A

  1. Plug the current clamp into Channel A of the PicoScope.
  2. Fit the clamp on one of the two battery cables (either positive or negative), whichever allows the easiest connection, or directly on the glow plug supply from the timer relay.
  3. Switch the current clamp on.
  4. Ensure that the clamp is facing the correct way (if it is reversed, the waveform will be inverted).

With the example waveform displayed on the screen, press the space bar to start looking at live readings.

Example waveform

Waveform notes

This test is to evaluate the condition of the glow plugs (in this case, in a 4 cylinder engine) and to measure the on-time, which is controlled by the timer relay.

The current draw of a typical glow (or heater) plug begins at a high level, then gradually drops to a final stable level. The final current depends on 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

The on-time is measured from the initial drop in current to the switch-off point. In this case it is around 11 seconds.

glow plug heater

Figure 2

Technical information

Glow (or heater) plugs are used to aid cold starting, and are activated only under certain engine conditions. Glow plugs can be controlled in several ways, the simplest being to switch them on with the ignition supply or during cranking. Other systems may keep them on until the engine coolant reaches a predetermined temperature. The glow plugs are traditionally wired in parallel and fed with battery voltage, although in some systems they are pulsed alternately in sets of two. The glow plug timer relay switches the plugs off after a set 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.

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.


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


Share your experience

If you have any suggestions to improve this guided test please do so using the 'add comment' button.

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