Glow plug system current

The purpose of this test is to evaluate the condition of the glow plugs and to check their operation time.

How to perform the test

View connection guidance notes.

1. Use manufacturer’s data to identify the glow plug supply circuit.
2. Connect the high amp clamp to PicoScope Channel A
3. Switch on and zero the clamp.
4. Place the clamp around to the glow plug supply circuit.
5. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
6. Start the scope to see live data.
7. Switch on the ignition, wait for the glow plug warning light to extinguish, then start the engine and let it idle.
8. Depending on the system type and engine operating conditions, the glow plugs may be activated for a period of time after the engine has started.
9. Stop the engine and turn off the ignition.
10. With your waveform on screen stop the scope.
11. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Note

The glow plugs may not activate if the ambient and engine temperature conditions are not correct.

The orientation of the clamp relative to the wire will determine whether it has a positive or negative output. If a live waveform does not appear on your screen, or appears to be inverted, try reversing the orientation of the clamp.

Example waveform

Waveform notes

This known good waveform has the following characteristics:

• A current spike up to around 100 A when the glow plugs are switched on, followed by an immediate decrease to around 70 A, and then a more gradual decrease to about 45 A over the next 5 s.
• The current remains stable at 45 A until the glow plug relay is switched off.
• The total on time is just over 11 s.

Waveform Library

Go to the drop down menu in the lower left corner of the Waveform Library window and select Glow plug current.

Further guidance

Glow plugs support diesel fuel combustion and emissions control processes.

Injected diesel fuel ignites if the cylinder charge temperature reaches 850° C during compression. However, this temperature may not be achieved with cold ambient air conditions and engine components. In these circumstances, the glow plugs are activated to heat the cylinder charge and ensure adequate combustion.

Glow plugs are designed to operate within temperatures from 850° to 1100° C. They are controlled by a relay switched by either the Engine Control Module (ECM) or by a dedicated glow plug control unit/timer. The system switching and on time characteristics will vary with vehicle.

Glow plugs also heat the cylinder charge to support the operation of diesel particulate filter (DPF) systems: a DPF requires very high exhaust temperatures during passive and active (either running or forced) regeneration processes. Any fault in the glow plug system will prevent regeneration and will inevitably lead to the excessive build-up of particulates in the filter and eventual blockage.

One of the main causes of glow plug failure is overheating. Therefore, some systems use Pulse Width Modulation (PWM) of the supply voltage to regulate the circuit current and to control glow plug temperature.

The expected circuit current is calculated by dividing the total power consumption of all glow plugs (individual plug wattages are available in the appropriate technical literature) by the circuit feed voltage (current = power / voltage).

For example, if a 12 V, 4-cylinder diesel engine with 150 W glow plugs, has a total power consumption of 600 W (4 x 150 W), the expected steady-state circuit current will be around 50 A (600 W / 12 V).

It is important to be able to identify control system failures, such as those that might occur in the ECM, glow control module, or other relays, as a cause of glow plug failure.

Glow plug circuits are susceptible to a variety of faults, such as:

• Short or open circuits or high resistances within the circuitry or connections
• Control failures, such as ECM, control module or relay failures
• Application of 12 V directly onto a glow plug outside of ECM control (incorrect testing)
• Thermal failures, such as those caused by:
  • excessive carbon build-up (causing overheating)
  • an incorrect injector spray pattern (over cooling)
  • incorrect timing (and fuel delivery)

Symptoms of failed glow plugs:

• Malfunction Indicator Lamp (MIL) or glow plug warning lamp illumination
• Diagnostic Trouble Codes (DTCs)
• Engine non-starting
• Excessive emissions when cold
• Excessive cranking time
• Rough running when cold
• Excessive particulate build-up in the Diesel Particulate Filter (DPF), Exhaust Gas Recirculation (EGR), Selective Catalytic Reduction (SCR) and intake (via EGR) systems

GT006-EN