The purpose of this test is to evaluate the condition of the glow plugs and to check their operation time.
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.
This known good waveform has the following characteristics:
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:
Symptoms of failed glow plugs:
Diagnostic trouble codes
Selection of component related Diagnostic Trouble Codes (DTCs):
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