Diesel Glow Plugs
Using a 0 to 600 amp high-current clamp
 How to connect the oscilloscope |
| Example waveform and notes |
| Technical information |
How to connect the oscilloscope when testing:-
diesel glow plugs
Plug the 600 A current clamp into channel A using the 4 mm sockets on the TA000 test lead, as shown in Figure 6.0.
Fig. 6.0
The clamp should be positioned on one of the two battery connections (live or earth), whichever allows the easiest connection, or directly onto the glow plug supply from the timer relay. The current clamp needs to be switched on and facing the correct way as, if reversed, an inverted picture will be seen.
With the example waveform displayed on the screen you can now hit the space bar to start looking at live readings.
Fig. 6.1
Figure 6.1 shows the amps clamp connected to the supply wire of the glow plugs.
Example diesel glow plug waveform
Glow plugs / timer relay waveform notes
This test is conducted to evaluate the condition of the glow plugs (this example is a 4 cylinder engine) and to measure the 'on time', which is controlled by the timer relay.
A typical glow (or heater) plug will have a high initial current draw that will gradually drop, stabilising at a constant amperage. The current draw will be dependent on the wattage rating of the glow plug. This data is available in the appropriate diesel data books.
Once the wattage has been ascertained, multiply it by the number of cylinders and then divide by the voltage to calculate the expected stabilised current. Example: -
Each glow plug = 150 watts so 4 glow plugs = 600 watts
current = watts divided by volts: 600 watts divided by 12 volts = 50 amps
The length of time that the glow plugs are operational can be measured from the initial drop in current to the switch off point. In this case it is around 17 seconds.
Technical information - glow plugs
Glow (or heater) plugs are used to aid cold starting, being activated in certain engine conditions. The glow plugs can operate in several ways, the most simple being to switch them on during cranking or while the ignition is switched on. The glow plugs are wired in series and are fed with battery voltage for a set period of time, which is determined by the 'glow plug timer relay'.
The components heat to their operating temperature in a matter of seconds and glow white hot when tested out of the engine. The plug should heat from the tip backwards, as illustrated in figure 6.2. Failure to do this means that the plug needs replacing.
Fig. 6.2
Other systems may remain on until the engine's water temperature reaches a predetermined temperature, while others may operate in a similar way but will be pulsed alternately in sets of two.
The testing of the glow plugs can be achieved in one of two ways, by either leaving the plugs in place and monitoring the current draw of them all, or by removing them and analysing their performance visually as they heat up, then measuring their individual current draw, at the same time. It is possible to use the Pico scope to measure this either with this set-up or by selecting 'view' then 'new meter'.