Plug the current clamp into Channel A of the scope. Set the clamp to the 20 amp setting if required and auto-zero it. Place the clamp around one of the injector's multi-plug wires. It may be necessary to carefully remove some of the loom tape to allow adequate space for the clamp's jaws.
The connection is illustrated in Figure 1.
Note: If the waveform does not start with a positive pulse, the current clamp has been connected around the wrong way. Remove it and replace facing the opposite direction.
These example waveforms show the injector current during different operating conditions. The positive signals are the injector 'open' commands and the negative signals are the injector 'close' commands.
Example 1: The first two injector phases are pre-injection and the third is the main injector phase.
Example 2: As the engine speed is raised, the control module increases the duration of the main injector phase.
Example 3: On over-run, only the first pilot injection phase remains.
Figure 2 shows a Bosch piezo injector, as used in most Bosch systems since 2007. Similar injectors are made by Siemens.
Piezo injectors have the advantage over conventional electromagnetic injectors of a very quick reaction time - some five times faster.
Piezo injectors operate by means of a stack of about 300 wafer-thin ceramic plates. When the switching voltage is applied, the plates expand and push upon the injector nozzle, opening the needle and injecting the fuel into the cylinder.
As with all common-rail systems, there may be various injection phases:
Figure 3 shows a piezo element in its two different states. Illustration 1 shows the stack with no supply voltage and illustration 2 shows the increase in height of the stack when the supply voltage is switched on.
Important note: Once the stack is in the injector open position, it does not need to have a continuous supply voltage and will remain in the open state until the close command. Hence the positive and negative current spikes in the waveform. For this reason, never disconnect the injector's multi-plug with the engine running. If the injector is in the open state it could hydraulic-lock the engine with the diesel fuel. The engine will cut out if the control module detects any faults with the injectors or driver circuit.
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.
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