CRD (Bosch) solenoid injector circuit current

The purpose of this test is to check the actuation current within a Bosch-type Common Rail Diesel (CRD) solenoid injector circuit across a range of engine load and demand conditions.

How to perform the test

View connection guidance notes.

1. Connect the low amp clamp into PicoScope Channel A.
2. Select the 20 A scale and zero the clamp before connecting to the injector supply circuit.
3. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
4. Start the scope to see live data.
5. Start the engine and allow it to idle.
6. Vary the accelerator pedal to see idle, acceleration and overrun injector waveforms.
7. With your waveforms on screen stop the scope.
8. Turn off the engine.
9. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Note

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

Engine at idle.

Engine at high speed.

Engine on overrun.

Waveform notes

These known good waveforms have the following characteristics:

• All injection events begin with a peak current in the range 16 to 18 A to open the injector.
• With the engine idling there are one or two pilot injections and one main injection.
• The pilot and main injection events have an additional phase of lower current, which modulates from 12 to 14 A. This retaining current holds the injector in its open position.
• The retaining current of the main injection increases in duration with increasing torque demand (as signalled by changes in the accelerator pedal position); however, the pilot injection retaining current does not.

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select Injector current.

Further guidance

A common rail diesel injector delivers atomised fuel directly to the combustion chamber.

Injection timing and quantity is controlled by the Engine Control Module (ECM). Common rail diesel solenoid injectors can be actuated independently of the pressure generating mechanism, unlike their counterparts within distributor pump or unit injector (i.e. Pumpe-Düse or PD) systems, which can only operate within periods of high pumping pressure. This characteristic facilitates multiple injection events per engine cycle and permits additional functionality:

• Pilot injection which delivers a small quantity of diesel prior to the main injection event to better regulate the build-up of combustion pressure and reduce diesel knock; and
• Post injection to facilitate exhaust gas treatment processes.

In order to facilitate their rapid operation, injector switching circuit voltages are quite high, typically 50 to 90 V.

Furthermore, the injector solenoids are rapidly energised using charge drawn from a combination of the vehicle power supply system and capacitors. The capacitors are charged by the circuit voltage induced by the solenoids when the supply voltage is removed.

Common rail diesel injectors are susceptible to mechanical and electrical faults, producing a variety of symptoms:

• Cylinder sealing: if the interface between the injector and the cylinder head is not properly sealed, unburnt or partially burnt gasses and soot can escape from the cylinder during compression and combustion, causing blow-by and reduced cylinder pressures/combustion temperatures, which, in turn, can cause excessive exhaust smoke.
• Internal sealing: if an internal seal has failed, fuel will leak from the supply side through to the return side, causing possible high pressure supply issues, such as a reduced pressure during cranking (where the high pressure pump speed and its ability to generate sufficient flow is limited).
• Electrical circuits: if an injector solenoid cannot energise sufficiently then it’s valve may not open properly, which may cause misfire symptoms.
• Contamination: engine issues causing excessive soot or contamination within the engine may affect the injector outlet, which can affect their flow and spray patterns.

GT039-EN