Multi-point injection - injector voltage and current

The purpose of this test is to monitor the injector voltage and current to determine the integrity of the injector circuit and the operation of the injector.

How to perform the test

View connection guidance notes.

1. Connect PicoScope Channel A to the injector switched earth circuit.
2. Connect the low amp clamp to PicoScope Channel B.
3. Select the 20 A scale and zero the clamp before connecting it to the injector power supply circuit.
4. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
5. Run the engine.
6. Start the scope to see live data.
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

Waveform notes

These known good waveform(s) have the following characteristics:

• Initially, the switched-earth circuit is at battery voltage, 14 to 15 V, no current is flowing, and the injector is off.
• When the switched earth circuit switches to 0 V, current begins to flow and the injector valve starts to open. This marks the start of injection.
• The current increases in two phases. The change point, at around 1.5 ms, marks when the injector valve is fully open.
• Around 3 to 4 ms after the start of injection, the circuit current reaches its peak value, which is sustained for the remainder of the injection period.
• Both the injector voltage and current show that the injection duration is about 4.25 ms.
• When the injector is switched off at 4.25 ms and the current flow stops, there is a spike in the waveform voltage, which peaks at about 85 V, and the injector begins to close.
• At about 1 ms after injector switch-off, there is a hump in the decaying voltage spike.
• The waveform voltage remains at battery positive volts until the next start of injection.

Waveform Library

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

Further guidance

An indirect injector delivers the correct quantity of atomised fuel to the air in the inlet tract as it is drawn through to a cylinder.

A multi-point injection system has one injector per cylinder supplied by a common fuel rail. As the rail pressure regulator maintains a constant pressure, the injected fuel quantity depends only on injection duration.

The ECM uses input signals from a range of sensors, dependent on both the system type and manufacturer application, to calculate injection duration.

Indirect injectors employ solenoid-controlled valves, working against a spring force acting to close them. The valves open when sufficient current flows through their circuit. The injector valve will not open fully if there is insufficient current.

An ECM, or dedicated control module, dictates the current flow in each injector circuit by switching in and out the individual injector earth paths.

When current flows, an injector solenoid builds and stores energy until it is saturated. When the current flow stops, the stored energy is returned to the circuit, inducing a large voltage spike. The voltage spike varies from system to system; some injector circuits include a Zener diode or a resistor-capacitor combination that limits, or squares-off, its peak.

The decay of the voltage spike is sometimes momentarily interrupted by the action of the valve as its spring forces it rapidly back to its closed position. This indicates normal movement of the injector valve.

There are two types of multi-point injection system:

• Sequential systems fire individual injectors every 720° of crankshaft rotation to coincide with the opening of each cylinder inlet valve. Injection durations range from 4 to 5 ms at engine idle.
• Simultaneous systems fire all the injectors together in an inline engine arrangement, or each bank of injectors in a ’V’ arrangement, twice every 720° of crankshaft rotation. In these systems, less fuel is injected per injection and therefore their injection durations are reduced to around 2.5 ms at engine idle.

GT388-EN