Multi-point injection - injector voltage

The purpose of this test is to monitor the injector voltage within a multi-point injection system.

How to perform the test

View connection guidance notes.

1. Connect PicoScope Channel A to the injector’s switched-earth circuit.
2. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
3. Run the engine.
4. Start the scope to see live data.
5. With your waveform on screen stop the scope.
6. Turn off the engine.
7. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Example waveform

Waveform notes

This known good waveform has the following characteristics:

• Initially, the waveform voltage on the switched-earth side of the injector is at battery voltage, 14 to 15 V, and the injector is off.
• At 0 ms, the Engine Control Module (ECM) provides an earth path to battery negative, 0 V, which marks the start of injection.
• With the engine at normal operating temperature and idling, the injection duration is about 4.25 ms.
• When the injector is switched off at 4.25 ms, there is a spike in the waveform voltage which peaks at about 85 V.
• At about 1 ms after injector switch-off, there is a hump in the decaying voltage spike.
• The waveform voltage remains at battery positive 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.

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 system type and manufacturer application in order to calculate injection duration.

Indirect injectors employ solenoid-controlled valves, which work 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 vehicle to vehicle; 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 injection pulses each 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, therefore their injection durations are reduced to around 2.5 ms at engine idle.

GT382-EN