Single point injection - injector: voltage [20:1 attenuation]

The purpose of this test is to evaluate the correct operation of a Single Point Injection (SPI) injector based on the switching voltage, pulse width, and formation during engine run conditions.

Note: This help file refers to a 20:1 attenuator. If you are using a 10:1 attenuator please adjust the Probe settings for the relevant channel. These settings can be found under the Channel Options button, then: Probe > 10:1 Attenuator.

Connection guidance

Connection for diagnostic work will vary dependent on application.

Technicians should whenever possible gain access to the test circuit without damage to seals and insulation. If this is not possible then make sure appropriate repairs are completed.

General connection advice

PicoScope offers a range of options within the test kits.

Dependent on difficulty of access, choose from:

  1. Breakout leads.
  2. Back-pinning probes.

Testing sensors and actuators (to include relevant circuit/connectors):

  • When testing a sensor, it is desirable to gain access at the control unit.
  • When testing an actuator, it is desirable to gain access at the actuator.

How to perform the test

Plug the 20:1 Attenuator into Channel A on the PicoScope and the BNC test lead into the other end of the attenuator. Place a black clip on the test lead with the black moulding (negative) and a Back-pinning Probe onto the test lead with the red moulding (positive). Attach the black clip onto the battery negative terminal and probe the switched earth side of the injector with the Back-pinning Probe as illustrated in Figure 1. If you cannot reach the terminal or plug with a probe, then you may be able to use a breakout box or lead if you have one available.

Please note that neither wire of the injector should be connected to the negative (ground) input of the oscilloscope as this could cause a short circuit.

The 20:1 Attenuator is used to monitor the induced voltage that is created when the earth path to the injector is removed. This voltage will be in the region of 60 to 80 volts.

With the example waveform displayed on the screen you can now hit the space bar to start looking at live readings. Snap the accelerator quickly from idle to full throttle and observe the supplementary injection of the waveform expanding under acceleration.
The waveform is stabilised by using the falling voltage to trigger the oscilloscope.

Example waveform

Waveform notes

Single Point Injector (SPI) is also sometimes referred to as Throttle Body Injection (TBI).

A single injector is used (on larger engines two injectors can be used) in what may have the outward appearance of a carburettor housing.

The resultant waveform from the SPI system will show an initial injection period followed by multi-pulsing of the injector in the remainder of the trace. This section of the waveform is called the supplementary duration and is the only part of the injection trace to expand.

Further guidance

The reason that a single point injector is used rather than a multi-point configuration is sometimes hard to justify, and can only be due to a consideration towards costing and ease of application. A single injector is used (on larger engines two injectors can be used) in what may appear to be a carburettor housing.

It has a very low operating pressure (usually around 1 bar) and the atomising of the fuel can only be described as minimal, relying on the air movement within the inlet manifold to break the fuel down into smaller particles, ready for combustion.

Due to its design, the main advantage over a carburettor is that a lambda sensor can be employed ensuring that closed-loop control is maintained. Multi-point will undoubtedly ensure that the vehicle's engine has a higher power output with fewer exhaust emissions.

Due to the design of the system, a conventional air flow meter cannot be used and a map sensor is often employed.


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.

Suitable accessories

  • Premium 6-way breakout lead set


  • Back-pinning Probe Set


  • Flexible Back-pinning Probe


  • PicoScope Battery Clip


  • 10:1 Attenuator


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Guided test: Single point injection - injector: voltage [20:1 attenuation]