Throttle position potentiometer

You will require a PicoScope to perform this test. A list of suitable accessories can be found at the bottom of this page.

How to perform the test

Plug a BNC test lead into Channel A on the PicoScope, place a black clip on the test lead with the black moulding (negative) and a Back-pinning Probe onto the test lead with the coloured moulding (positive). Place the black clip onto the battery negative terminal and probe the throttle potentiometer with the Back-pinning Probe 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 lead or box if you have one available. The variable voltage output is normally the centre terminal (check with technical data). Figure 1 shows the sensor connection.

When testing the throttle potentiometer, it may take several attempts to 'centralise' the waveform when capturing the output.

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 waveform.

Example waveform

Waveform notes

This sensor or potentiometer is able to indicate to the Electronic Control Module (ECM) the exact amount of throttle opening due to its linear output.
The majority of modern management systems will employ this particular sensor, and is located on the throttle butterfly spindle. This is a 3 wire device having a 5 volt supply, an earth connection and a variable output from the centre pin. As the output is critical to the vehicle's performance, any 'blind spots' within the internal carbon track's swept area, will cause 'flat spots' and 'hesitations'. This lack of continuity can be seen on an oscilloscope and will enable the operator to plot the output voltage over its operational range, showing any faulty areas.

A good throttle potentiometer should show a small voltage at the throttle closed position, gradually rising in voltage as the throttle is opened and returning back to its initial voltage as the throttle is shut. Although many throttle position sensor voltages will be manufacturer specific, many are non-adjustable and the voltage will be in the region of 0.5 to 1.0 volts at idle rising to 4.0 volts (or more) with a fully opened throttle. For the full operational range, a time scale around 2 seconds is used.

The picture should be clean with no voltage 'drop out' at any particular point, as this small discrepancy will be sufficient to cause a 'flat spot' under initial acceleration.

throttle position potentiometer

Figure 2

Technical information

This sensor or potentiometer is able to indicate to the ECM the exact amount of throttle opening. A throttle switch is unable to give precise increments of opening; however, a throttle potentiometer will be able to give exact openings due to its linear output. The majority of modern management systems will employ this particular sensor, and like the throttle position switch, it is also located on the throttle butterfly spindle.
This is also a 3 wire device having a 5 volt supply, an earth connection and a variable output from the centre pin.

As the output is critical to the vehicle's performance, any 'blind spots' within the internal carbon track's swept area, will cause flat spots and hesitations. This lack of continuity can be seen on a multimeter, however the use of an oscilloscope will enable the operator to plot the output voltage over its operational range, showing any faulty areas. A good throttle potentiometer with no faults should provide the waveform indicated above.

Figure 2 shows a throttle potentiometer.

AT029-2

Disclaimer
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

  • Back-pinning Probe Set

    £48.00

  • Flexible Back-pinning Probe

    £7.00

  • Large Dolphin/Gator Clips

    £10.00

  • Connector breakout lead kit A

    £329.00

  • Premium Test Lead: BNC to 4 mm, 3 m

    £48.00

  • Premium Test Leads: Set of four leads 3 m (TA125 - TA128)

    £179.00

  • PicoScope Battery Clip

    £5.00

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Guided test: Throttle Potentiometer