Throttle position switch

The purpose of this test is to investigate the operation of the idle and full throttle position switches.

How to perform the test

View connection guidance notes.

1. Use manufacturer’s data to identify the throttle switch circuit.
2. Connect PicoScope Channel A to the idle switch circuit.
3. Connect PicoScope Channel B to the full load switch circuit.
4. Turn the ignition on, engine off.
5. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
6. Start the scope to see live data.
7. Slowly fully depress then release the accelerator pedal to activate the switches.
8. With your waveforms on screen stop the scope.
9. Turn off the ignition.
10. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveforms.

Example waveform

Waveform notes

These known good waveforms have the following characteristics:

Channel A indicates the operation of the (normally closed) idle position switch.

• At ignition on, it has a constant, unbroken, output, just above 0 V.
• The output instantly switches up to just below 5 V when the throttle moves from its idle position and remains there until the throttle position returns.
• The output instantly switches back to just above 0 V when the throttle returns to its idle (closed) position.

Channel B indicates the operation of the (normally open) full-throttle position switch.

• At ignition on, it has a constant, unbroken, output, just below 5 V.
• The output instantly switches down to just above 0 V when the throttle is at the full load (wide open) position and remains there whilst the throttle position is maintained.
• The output instantly switches back to just below 5 V when the throttle is released from the full load (wide open) position.

There is no drop-out, noise, or hashing in either output signal.

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select, Throttle position switch.

Further guidance

The throttle position switch signals the movement and position of the throttle valve to the Engine Control Module (ECM). The switch unit is installed at the throttle housing and attached to the throttle butterfly spindle.

Throttle position switches can be configured in different ways, so it will be necessary to check manufacturer specifications for terminal and switching data. However, the general principles of operation are similar.

Inside the unit there are two sets of contacts, with each having an open or a closed state. This allows it to signal three possible throttle position states, such as:

• Throttle closed – idle switch closed and full load switch open.
• Throttle part open – idle switch open and full load switch open.
• Throttle fully open – idle switch open and full load switch closed.

The throttle position switch is usually a three-wire device with a 5 volts supply (12 volts on some very early systems). A typical pin configuration might be (Hella manufactured switch):

• Pin 1 – voltage supply (5 V).
• Pin 2 – full load signal.
• Pin 3 – idle switch signal.

Throttle position switch faults can be both electrical and mechanical:

• Open, short and high resistance faults in connectors and circuits.
• Corrosion and stress fractures in the switches.

A faulty throttle valve switch can cause:

• Malfunction Indicator Lamp (MIL) illumination.
• Diagnostic Trouble Codes (DTCs).
• Erratic or out of tolerance idle speeds.
• Stalling.
• Poor or no acceleration.
• Full load driveability issues.
• General erratic running.

GT028