A Ford Transit was presented with a fault code stored for accelerator pedal performance.
DTC P0121- Throttle/Pedal Position Sensor/Switch 'A' Circuit Range/Performance.
Information systems suggest the causes for such a fault are;
- Faulty throttle position sensor
- Throttle position sensor harness is open or shorted
- Throttle position sensor circuit poor electrical connection
- Faulty Engine Control Module (ECM)
The easy option is to replace the pedal, but we prefer to test the component plus the power, ground and signal wires before condemning any parts or control modules.
It may take longer than picking up the phone and ordering the suspected part but it is a vital step in the diagnostic process.
The easiest way of testing the complete circuit is to use an oscilloscope.
Most older pedal position sensors use a variable resistance track that changes the voltage sent back to the module which is converted into a pedal position.
This method uses an analogue signal, which must be converted inside the module into a digital signal.
Some newer vehicles employ a digital sensor which can be utilised by the module without any further processing of the signal.
The sensor output is a fixed frequency variable duty signal which can be tricky to interrupt using the oscilloscope.
Check out the scope trace of the pedal signal.
This one test shows that the power and ground is good as well as proving the change in pulse width.
The problem is checking the smooth transition from the idle position to Wide open throttle.
The use of a built in maths channel to display duty cycle is very useful.
To display the Duty Cycle on a Pico Scope you need to select Tools> maths channels>Create>Advanced>Duty>A>Next>Next>Next>Finish>Ok>
You should then end up with a trace like this;
It is now much easier to analyse the performance of the pedal sensor. Note the smooth transition from idle to WOT.
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