You will require a PicoScope to perform this test. A list of suitable accessories can be found at the bottom of this page.
Plug the BNC test lead into Channel A on the PicoScope. Place a black clip on the black probe (negative) and connect this to a suitable ground point. Connect a Back-pinning Probe to the coloured lead (positive) to backpin one of the potentiometer connections within the accelerator pedal position sensor multiplug as shown in Figure 1.
Plug the BNC test lead into Channel B on the PicoScope. Connect a Back-pinning Probe to the coloured lead (positive) to backpin the other potentiometer connection within the accelerator pedal position sensor multiplug. If a suitable automotive breakout lead is available, this may be used instead of the back-pinning method.
In this example the Accelerator Pedal Position (APP) sensor is of the potentiometer type. It receives two reference voltages from the Powertrain Control Module (PCM), having two ground wires and two signal wires that send a varying voltage back to the PCM relating to accelerator pedal position. The signal voltage sent back to the PCM may vary from manufacturer to manufacturer but will probably never be greater than 5 volts.
With the increasing level of electronic control and the subsequent decrease in moving mechanical parts it is inevitable that we will see more items being controlled in a "fly by wire manner".
One example of this is throttle control. The majority of vehicles currently being produced no longer use an accelerator cable but instead use an APP in conjunction with an electronic throttle control actuator (ETC) incorporating an electronic throttle motor and a throttle position sensor (TPS).
The APP is quite simply one or, more commonly, two potentiometers attached to the accelerator pedal. As the accelerator is depressed, a voltage signal is sent to the PCM relaying the actual position of the accelerator pedal and thus the driver's physical demand. As a result of this input, the PCM then generates an output to the relevant actuator; in this case the ETC. As previously mentioned, the APP commonly has two potentiometers. These are employed to act as a plausibility test and also to ensure a degree of failsafe operation.
Several methods are used to generate the signal. The great majority use the common 5 volt reference as is used throughout the engine management system. The two most common methods of signal generation are as follows:
Figure 2: Potentiometer 1 generates a signal of 0.3 to 4.8 volts (red trace in Figure 2) and potentiometer 2 generates a signal of 0.5 to 4.8 volts (blue trace in Figure 2). With an accelerator pedal position of 45 degrees, potentiometer 1 may be outputting a signal of 2 volts and potentiometer 2 a signal of 3 volts, for example.
Figure 3: Potentiometer 1 generates a signal of 0.3 to 4.8 volts (red trace in Figure 3) and potentiometer 2 generates a signal of 4.8 to 0.3 volts (blue trace in Figure 3). With an accelerator pedal position of 0 degrees, potentiometer 1 may output a signal of 0.5 volts and potentiometer 2 may output a signal of 4.5 volts.
Upon receiving signals in this manner the PCM is able to ensure that the information is correct; for example, if APP angle is 45 degrees, then potentiometer 1 outputs 2 volts and potentiometer 2 outputs 3 volts. If there is any deviation from this then the PCM detects a possible fault and logs a relevant fault code. If one potentiometer track should fail then once again the PCM is able to detect this and run in a failsafe or emergency mode, often raising the idle and limiting throttle operation and lighting the malfunction indicator lamp (MIL). The use of two potentiometers also enables the PCM to monitor the speed at which the accelerator is depressed and closed, the throttle position thus controlling fuelling accordingly.
Should you suspect a fault with the signal, test the wiring from the PCM to the APP.
Ensure that the PCM has good power supplies and grounds where required.
Test the APP (disconnected) with an ohmmeter.
Example pin data
Tested from a Smart Forfour 1.1 petrol 2005 MY.
6 pin connector
Pin 1= 2.5 V reference voltage (yellow/red)
Pin 2= 5.0 V reference voltage (yellow/green)
Pin 3= Signal voltage, approx 1 V closed throttle & 3.8V open throttle (grey)
Pin 4= 0 V ground (brown/white)
Pin 5= 0 V ground (brown)
Pin 6= Signal voltage, approx 0.5 V closed throttle & 1.8 V open throttle (pink/black)
All figures quoted are approximate and measured by back pinning with ignition on and the multiplug connected.
This analogue/digital APP sensor produces an analogue voltage (Channel A, blue trace) and a digital output (Channel B, red trace). The analogue voltage is simply proportional to the pedal position, as in the previous waveform. The digital voltage is a sequence of approximately 12-volt pulses of varying width. The width of each pulse is proportional to the analogue voltage at that time, as shown in Figures 5a, 5b, 5c.
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.
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