Accelerator Pedal Position Sensor

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Purpose

The accelerator pedal position sensor (APPS) sends an electronic signal to the engine or powertrain control module (ECM/PCM) indicating the position of the accelerator pedal. The ECM/PCM uses that signal to determine the throttle position demanded by the driver. This sensor is most commonly used in vehicles with "drive by wire" throttle systems, which do not have a cable connecting the accelerator pedal to the throttle.

Location

The accelerator pedal position sensor is located on the accelerator pedal assembly, usually mounted above and ahead of the pedal itself. (On some vehicles with early versions of electronic throttle control, the accelerator pedal is connected to the throttle by a cable and an 'accelerator pedal position sensor' is mounted to the throttle body. These are properly considered as throttle position sensors.)

Operation

The typical accelerator pedal position sensor consists of two or three potentiometers, a type of variable resistor. Resistance to voltage passing through the resistor varies with the position of the accelerator pedal. 

Each potentiometer has three electrical circuits, each with its own wire: a 5 volt reference circuit that supplies voltage to the sensor, a signal voltage circuit to the ECM/PCM, and a ground circuit. Throttle position sensors located on the throttle body (usually two in number) are employed to verify that throttle position corresponds to accelerator pedal position.

On many vehicles, including many General Motors and Ford vehicles, two first potentiometers operate in opposition: pressing the accelerator pedal increases signal voltage in one potentiometer while decreasing it in the other, with a with a third voltage signal using a different voltage range for its signals. 

In other vehicles, signal circuit voltage increases in both circuits as the pedal is pressed, but each circuit employs a different voltage range. The ECM/PCM precisely determines pedal position by comparing these signals.

Drivability Symptoms

Symptoms of accelerator pedal position sensor malfunction vary with the design of the sensor. If the sensor has three separate signal circuits, malfunction of a single sensor circuit usually creates no drivability symptom, merely setting and storing a Diagnostic Trouble Code (DTC).

If, however, two sensor signals disagree by more than a specified percentage, the vehicle enters a 'limp-home' mode, operating with reduced engine power. Failure of all three circuits prevents the engine operating above idle. The instrument panel Malfunction Indicator Light (MIL) may not illuminate. However, a different warning light or message usually will appear. 

In vehicles with only two APPS circuits, malfunction of the APPS may cause stumbling during acceleration, stalling when stopping, jerky throttle response, and loss of engine power at cruising speeds, and may illuminate the MIL. The vehicle may also enter a "limp home" mode.

In vehicles equipped with traction control, malfunction of the APPS may also cause the warning light for that system to illuminate because the ECM/PCM relies upon input data from the APPS to activate traction control. 

In Dodge light trucks equipped with Cummins diesel engines, a "dead pedal" condition, in which the throttle does not respond to the pedal at cruising speed until the pedal is slammed to the floor, commonly indicates APPS malfunction.

Inspection, Test, and Diagnosis

Malfunction of the accelerator pedal position sensor sets both "generic" and "enhanced" OBD2 Diagnostic Trouble Codes. Generic codes are used by all vehicle manufacturers in common. Enhanced codes are unique to the specific vehicle manufacturer only, and are often adopted by manufacturers to refine diagnostic procedures.

The first diagnostic step when APPS malfunction is suspected is retrieving these codes with a suitable Code Reader or Scan Tool. The stored "freeze frame" data showing the throttle position, vehicle speed, engine temperature, and engine load at the moment the code was stored should also be retrieved. This allows duplicating the conditions that set the code to verify the repair corrected the problem.

Common generic accelerator pedal position sensor codes are P2120 through P2134, all of which indicate circuit faults (high or low voltage, intermittent or short circuit) on specific APPS circuits, and P2135 through P2140, indicating the voltage on specific APPS circuits does not correlate with the expected voltage, based on voltage from another APPS circuit or from a throttle position sensor. A single fault may set multiple codes.

To test the accelerator pedal position sensor, first identify the circuit terminals in the pigtail connector to the APPS for each potentiometer. If you do not have a service manual with this information, you can identify these terminals by elimination using a digital multimeter (DMM) set to the voltage scale. 

With the pigtail connector removed from the APPS, the ignition key on but the engine off, connect the negative black lead to a good ground and touch the tip of the red lead to each terminal in the pigtail. Terminals reading 5 volts are reference circuits supplying voltage to the APPS. If you do not find one terminal reading 5 volts for each group of three circuits, i.e., for each potentiometer, there is a fault in the wiring leading to the APPS. Manipulate the wiring at the pigtail connector. If the reading changes, the fault is in the pigtail or its connector.

The remaining two terminals for each potentiometer are the signal and ground circuits, which should be adjacent to the reference circuit terminal. To verify these circuits, it is necessary to reinstall the pigtail connector to the APPS and then "backprobe" the remaining two circuits inside the connector with the key on and engine off. Backprobing is simply slipping a "backprobe pin" into the connector, sliding it between the wire where it enters the connector body and the neoprene or rubber seal around the wire that keeps moisture out of the connector. 

You can use a commercially available "backprobe pin" designed to slip onto the DMM lead or use a "T" pin or small paperclip bent straight and clip the DMM lead to it. Be careful not to pierce or damage the wire's insulation. Insert the pin sufficiently into the connector to touch that wire's electrical contact within the connector. In some vehicles, it may be easier to do this if the accelerator pedal assembly is removed from the firewall.

The objective is connecting the red positive lead to the signal circuit and the black negative lead to the ground circuit to measure voltage change in that circuit as the accelerator pedal is operated. If you are unsure which circuit is the signal and which is the ground, connect the red DMM lead to one and the black lead to the other. With the key on and the engine off, slowly and smoothly press the accelerator pedal. The voltage reading should change (up or down, depending on APPS design) correspondingly. 

If there is no reading, reverse the leads, and repeat the test. If there is no voltage change with either configuration, the APPS is defective or there is a bad ground. To eliminate that possibility, repeat the test for each of the two terminals with the DMM black lead connected to an external ground, such as the door frame. If the voltage reading changes on one terminal as the pedal is operated, that is the signal and the ground is good. Replace the APPS.

General Motors vehicles manufactured for model years in the late 1990's and early 2000's using electronic throttle control may set APPS codes when water intrudes into connectors to the throttle actuator control module. GM issued a service bulletin recommending inspecting these connectors for moisture before replacing the APPS.

Reprogramming

In most vehicles, replacement of an accelerator pedal position sensor does not require reprogramming. However, a relearn procedure must be performed on Nissan vehicles whenever the APPS is replaced, consisting of turning the ignition key on and off and operating the accelerator pedal in a specific sequence varying with model.

Repair/Replacement

The APPS is part of the accelerator pedal assembly, so replacement requires replacing the entire pedal assembly. To remove the pedal assembly, disconnect the APPS connector pigtail and remove the bolts positioning the assembly to the firewall. Remove the rubber pad from the accelerator pedal, transfer it to the new assembly, install the new assembly, and reinstall the connector.

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