The electronically controlled throttle valve replaces the mechanical link between the accelerator pedal and throttle valve.
As a result of this technology, the throttle valve is controlled by data from a variety of sources, thus allowing it to continually adjust itself based on input from the traction control, stability control and adaptive cruise control, among others.
The two main sources of input data are the accelerator pedal position sensors (APP1 and APP2) and throttle position sensors (TPS). These sensors consist of a combination of two or more sensors, while the electronic control unit (ECU) uses an algorithm to validate the integrity of the signals.
Most APP sensors use an incremental voltage value when the accelerator pedal is applied. The incremental voltage of APP1 always represents half of the APP2 voltage. In most vehicles, the corresponding APP algorithms are as follows: APP2 X 2 = APP1. For illustration purposes: if APP2 displays a reading of 1.5 volts, the APP1 reading should be approximately 3 volts, with a variance of more or less 50 mV. See Figure 1.
Both APP1 and APP2 sensors use an independent 5-volt power source, as well as an independent ground for safety reasons, as illustrated in the wiring diagram in Figure 2.
If the corresponding voltage value in these sensors is 100 mV higher than the normal algorithm range, the ECU produces a diagnostic trouble code (DTC), which usually triggers a fail-safe mode and symptoms such as the absence of a response by the throttle valve.
In some instances, the problem can be traced back to water infiltrating into the wiring harness, producing intermittent short-circuiting in adjacent wires. If this is the case, replacing the defective wiring harness should solve the problem.
Case study
Vehicle: 2006 Acura TL , 3.2 L V6, automatic transmission
Odometer: 182,336 km
Problem: The vehicle has an intermittent problem; the throttle valve occasionally fails to respond. The technician has detected a diagnostic trouble code, DTC P2138: Accelerator pedal position sensor D/E – Incorrect corresponding voltage value.
Background: Older Acuras and Hondas employ a throttle valve cable mounted between the accelerator pedal and APP sensor, which, in turn, sends the position signal to the electronic throttle control system (ETC) via the PCM, then adjusting the throttle valve angle using the input data from the APP sensor.
Simple Trick: Begin by using the scan tool to confirm that the APP sensor reading is lower than 2 percent at idle. If this isn’t the case, check the throttle valve free play and adjust as required.
Scan tool test terminals 3 and 4.
Ignition switch ON engine OFF.
Measure the voltages as illustrated on the oscilloscope. The APP2 x 2 should display a voltage reading of ± 50 mV of the APP1’s range, in every position, without interruption.
If the results are as expected, inspect other possible sources of intermittent problems such as loose connectors, damaged harnesses, or water infiltration in the circuit. Check for possible service bulletins issued for this particular vehicle and apply the required corrections.
Repair confirmation: The technician replaced the defective APP sensor. He adjusted the throttle valve cable according to specifications and erased the diagnostic trouble code. The road test confirmed that the problem was fixed.
