The Powertrain control module (PCM) is the control centre for the throttle actuator control (TAC) system.
In modern vehicles with drive-by-wire throttle systems, the ECM determines the driver’s intent based on input from the accelerator pedal position sensors, then calculates the appropriate throttle response based on the dual throttle position sensors (often coded) as TP1 and TP2 or TPA and TPB.
The PCM achieves throttle positioning by providing pulse width modulated voltage to the throttle actuator motor. The throttle blade is spring-loaded in both directions, and the default or fail position safe is slightly open.
The PCM also considers multiple inputs to control the TAC for multiple safety or convenience applications such as (Fig. 1):
- RPM, to avoid Engine over rev.
- Park/Neutral Switch for idle control
- Info from the ABS sensor and this module to assist with traction control
- Cruise Control request to control speed
- Collision avoidance & system safety
- Self-driving control system on new autonomous vehicles
For safety reasons, throttle by wire systems use a dual TPS sensor. Unlike the APP sensor, the TPS sensors usually have a common five-volt supply and a common ground. The ECU monitors both TP1 and TP2 signals while making sure it keeps to a specific algorithm. TP1 is often used to inform the PCM of the throttle angle position, while TP2 is used in combination with TP1 to verify the integrity of the signal while referencing specific algorithms. The algorithms tend to vary from vehicle to vehicle.
The most common algorithms are inverted voltage ones. In this case, when the throttle is open, the TP1 voltage increases as the TP2 voltage decreases. Regardless of the throttle angle, the TP1 and TP2 should always equal five volts (Example: TP1 1.5 volts + TP2 3.5 volts = 5 Volts).
When the voltage of the TP1+TP2 is 200mv above or below five volts, the PCM will detect a fault and trigger a code; this particular code will most likely force the ECU to go in to failsafe mode causing a ‘no throttle’ response symptom.
Figure 2 shows a familiar pattern while testing the TP1 and TP2. The Red trace represents the TP1, and the Blue trace represents the TP2. The pink trace is the A+B scope function that represents the addition of the TP1+TP2 that should always equal five volts.
Some vehicles, such as Hondas, use an out of phase voltage increment for the TPA and TPB. The voltage differential between the TPA and TPB is used by the PCM to validate the accuracy of the signals. We have to refer to an OE diagnostic chart and graphic to perform diagnostics in these cases (Fig. 3).
Using a lab scope or scan tool in graph mode will help identify an irregular signal or intermittent glitch that could lead to a trouble code or symptoms. In some cases, water intrusion in the wiring harness can be the cause of intermittent shorts, affecting the adjacent wire enough to cause a problem. In such cases, replacing the harness could be the solution. Figure 4 shows a familiar pattern of Honda throttle position sensors.