9.6 Electronic Throttle Testing and Learning

Throttle Body & Electronic Throttle Actuator Control (TAC / ETC) (Task F.7)

Dual Redundant APP and TP Sensors & Limp-Home Spring Operation

Heavy-duty drive-by-wire TAC systems eliminate the mechanical throttle cable and separate Idle Air Control (IAC) valve. Instead, the PCM controls all idle, cruise, PTO speed-up, and WOT airflow using three assemblies:

1. Accelerator Pedal Position (APP) Assembly: Contains two (or three) independent non-contact Hall-effect or potentiometric sensors (APP1 and APP2), each supplied with its own dedicated 5.0 V reference and ground circuit from the PCM. One common arrangement uses offset/proportional voltage slopes (such as a 2:1 voltage ratio, where APP1 sweeps from 0.8 V at rest to 4.4 V at WOT while APP2 sweeps from 0.4 V at rest to 2.2 V at WOT). If APP1 does not equal 2 × APP2 within ~0.15 V at any pedal position (P2122, P2127, P2138), the PCM flags an APP correlation fault.
2. Throttle Position (TP) Sensor Assembly (TP1 & TP2 Inverse Slopes):
   • Built into the electronic throttle body housing to report actual throttle plate angle back to the PCM:
   • TP1 (Ascending Voltage): Starts at ~0.5 to 0.8 V at closed throttle and rises smoothly toward ~4.3 to 4.5 V at WOT.
   • TP2 (Descending Voltage): Starts at ~4.2 to 4.5 V at closed throttle and falls smoothly toward ~0.5 to 0.7 V at WOT.
   • The Example 5.0 V Sum Relationship: Because TP1 and TP2 move in exact opposite directions at identical rates, adding TP1 + TP2 at any throttle opening equals on the specified example design ~5.0 V (±0.2 V)! A deviation outside the calibrated tolerance can set P2135 (TP1/TP2 Voltage Correlation).
3. Bidirectional DC TAC Motor & Default Limp-Home Torsion Springs:
   • With the ignition Key OFF (0% TAC motor PWM drive), dual opposing return torsion springs hold the throttle blade slightly cracked open at a 7° to 15° mechanical default (limp-home) stop.
   • During normal curb idle, the PCM actually reverses the H-bridge polarity to drive the blade closed against spring tension down to ~2° to 4° (650 RPM). If a severe TAC fault occurs (P2101, P2135, P2138), the PCM illuminates the Wrench / Reduced Engine Power warning, cuts power to the TAC actuator motor so the return springs snap the blade to the 7°–15° limp-home stop, and regulates engine speed between 1,000 and 1,400 RPM by selectively disabling fuel injectors.

Applied Diagnostic Check

Before cleaning, establish whether deposits are the cause rather than an incidental finding. Compare actual blade movement, requested angle, idle correction and the reported symptom. After cleaning, a new high idle can come from a disconnected intake path as well as retained adaptive values. Inspect the assembled duct, gasket and vacuum connections before executing a learning routine. Verify the supported routine's prerequisites, which can include temperature and accessory state. A routine that did not complete is not a completed repair. Record the result and check stable operation through the relevant idle and off-idle conditions.

Checking the Pedal-to-Throttle Relationship

Pedal position requests torque; it is not a direct mechanical command that the blade must copy at every instant. The PCM can change blade angle for idle control, cruise, PTO, traction requests and engine protection. Compare desired and actual throttle position with the applicable operating state. If they differ, check the motor circuit and mechanism before assuming the pedal sensor is inaccurate.

Graph both channels over the permitted sweep and compare their specified relationship. A dropout in one channel may arise from its reference, ground, connector or sensor. A valid signal relationship does not prove the blade moves freely. After cleaning, inspect all reconnected intake and vacuum joints, then perform the stated learning procedure and verify idle both with and without accessory load. Keep hands clear whenever the controller can energize the motor.