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Old 04-05-2014, 04:39 PM   #4
fdryer
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Join Date: Jan 2006
Location: NYC
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2003 L-Series 3.0L Sedan
Default Re: Throttle Position Relearn

A partial reprint from the service manual; (no tools needed)

Throttle Body Relearn Procedure

The engine control module (ECM) stores values that include the lowest possible throttle position (TP) sensor positions--0 percent, the rest positions--7 percent, and the return rate of both springs. These values will only be erased or overwritten if the ECM is reprogrammed or if a throttle body relearn procedure is performed. Observe, if the battery is disconnected, the ECM will immediately perform a throttle body relearn procedure when the ignition is turned ON.

A throttle body relearn procedure is performed anytime the ignition is turned ON, with the engine OFF for longer than 29 seconds. After 29 seconds, the ECM commands the throttle plate from the rest position to full closed, then to around 10 percent open. This procedure takes about 6-8 seconds. If any faults occur in the throttle actuator control (TAC) system, a DTC sets. At the start of this procedure, the scan tool TAC Learn Counter parameter should display 0, then count up to 11 after the procedure is completed. If the counter did not start at 0, or if the counter did not end at 11, a fault has occurred and a DTC should set.

DTC P0121-P0123 or P0221-P0223

DTC P0121: Throttle Position (TP) Sensor 1 Performance

DTC P0122: Throttle Position (TP) Sensor 1 Circuit Low Voltage

DTC P0123: Throttle Position (TP) Sensor 1 Circuit High Voltage

DTC P0221: Throttle Position (TP) Sensor 2 Performance

DTC P0222: Throttle Position (TP) Sensor 2 Circuit Low Voltage

DTC P0223: Throttle Position (TP) Sensor 2 Circuit High Voltage

Circuit/System Description

The throttle body assembly contains 2 throttle position (TP) sensors. The TP sensors are mounted to the throttle body assembly and are not serviceable. The TP sensors provide a signal voltage that changes relative to throttle blade angle. The engine control module (ECM) supplies the TP sensors with a common 5-volt reference circuit , a common low reference circuit, and two independent signal circuits. The TP sensors have opposite functionality. TP sensor 1 signal voltage decreases from greater than 4 volts at idle to less than 1 volt at wide open throttle (WOT). TP sensor 2 signal voltage increases from less than 1 volt at idle to greater than 4 volts at WOT.


Our drive by wire systems uses the electronically controlled throttle and pedal coupled to the ecm as a three part system. Dual feedback circuits in throttle and pedal ensure 100% safety otherwise the reduced power mode is invoked to prevent any engine runaway issues from occurring. All electrical connections must be clean and free of corrosion in their weather proof boots to ensure low voltage feedback circuits maintain constant communications for trouble free reliability.

If reduced power occurred, the usual problems are either throttle or pedal as they're the moving parts with external sensors and motor to wear down. Wiring and connections can also contribute to problems. Try the relearn procedure. Normal throttle operation; with ignition on throttle plate movement should mimic pedal travel.

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