• | The throttle actuator control (TAC) system is used to improve emissions,
fuel economy, and driveability. The TAC system removes the mechanical link between
the accelerator pedal and the throttle plate. The TAC system removes the need for
a cruise Control Module and idle air control motor. The following is a list of TAC
system components: |
• | The accelerator pedal assembly includes the following components: |
- | The accelerator pedal position (APP) sensor 1 |
• | The throttle body assembly includes the following components: |
- | The throttle position (TP) sensor 1 |
- | The throttle actuator motor |
• | The engine Control Module (ECM) |
- The ECM monitors the driver demand for acceleration with 2 APP
sensors. The APP sensor 1 signal voltage range is from about 0.98-4.16 volts
as the accelerator pedal is moved from the rest pedal position to the full pedal travel
position. The APP sensor 2 range is from about 0.49-2.08 volts
as the accelerator pedal is moved from the rest pedal position to the full pedal travel
position. The ECM processes this information along with other sensor inputs to command
the throttle plate to a certain position.
- The throttle plate is controlled with a direct current motor called a
throttle actuator control motor. The ECM can move this motor in the forward or reverse
direction by controlling battery voltage and/or ground to 2 internal drivers.
The throttle plate is held at a 7 percent rest position using a constant force
return spring. This spring holds the throttle plate to the rest position when there
is no current flowing to the actuator motor.
- The ECM monitors the throttle plate angle with 2 TP sensors. The
TP sensor 1 signal voltage range is from about 4.86-0.86 volts
as the throttle plate is moved from 0 percent to wide open throttle (WOT).
The TP sensor 2 voltage range is from about 0.82-4.14 volts as
the throttle plate is moved from 0 percent to WOT.
- The ECM performs diagnostics that monitor the voltage levels of both APP
sensors, both TP sensors, and the throttle actuator control motor circuit. It also
monitors the spring return rate of both return springs that are housed internal to
the throttle body assembly. These diagnostics are performed at different times based
on whether the engine is running, not running, or whether the ECM is currently in
a throttle body relearn procedure.
- Every ignition cycle, the ECM performs a quick throttle return spring
test to make sure the throttle plate can return to the 7 percent rest position
from the 0 percent position. This is to assure that the throttle plate can
be brought to the rest position in case of an actuator motor circuit failure. Observe,
under cold conditions, the ECM commands the throttle plate to 7 percent with
the ignition ON and the engine OFF to release any ice that may have formed on the
throttle plate.
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 any time the ignition is turned
ON, with the engine OFF for longer than 29 seconds when the following conditions
have been met:
• | The engine speed is less than 40 RPM. |
• | The vehicle speed is 0 km/h (0 mph). |
• | The engine coolant temperature (ECT) is between 5-85°C
(41-185°F). |
• | The intake air temperature (IAT) is between than 5-60°C
(41-140°F). |
• | The accelerator pedal position (APP) sensor angle is less than 14.9 percent. |
• | The ignition 1 voltage is more than 10 volts. |
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.
TAC System Default Actions/Reduce Power Modes
- There are 2 reduce power modes that the engine Control Module (ECM)
can default to if an error is detected in the throttle actuator control (TAC) system.
If an accelerator pedal position (APP) sensor 1 or APP sensor 2 circuit
fault, throttle position (TP) sensor 2 circuit fault, or if a TP sensor 1
circuit fault is detected with some APP angle, the ECM goes into one of the 2 reduce
power modes. In this mode, the engine torque is limited so that the vehicle cannot
reach speeds of more than 100 km/h (60 mph). The ECM remains in this
reduce power mode during the entire ignition cycle even if the fault is corrected.
- If there is a condition with the throttle actuator control circuits,
throttle actuator command vs actual position fault, return spring check fault, or
a TP sensor 1 circuit fault, the ECM goes into the other reduce engine power
mode. In this mode, the engine speed is limited to 2,500 RPM and the 3-6 fuel
injectors are randomly turned OFF. At this time the Reduced Power indicator is commanded
ON. The ECM remains in the reduce power mode during the entire ignition cycle even
if the fault is corrected. Observe, if a TP sensor 1 or throttle actuator control
circuit fault is present at the time the vehicle is at idle, with no accelerator pedal
angle, the engine may stall.