Electronic Throttle Control (ETC), or "Throttle Actuator Control" (TAC), is replacing the throttle linkage on more and more late model vehicles. The mechanical linkage or cable between the accelerator pedal and throttle body has been replaced with a gas pedal position sensor and an electronically-operated throttle.
Throttle-By-Wire eliminates the sticking and binding problems that sometimes occur with mechanical linkages, and return spring failures that may prevent the throttle from closing (a runaway engine). Electronic throttle control also helps reduce emissions and improves fuel economy. But the main advantage is that Throttle-By-Wire allows the engine computer to integrate torque management with cruise control, traction control and stability control.
The computer is a lot smarter than the driver when it comes to choosing the optimum throttle opening, especially in situations where a vehicle is starting to lose traction or spin out of control. The driver may not react quickly enough, so the computer backs off the throttle to regain traction and reduce wheel spin or skidding.
With cruise control, integrating throttle control into the engine management system eliminates the need for external vacuum servos or motors to maintain a constant vehicle speed. It also makes "adaptive" cruise control possible (with some additional sensors) so a vehicle can match the speed of the vehicle ahead of it and maintain a safe following distance.
Idle speed on some applications with Throttle-By-Wire is still controlled by a separate Idle Air Control (IAC) valve on the throttle body that allows air to bypass the throttle plate. But on others (Ford Mustang, for example), there is no IAC valve and idle speed is controlled by the position of the throttle shaft.
Electronic throttle control also provides some warranty advantages for the vehicle manufacturer, too, by limiting "abusive driving" by lead-footed motorists. If you put the pedal to the metal on a car with Throttle-By-Wire, you won't impress anybody by smoking the tires. It won't happen -- unless there is an over-ride switch to deactivate traction control. Even then, the computer may limit engine torque to certain limits to protect the transmission and drivetrain from possible damage. On some applications (Mustang), the computer "feathers" the throttle opening slightly between gear shifts to reduce harshness and shock loading on the transmission and drivetrain.Throttle-By-Wire on a 2008 Ford Crown Vic Police Car.
In 1988, BMW was the first vehicle manufacturer to offer electronic throttle control on its 7-Series cars. In 1997, Chevrolet opted for "Throttle Actuator Control" (TAC) on its C5 Corvette. Today, you will find electronic throttle control on the Ford Crown Vic, Focus, Five Hundred, Fusion and Mustang, Saturn ION and L-Series, Cadillac CTS, STS & XLR models, Toyota Camry and Prius, various Mercedes models, Nissan Maxima, Volkswagen Jetta and Passat, and many other cars, even diesel-powered pickup trucks.
There are three basic components in electronic throttle control:
* A position sensor on the accelerator pedal (actually there are 2 or 3 position sensors for redundancy).
* An electronically-controlled throttle body with a small electric motor to open/close the throttle (most use reduction gears to rotate the throttle shaft).
* A control module (the PCM or a separate ETC module that talks to the PCM via the CAN-bus or serial data link).
For redundancy backup and failsafe operation, the accelerator pedal usually has two or even three position sensors. In a 2005 Mustang, three sensors are used. Two decrease their resistance (increase voltage) when the pedal is depressed, and the third increases its resistance (decreases voltage). As long as the position sensor signals increase and decrease by the same amount (mirror each other), the circuit is assumed to be working correctly. But if any of the position signals disagree, it indicates a fault and the module sets a code and turns on the Malfunction Indicator Lamp (MIL). On the Mustang, an electronic throttle control fault will also illuminate a little yellow wrench warning light.
On most vehicles, an electronic throttle fault will put the system into a "limp-in" mode that will limit engine speed. On the Mustang, that means idle speed only. On a Corvette, the limit is a maximum vehicle speed of 30 mph. The throttle control system will remain in the limp-in mode until the fault can be diagnosed and repaired.
Most of the faults that occur in a Throttle-By-Wire control systems are things you would expect, like pedal or throttle position sensors that wear out and skip or produce erratic signals, motor failures in the throttle body, and electrical problems like loose or corroded wiring connectors.
A code reader or scan tool is required for diagnostics. Generic OBD II trouble codes for possible pedal position sensor faults include P0120 through P0124, P0220 through P0229, plus any OEM enhanced P1 or P2 series codes for that specific vehicle.
If a fault occurs in the motor on the throttle body, it will be detected by the feedback signals from the throttle position sensors. Generic OBD II codes for this kind of problem include P0638 & P0639, plus any OEM enhanced P1 or P2 series codes for that specific vehicle.
The Throttle-By-Wire system also monitors the throttle position sensors on the throttle body. A fault here may set any of the same OBD II codes just listed for the pedal position sensor, or OEM enhanced P1 or P2 series codes for that specific vehicle.
Diagnosis involves reading the fault code(s) to determine the circuit that is experiencing the problem, then checking the voltage or resistance of the pedal or throttle position sensors with a DVOM, or checking the operation of the throttle control motor (visual observation of the throttle when the motor is commanded to move, and/or checking the duty cycle commanded by the control module using a scan tool).
When the fault has been identified, the faulty part can then be replaced. The codes can then be cleared, and hopefully everything will work correctly again.
On some systems, a special relearn procedure is required when parts have been replaced or the throttle control wiring harness has been disconnected. The relearn procedure is necessary so the control module can learn the rest positions of the gas pedal and throttle. On some vehicles this occurs automatically every time the key is turned on. But on others it requires a scan tool or special manual procedure. Refer to the OEM service literature for specific instructions.
The pedal position sensor may be part of the pedal assembly, or a separate component mounted at the top of the pedal.
The throttle body is usually replaced as a complete assembly if the motor has failed. The throttle position sensors, however, can usually be replaced separately. Throttle body units are quite expensive (OEM units typically cost $700 or more), so accurate diagnosis is essential to prevent unnecessary parts replacement and warranty returns. The same goes for the ETC control unit (if used) or the PCM.
Note: It is NOT possible to convert an electronic throttle back to an ordinary mechanical throttle linkage. The electronics are so integrated on today's engines that modifications such as this would upset the operation of the entire system.
* The trace of the two gas pedal potentiometers will be different when seen on a DSO (scope). They should mirror each other. One should respond with a higher voltage swing while the other goes low from idle to WOT. It is these two signals that are used by the ECM to calculate the mean voltage used to establish ultimate throttle position.
* On a Ford Mustang, the TPS sensor has 4 circuits:
* 5-volt reference voltage.
* Signal return (ground).
* TP1 voltage with negative voltage slope (5-0 volts).
* TP2 voltage with positive voltage slope (0-5 volts).
* The accelerator pedal position (APP) sensor in a Ford Mustang has three sensors and a total of 7 circuits:
* 2 reference voltage circuits (5 volts).
* 2 signal return (ground) circuits.
* APP1 voltage with negative voltage slope (5-0 volts).
* APP2 voltage with positive voltage slope (0-5 volts).
* APP3 voltage with positive voltage slope (0-5 volts).
The pedal position signal is converted to pedal travel degrees (rotary angle) by the PCM. The software then converts these degrees to counts, which is then used to command throttle position.
* The use of a DSO is very valuable when diagnosing electronic throttle control problems. By looking at the TPS voltage and the servomotor square wave duty cycle, the operation of the system can be viewed. When the TPS voltages change, there should be a corresponding change in the duty cycle of throttle servomotor. This relationship will show if the system is working properly or if there are problems in the feedback or control circuits.