How Electronic Throttle Control Systems Work

car pedals
Image Gallery: Car Safety With electronic throttle control, electrical signals, rather than manual pressure, carry messages from the gas pedal to the car's throttle. See more car safety pictures.

Up until the late 1980s, most cars had a fairly straightforward throttle control. You stepped on the accelerator pedal, the throttle opened, and air flowed into the engine, where it mixed with gasoline and burned. This burning gas powered the car's wheels, getting you down the road. If you wanted to go faster, all you had to do was step down harder -- the throttle would open wider, giving the car more power.

But electronic throttle control, which is sometimes called drive-by-wire, uses electronic, instead of mechanical, signals to control the throttle. That means that when you step on your car's gas pedal, instead of opening the throttle, you're activating an accelerator pedal module, which converts the pressure you put on the pedal into an electric signal. That signal is then sent to an electronic control unit, which takes your inputs into account, as well as outside variables, to open the throttle for optimum efficiency and performance.


It's a complex system, but one that has a lot of benefits for engine wear, performance and efficiency. However, like any complex system, it's not perfect, and drivers have raised a lot of questions about them. Can outside signals interact with the ETC? What kind of failsafes are in place if such interference happens? Read on to learn the answers.

Benefits of Electronic Throttle Control

Electronic throttle control systems may seem a little silly. After all, if a mechanical throttle control system works, why make it more complicated?

While it's true that electronic throttle control adds complications, it also adds a number of benefits. The first is decreased maintenance. Mechanical throttle systems, because they are made up of a lot of moving parts, are subject to a lot of wear. Over the life of the car, the various components can wear out. By comparison, an electronic throttle control system has comparatively few moving parts -- it sends its signals by electric impulse, not moving parts. That reduces wear and the amount of maintenance needed on the system.


Beginning in 2009, electronic throttle control systems made headlines as a result of a large-scale recall of Toyota vehicles due to acceleration control problems. Given the negative press electronic throttle control systems received, you may be surprised to know that electronic throttle controls add a number of safety benefits over mechanical systems. In a mechanical system, the throttle relies only on driver input to decide how far to open or close. With an electronic throttle control system, the main control unit not only reads input from the driver's foot on the accelerator, but it also examines input from wheels that are slipping, wheels that have grip, the steering system and the brakes, helping correct driver error and keep the car under control. In other words, a throttle control system can balance several factors that affect a car's speed and direction -- not just a foot on the pedal. Plus, electronic throttle control is a key component in most cruise control systems.

Electronic throttle control may be a complex system, but it makes driving a car easier and safer, and it can reduce maintenance. However, one of the concerns raised during the 2009-2010 Toyota recall was whether outside signals can interfere with electronic throttle control. Keep reading to learn whether this is true.


ETC Systems and Outside Signals

car driving by power lines
Critics allege that power lines and electronic devices can interfere with electronic throttle control.

Imagine driving along the route you usually take to work, when your car suddenly starts surging through traffic. It's what some drivers claim has happened to them, and some of them blame the car's electronic throttle control system.

According to some drivers who have experienced unintended acceleration, as well as some experts in automotive engineering, electromagnetic interference can cause electronic throttle control systems to malfunction. In some scenarios, interference from things like cell phones and power lines has been blamed for causing a short circuit in the electronic throttle control, leading to sudden, unintended acceleration.


In the most publicized case, Professor David Gilbert, a professor of engineering at Southern Illinois University, showed on ABC news how he was able to create a short circuit in a Toyota Avalon that caused the engine to rev, accelerating the car without driver input -- and in spite of the application of the brakes [source: Ross].

However, Toyota and other experts fired back that Gilbert's example was contrived and unlikely to occur in the real world. According to Gilbert's critics, he had to cut and reconnect several wires in the system, something that is extremely unlikely to occur in a car that's been used normally [source: Toyota].

While a short circuit could, in theory, cause an electronic throttle control to open the throttle and rev the engine, many experts point out that the systems are well insulated to prevent electromagnetic interference from compromising the system.

However, just because short circuits and interference are unlikely doesn't mean that automakers have ignored the possibility of them happening. Keep reading to learn about failsafes and backups that have been built into electronic throttle control systems.


ETC Failsafe Modes

Like most complex systems, electronic throttle control systems have a number of failsafes. These are designed as redundancies and backups to keep the system running, or provide a safe shutdown if something goes wrong.

Generally speaking, at the first sign of a problem, most electronic throttle controls are designed to close the throttle and return to idle. So, for example, if the engine control unit detects a problem with a sensor, the system reverts to idle, preventing the throttle from opening.


Similarly, there are a number of redundancies built into the system. For example, just one sensor isn't used to detect driver inputs or other factors. Each sensor position uses two sensors. If a sensor malfunctions, or the two sensors in a given position report different readings, the system closes the throttle, idling the engine.

What about outside interference causing power surges or short circuits? Most systems use a smart throttle motor. The throttle motor is the final gatekeeper that throttle signals need to go through before the throttle actually moves. If the throttle motor detects voltage or signals that didn't come from the engine control module, it's designed to shut the engine down. If electromagnetic interference were strong enough to affect electronic throttle controls, the throttle control system is designed to shut down, not surge ahead.

That's not to say that electronic throttle control systems are problem-free; rather, they've been designed with a number of failsafes that, if working properly, should prevent unexpected engine surges and acceleration.

Still, in the wake of new consumer awareness about unintended acceleration and questions about electronic throttle controls, car makers are adding another failsafe: brake overrides. These systems, which are already available on a number of cars from German manufacturers, allow driver inputs to step in and override the throttle system. So, if the system malfunctions somehow and the throttle opens on its own, stepping on the brakes will close it.

Electronic throttle control is just one of the electronic components under the hood. Learn about others by reading the links on the next page.


Lots More Information

Related HowStuffWorks Articles

  • Bergholdt, Brad. "Under the Hood: Electronic Throttle Control is Complicated, But Precise." McClatchy-Tribune Information Services. February 26, 2010. [March 8, 2010].
  • Lienert, Anita. "Toyota's Electronic Throttle Control and Electromagnetic Interference Testing Presentation." February 23, 2010. [March 8, 2010].
  • O'Donnell, Jayne. "Could electronics be what's causing runaway cars?" USA Today. February 23, 2010. [March 8, 2010].
  • Pico Technology. "Electronic Throttle Control." Pico Technology. [March 9, 2010]
  • Rhee, Joseph. "Toyota Slams Sudden Acceleration Research of Auto Expert as Unrealistic." ABC News. March 8, 2010. [March 8, 2010].
  • Ross, Brian. Expert Recreates Sudden Acceleration in Toyota." March 4, 2010. [March 8, 2010].
  • Thomas, Ken and Stephen Manning. "Toyota disputes critic who blames electronics." The Associated Press. March 8, 2010. [March 8, 2010].
  • Toyota. "Comprehensive Analysis Raises Concerns About Gilbert Congressional Testimony, ABC News Segment." Toyota. March 8, 2010 [March 8, 2010].