Electronic Throttle Control - Failure Modes

Failure Modes

Occasionally after servicing or repair, the wire or cable between the accelerator and throttle would not be correctly reinstalled causing sudden acceleration. However, with the ETC, the movement is all done by electronic controls moving an electric motor. But just moving the throttle by sending a signal to the motor is an open loop condition and leads to poor control. Most if not all current ETC systems have a closed loop system whereby the ECU tells the throttle to open a certain amount according to an algorithm based on the geometry of the throttle. Then, if due to dirt build up in the throttle bore or a damaged TPS a signal is sent from the TPS to the ECU, the ECU can make appropriate adjustments to compensate, though it might result in surging, hesitation or uneven idle.

There are two primary types of throttle position sensors: a potentiometer or a Hall Effect sensor (magnetic device). A potentiometer is a satisfactory way for non-critical applications such as volume control on a radio, but as it has a wiper contact rubbing against a resistance element, and dirt and wear between the wiper and the resistor can cause erratic readings. The more reliable solution is the magnetic coupling that makes no physical contact, so will never be subject to failing by wear. This is an insidious failure as it may not provide any symptoms until there is total failure. All cars having a TPS have what is known as a 'limp-home-mode'. When the car goes into the limp-home-mode it is because the accelerator and engine control computer and the throttle are not talking to each other in a way that they can understand. The engine control computer shuts down the signal to the throttle position motor and a set of springs in the throttle set it to a fast idle, fast enough to get the transmission in gear but not so fast that driving may be dangerous.

Another possible ETC failure mode can originate from single-bit soft errors caused by ionizing radiation in the vicinity of transistors in the digital hardware from cosmic ray neutrons, so the only acceptable way to prevent this failure mode is to implement a minimum of triple redundancy in the digital electronics that comprise the ETC system.

Recently, ETC was initially suspected by some to be responsible for alleged incidents of unintended acceleration in Toyota and Lexus vehicles. No evidence of this has been demonstrated, and Toyota has been exonerated by the U.S. National Highway Traffic Safety Administration (NHTSA).