Automotive Repair Library, Auto Parts, Accessories, Tools & Equipment, Manuals & Books, Car BLOG, Links
Toyota Unintended Acceleration
By Larry Carley November 4 2009
By Larry Carley November 4 2009
Recent news reports of Toyota sudden unintended acceleration being caused by mispositioned floor mats will probably turn out to be bunk. I suspect the real cause will turn out to be an electronic glitch in the throttle-by-wire system.
Like many other vehicle manufacturers, Toyota dropped the old fashioned throttle cable that used to link the gas pedal to the engine throttle. In its place is a computer-controlled throttle-by-wire system that uses an accelerator pedal position sensor. When the driver steps down on the pedal, the request to "go faster" is sent to the computer. The computer then commands a small electric motor on the throttle to open the throttle wider. A throttle position sensor on the throttle monitors the throttle opening, and the corresponding increase in engine speed is noted by the computer.
Is throttle-by-wire failsafe?
Throttle-by-wire is supposed to be failsafe, but apparently it is not. The accelerator pedal sensor actually contains two sensors, not one. If both sensors do not report the same amount of pedal movement to the computer, it is supposed to set a fault code and go into a default mode that limits the throtle opening. Likewise, if the engine throttle fails to open or close as commanded, that should also set a fault code and put the system into a default mode.
But strange things can happen in electrical circuits as well as the control electronics. In the case of the Toyota sudden unintended acceleration problem, something is apparently happening in the control circuit that is commanding full throttle even though the driver may not be touching the gas pedal at all.
I can only speculate as to what might be causing this problem. But I suspect that it will turn out to be an electronic fault in the accelerator pedal sensor circuit wiring harness that is giving a false "full throttle" command to the computer. Or, something could be shorting battery voltage to the throttle control motor causing it to go full open. Or, there is an electronic glitch or programming fault in the cruise control system that is commanding full throttle while driving.
Cruise Control Involved?
Since most of the reported instances of sudden unintended acceleration have occurred while highway driving, I would suspect a link between the cruise control system and the throttle control system.
What happens if the cruise control system loses input from the vehicle speed sensor? Does it shut down or does it try to increase speed by opening the throttle all the way? I don't know.
When you step on the brakes while driving in cruise control mode, most cruise control systems disengage until you press the RESUME button. The drivers who have experienced sudden unintended acceleration say standing on the brakes has had little or no effect in slowing the vehicle or returning the engine to normal speed. The throttle stays wide open and the vehicle continues to accelerate. Again, this might imply some kind of fault in the cruise control system logic, or possibly in the brake pedal switch circuit that tells the system when the driver is depressing the brake pedal.
Is it a Control Logic Fault?
Because the sudden unintended acceleration problem has been reported in a number of different Toyota and Lexus models, the cause my turn out to be the programming of the throttle-by-wire control system. One difference between Toyota/Lexus and most other makes is that Toyota does NOT have a back-to-idle failsafe mode if the control system detects a conflict between the brake and accelerator pedals. In other vehicles, if the driver is pressing down hard on both pedals at the same time (like old fashioned power braking), or the computer receives inputs from the brake pedal switch and accelerator pedal position switch that both are being pressed simultaneously, it commands the engine throttle to close to the idle position. This allows the vehicle to stop in a shorter distance, and also averts the possibility of a runaway car if something is amiss. Why this is not part of the Toyota/Lexus control logic, I don't know.
In any event, if floor mats jamming the gas pedal is not causing the problem (which many of those who have experienced sudden unintended acceleration claim is NOT the cause), hopefully a close review of the wiring schematics and control logic will reveal a design flaw or programming glitch that is causing the runaway cars.
What To Do If Your Vehicle Accelerates Unintentionally
1. Keep a cool head and remain calm. But reacting quickly to the situation is absolutely essential to minimize your risk of an accident.
2. If the engine is accelerating uncontrollably, hit the brakes as hard as you can and maintain firm and steady pressure on the pedal. Your vehicle should slow down.
3. If the engine fails to respond, or the brakes are not having much effect, switch the ignition key to the ACCESSORY position (not all the way off as this may cause the steering column to lock!). If your vehicle has a STOP/START push button, push and hold the button in until the engine quits. On Toyota and Lexus, this can take up to three seconds, which may seem like an eternity if your vehicle is speeding out of control down the highway.
4. If you can't shut the engine off, shift the transmission into neutral (if you have an automatic) or depress the clutch pedal (if you have a stick shift). This will disengage the engine from the drivetrain and allow the brakes to slow the vehicle. However, the engine will continue to race so try to shut it off as quickly as posible to prevent possible engine damage.
5. Once you have the situation under control (engine off, car slowed or coasting to a stop), pull over, stop the car, get out and call a tow truck. Have the vehicle towed to a Toyota/Lexus dealer or a repair shop for diagnosis. Your vehicle may be unsafe to drive. I would not drive it again until a technician has had an opportunity to check the computer system for fault codes or other glitches.
6. If you have had a scary expeience with sudden unintended acceleration, but the new car dealership or other technician was unable to find anything wrong, I would be very leery about driving the vehicle home. The same thing could happen again. That leaves you in a real bind. Should you take your chances and drive a car that may suddenly speed up, or should you demand the dealership keep it until they can guarantee they have fixed it, or should you dump the car then and there and buy another car? None of these are very good options. Hopefully, Toyota can get this figured out.
Related articles: