field notes

Unintended Acceleration - Shift to Neutral!

It is déjà vu all over again, but with some new twists. In the 80s most automobiles had carburettors and cable linkages for the throttle. In response to reports of unintended acceleration, including an infamous fake by a major TV network, manufacturers interlocked the shift lever with the brake pedal making it impossible to shift an automatic transmission out of neutral unless the brake light switch was activated. The driver had to have a foot on the brake pedal before shifting into gear, and incidents became far less frequent.

Now fuel is injected, and many vehicles have no throttle cable; instead electronics link the pedal to the engine management computer, and it in turn operates the throttle plate with a servo. Complaints of unintended acceleration have resurfaced. Manufacturers of some vehicles have responded by interlocking the brake light switch with the engine management computer, and then if the brake pedal is depressed at all, the engine management computer closes the throttle.

The computer is programmed to prevent excessive engine speed, and one could shift to neutral with minimal risk of engine damage, even if the throttle was wide open. Few media stories emphasize that, or the fact that brakes can always overpower the engine - at least once.

Every time I have performance tested a vehicle for this problem, I have been able to stop by using the brakes while holding the accelerator pedal down. There are caveats. Some people sit too far back to properly apply the brake pedal. If the brakes are not fully applied when the throttle is wide open, they can overheat, lose effectiveness, and fail to stop the vehicle. Because minimal vacuum is developed when the throttle is wide open, pumping the brake pedal could exhaust the vacuum reserve and negate the power assist. And if the driver's right foot is on the throttle pedal and partly under the brake pedal, application of the brakes by the left foot could simply hold the throttle open.

Each unintended acceleration event needs a thorough investigation. It could have been as simple as a floor mat catching the throttle pedal, or the driver pushing the wrong pedal. It could have been evanescent such as some detritus or ice that interfered with the throttle plate, and was long gone by the time of inspection. Or a failed accelerator pedal position sensor. Or a faulty cruise control. Or a broken return spring. Or? There are a myriad of possible reasons, and only a thorough investigation can hope to discover the cause.

If the air bags have deployed, then in some vehicles there is a record of the speed and throttle position sensors, and whether the brake light switch had been activated.

Finally some practical advice if you are unlucky enough to experience this phenomenon personally. First as mentioned above, the brakes will always overcome the engine, if applied properly and firmly. But another option exists - shift to neutral! All modern engines incorporate a limiter which will prevent over-speeding of the engine, and minimize the chance of damage to the engine. It will not sound nice, but it is a safe way to regain control.

Brake and Shift to Neutral