Interpretation ID: 3194o
Product Safety & Environmental Control
Caterpillar Inc.
Peoria, IL 61629
Dear Mr. Carr:
This letter responds to your inquiry of April 29, 1988, asking this agency for an interpretation of Standard 124, Accelerator Control Systems, as it applies to the components of a diesel truck engine and accelerator control components described in your letter. I apologize for the delay in this response.
With your letter, you provided diagrams of three different accelerator control systems and fuel metering devices for a diesel engine. The first system (Type I in your letter) was a purely mechanical system. In this system, the linkage from the accelerator pedal connects to a mechanical governor. This governor is a device that responds to pedal displacement, and causes a rack to move, thereby controlling the flow of fuel from fuel injector pumps into the engine cylinders. The second and third systems described in your letter operate through an electrical-mechanical sequence. In the second system (Type II in your letter), mechanical linkage from the accelerator pedal connects to an electronic pedal position sensor. When the vehicle operator puts pressure on the accelerator pedal, this sensor converts that movement into an electrical signal. The electrical impulse travels through a wire to an engine control module (ECM). The ECM interprets pedal position and engine speed from the sensor impulse, and determines how much fuel must go to the engine cylinders either to maintain or reduce speed in consequence of pedal movement. In turn, the ECM sends a signal to a motor which moves a fuel rack to control the flow of fuel from the injector pumps into the engine cylinders. In the system you call Type III, the ECM sends a signal directly to individual fuel injector pumps without the intervention of a rack. Otherwise, Type II and III are the same.
On October 28, 1988, at the agency's invitation, you met with us to further explain the background information for this request and why Caterpillar believes that particular components were included in either the accelerator control system or the fuel metering device. During the course of that meeting, you agreed with us that Caterpillar's principal concern was not whether any particular component was part of the accelerator control system or the fuel metering device. Instead, you were concerned with the requirement that the throttle "shall return to idle" under specified conditions. You explained that your ECM is designed to shut off the engine, rather than return the throttle to idle, during many failure modes. We conclude that the requirement that the throttle "return to idle" is satisfied by a system that shuts off the engine in the specified circumstances.
As S2 of Standard 124 makes clear, the purpose of the standard is to prevent runaway vehicles when certain malfunctions occur in the accelerator control system. If such malfunctions do occur, the standard ensures that the engine will not continue at a high speed, but will return to idle, so that the driver can safely brake the vehicle and get it off the road. In both the Type II and III systems referred to in your letter, the agency understands that a failure in the ECM circuitry, or a failure of the components that respond to the ECM, will either return the throttle to idle or shut down the engine. Either of these results would serve the purpose of Standard 124. Therefore, we interpret the phrase "return to idle" to be satisfied by returning to idle or going beyond that throttle position to shut off the engine. Accordingly, based on the information you have provided, it appears that your Type II and III systems would comply with Standard 124 regardless of whether the ECM and other components are considered part of the acclerator control system or the fuel metering device.
I hope you find this information helpful. If you have further questions, please call Joan F. Tilghman of my staff at (202) 366-2992.
Sincerely,
Erika Z. Jones Chief Counsel
/ref:124 d:ll/9/88