Interpretation ID: 2794o
Manager, Engineering
Isuzu Motors America, Inc.
21415 Civic Center Drive
Southfield, MI 48076-3969
Dear Mr. Tokunaga:
This letter responds to your inquiry in which you ask a number of questions concerning Federal motor vehicle safety standard (FMVSS) 124, Accelerator Control Systems. I apologize for the delay in this response. In your letter, you describe a new accelerator control system that operates through electrical rather than mechanical signals.
You state that the moving components of this system are the accelerator pedal, stepping motor arm, linkage, and the throttle lever. When a driver depresses the accelerator pedal, a pedal sensor converts the displacement into a proportional electric signal. The signal goes through a control unit to a position switch, and then to a stepping motor. This stepping motor works to move the motor's arm and linkage, and they in turn work the throttle lever. Therefore, you say, the engine speed is controlled in proportion to the amount of accelerator pedal displacement.
You further inform us that Isuzu already has distributed vehicles equipped with this system in Japan, and that the company would like to market this kind of vehicle in the United States. You present three questions and a diagram of the system components, and request an agency response.
First, please be aware that in issuing this interpretation, NHTSA is neither approving, certifying, nor endorsing your new accelerator control system. Under the National Traffic and Motor Vehicle Safety Act, each manufacturer must certify that its product meets agency safety standards, or other applicable standards. However, based on the information you supplied in your letter, I have the following responses.
Question I: In this vehicle, Isuzu considers the battery that drives the stepping motor to be one of the energy sources under S5.1, and the return springs (accelerator pedal and throttle lever return springs) the other sources. Is this interpretation correct? We do not have enough information to state whether the battery that drives the stepping motor, or the return springs would be considered energy sources under S5.1. Section S5.1 of Standard 124 requires, among other things, that there be a minimum of two energy sources capable of returning the throttle to idle whenever the driver removes the opposing actuating force, or if there is a single severance or disconnection in the accelerator control system.
With respect to the battery, if all system elements are operating properly, then it would appear that removing the actuating force will cause the electrical circuit from accelerator pedal sensor to stepping motor to return the throttle to idle. On the other hand, if there is a failure caused by a severance or disconnection in the accelerator control system between the pedal and the stepping motor, it is not clear to me whether the stepping motor will return to zero, and bring the throttle springs back to idle; or lock the arm and linkage in an "open-throttle" position.
Similarly, it is not clear to me that the accelerator pedal and throttle return springs are capable of returning the throttle to idle in the event of a failure caused by an ACS severance or disconnection. (While you include the throttle lever in your description of the accelerator control system, the agency considers it as part of the fuel metering device. However, as NHTSA explained in the preamble to 124, an energy source under the Standard may be attached to the fuel metering device. [37 FR 20033, September 23, 1972.]) Ordinarily, the agency would have no difficulty in finding that either of the throttle return springs is an energy source capable of returning the throttle to idle. But I cannot tell from your description and diagram whether a severance or disconnection in the electrical system would cause the throttle to lock in a position other than idle.
I would make the same observation with respect to the accelerator pedal. I can not tell from the information you supplied what impact a severance or disconnection failure would have on the pedal. For example, it is not apparent whether some element in the electrical system senses a severance or disconnection in the accelerator control system, so that a sensor transmits a signal to the appropriate energy sources that the throttle should return to idle. If the pedal and return springs can operate mechanically and in concert to return the throttle to idle in the event of a failure in the accelerator control system caused by a severance or disconnection, then together they may be an energy source under the Standard.
Question 2a: Is a severance in electric wires in this system a severance or disconnection within the meaning of S5.2? Isuzu considers negative because electric wires are not a moving part.
A severance or disconnection of the electric wires in this system would be a severance or disconnection within the meaning of S5.2 of Standard 124.
Section S4.1 of Standard 124 defines a "driver-operated accelerator control system" as "all vehicle components, except the fuel metering device, that regulate engine speed in direct response to movement of the driver-operated control and that return the throttle to the idle position upon release of the actuating force."
You stated in your letter that, in this new system, when the driver depresses the accelerator pedal, the mechanical displacement is converted into electrical signals. These electrical signals are transmitted by wires to a control unit that regulates engine speed in direct response to pressure on the accelerator pedal, again by means of wires that connect the control unit's electrical signal to the appropriate components. Thus, the control unit, all of the components to which it is connected, and the wires that make those connections are "vehicle components ... that regulate engine speed in direct response to movement of the driver-operated control and that return the throttle to the idle position upon release of the actuating force." Under S4.1, then, the control unit, the components to which it is connected, and the wires that make the connection are components of the driver-operated accelerator control system.
Section S5.2 of Standard 124 requires that the throttle return to idle "from any accelerator position or any speed...whenever any one component of the accelerator control system is disconnected or severed at a single point." Please note that this language does not limit the requirement to disconnections or severances of components that are moving parts. Thus, all severances or disconnections of any component of the accelerator control system are within the ambit of the standard. In this case, since the wires are a component of the accelerator control system, the throttle must return to idle whenever a wire is disconnected or severed.
Question 2b: If a severance in electric wires were a severance or disconnection under S5.2, what about a short-circuiting that may result from such a severance? Does the Standard require that the throttle returns to the idle position even in such a condition?
Yes. Section S5.2 of Standard 124 requires the throttle to return to the idle position whenever any component of the accelerator control system is disconnected or severed at a single point, regardless of the other consequences of the disconnection or severance. In the case of this system, this language requires the throttle to return to idle when any wire is severed, even if the severance results in a short circuit.
Question 2c: Our understanding is that a failure (other than severance or disconnection) of a system component itself (i.e. a failure in the accelerator pedal sensor with pedal position switches, control unit, throttle valve position switch, or stepping motor) is not subject to the throttle return requirement under the Standard. Is this correct?
Your understanding is partially correct. Standard 124 addresses those circumstances where (1) the driver removes the opposing actuating force; and (2) a severance or disconnection in the ACS causes a failure. Therefore, you are correct that Standard 124 addresses only those failures resulting from a severance or disconnection within the system. However, for electrical systems, shorted or open circuits are the consequence of a change in one or more of the electrical components in the system. The agency would consider such a change a disconnection or severance in the context of this Standard.
Question 3: It is our interpretation that the battery and the electric wires from the battery to the control unit are not a part of the accelerator control system under this definition. (That is, the definition of "driver-operated accelerator control system.") Is this interpretation correct?
No, your interpretation is incorrect. We have set out the definition of "driver-operated accelerator control system" in section S4.1 above, in response to your Question 2a. With respect to your electrical accelerator control system, the electrical impulse that travels between the vehicle battery and the control unit is a direct consequence of the driver's applying an actuating force to the accelerator pedal. Given this aspect of your system's design, both the vehicle battery and the electric wires from the battery to the control unit fall within the definition of "driver-operated accelerator control system."
I hope you find this information helpful.
Sincerely,
Erika Z. Jones Chief Counsel
ref:124 d:8/8/88