Manager, Vehicle Regulations
Mail Code 4F02
3800 Hamlin Road
Auburn Hills, MI 48326
Dear Mr. Haenchen:
This responds to your request for an interpretation whether a single coil spring consisting of multiple strands of wire twisted together would meet the Standard No. 124 Accelerator Control Systems requirement that the control system have "at least two sources of energy capable of returning the throttle to the idle position within the time limit specified by S5.3." Since we do not agree that each wire strand constitutes a separate source of energy, the answer is no.
Under specified temperature conditions, S5. Requirements, of Standard No. 124 states that the vehicle shall meet the following requirements when the engine is running under any load condition:
S5.1 There shall be at least two sources of energy capable of returning the throttle to the idle position within the time limit specified by S5.3 from any accelerator position or speed whenever the driver removes the opposing actuating force.
In your letter, you expressed your belief that a single coil spring consisting of multiple strands of wire twisted together would constitute the "two sources of energy" capable of returning the throttle to the idle position. You stated: "Assuming in the spring consisting of three strands of wire, that if only one strand is broken, the remaining two would have sufficient force to return the throttle to idle, we believe such a spring would comply with S5.1." You stated that a seven wire spring would provide even greater redundancy as the spring would have sufficient force to return the throttle to idle if up to three of the wire strands were broken.
We do not agree that a single spring consisting of multiple strands of wire meets Standard No. 124. A spring consisting of multiple strands of wire may be more resistant to fatigue than a spring consisting of a single strand, but the spring remains a single component. S5.1 requires two sources of energy that are separate, independent components. I note that the purpose of the requirement for "two sources of energy" was explained in a September 23, 1972 Federal Register document, in which NHTSA concurred with a petitioner's statement: "The intent of paragraph S5.1 is to eliminate the driver's dependence on a single accelerator return spring." (See 37 FR 20033, at 20034).
The components must also be independent of each other in order to meet S5.2, which specifies a return to idle if "any one component of the accelerator control system is disconnected or severed at a single point." (Emphasis added.) The disconnection of a single spring consisting of multiple strands of wire would make the accelerator control system unable to meet S5.2.
I hope this information is helpful. If you need any further information, please contact Dorothy Nakama of my staff at (202) 366-2992.
Samuel J. Dubbin Chief Counsel ref:124 d:1/19/96