Interpretation ID: nht88-2.11
TYPE: INTERPRETATION-NHTSA
DATE: APRIL 29, 1988
FROM: J. E. CARR -- PRODUCT SAFETY AND ENVIRONMENTAL CONTROL
TO: ERIKA JONES -- CHIEF COUNSEL, NHTSA
TITLE: INTERPRETATION OF TERMS FOR FMVSS 124
ATTACHMT: ATTACHED TO MEMO DATED 11-9-88 TO J. E. CARR, FROM ERIKA Z. JONES -- NHTSA, STD 124
TEXT: The increasingly restrictive emissions requirements being specified make it imperative that Diesel Truck Engines have electronic engine controls. On many Caterpillar Truck Engines the traditional mechanical governor is being replaced by an Electronic Co ntrol Module (ECM). Caterpillar is asking NHTSA for interpretation of how the terms used in FMVSS 124 would be applied to an electronic system.
Regulation FMVSS 124, paragraph S5.2, concerns the throttle going to idle if there is a severance or disconnect in the accelerator control system. The accelerator control system is defined as all vehicle components, except the fuel metering device, that regulate engine speed in direct response to movement of the driver-operated control. When the mechanical governor and linkage is replaced by electronics and electrical wires, the distinction of what is part of the accelerator control system and what is part of the fuel metering device requires interpretation.
Attached is a sketch showing 3 types of accelerator control systems either in use or planned by Caterpillar.
Type I is a mechanical linkage pedal to a mechanical governor which moves a fuel rack. This is the mechanical system that has been in production for several years and is shown here for reference only.
Type II is a mechanical linkage connected to an accelerator pedal position sensor which is electrically connected to an Engine Control Module (ECM) and a Brushless Torque Motor (BTM) controling fuel injection by moving the fuel rack. The Caterpillar acce lerator pedal position sensor can be located at the OEM's option but is generally mounted on the engine compartment side of the firewall. Both the ECM and BTM are mounted on the engine. The ECM has a function similar to the mechanical governor in Type I in that it senses pedal position and engine speed along with several other factors and signals the BTM as to the desired amount of fuel to be injected.
Type III is a mechanical linkage to an accelerator pedal position sensor which is electrically connected to an ECM and to individual unit injectors. The ECM is mounted on the engine. The ECM senses pedal position and engine speed along with several oth er factors and signals the individual injectors as to the desired amount of fuel to be injected.
It is Caterpillar's position that in Type II and Type III Systems the term "Driver-Operated Accelerator Control System" includes all linkage and wiring up to the ECM. All other components shown in the sketch (except the battery) are part of the "Fuel Me tering Device".
Caterpillar would like confirmation from NHTSA that our interpretation of the definitions used in FMVSS 124 is correct. We would appreciate a prompt response. If you have any questions, please contact me or Jim McCollum on 309-675-5377.
SYSTEM COMPARISON
TYPE I
GOV RACK
Accelerator pedal
linkage
TYPE II
Accelerator pedal position sensor
Accelerator pedal
Bat.
ECM
BTM
RACK
TYPE III
Accelerator pedal position sensor
Accelerator pedal
ACCELERATOR CONTROL SYSTEM
ECM
Bat.
FUEL METERING DEVICE