Interpretation ID: nht75-3.42
DATE: 10/10/75
FROM: AUTHOR UNAVAILABLE; F. Berndt; NHTSA
TO: Kelsey-Hayes Company
TITLE: FMVSS INTERPRETATION
TEXT: This responds to Kelsey-Hayes Company's July 28, 1975, question asking whether the requirements of S5.1.6 of Standard No. 121, Air Brake Systems, that specify a warning signal "in the event of a total electrical failure of the antilock system" would permit installation on a vehicle of an antilock-equipped axle that has no capability to signal electrical failure of its antilock system. You state that the vehicle would be equipped with antilock systems on other axles that would provide a warning signal in the event of their electrical failure.
The answer to your question is yes. As you noted in your letter, the National Highway Traffic Safety Administration has interpreted the specifications of S5.1.6 to require a signal only in cases where electrical failure within the antilock electrical system circuitry causes loss of antilock control of every wheel on the vehicle. In the design you describe, the signal which activates upon loss of antilock control at one or more wheels on the vehicle would fulfill this requirement, because it would always activate by the time antilock control had been lost at every wheel on the vehicle. Under our interpretation of S5.1.6, a failure of antilock only on the axle described by you would not constitute "loss of antilock control of every wheel on the vehicle" and would not be required to be signaled.
SINCERELY,
KELSEY-HAYES COMPANY
July 28, 1975
National Highway Traffic Safety Administration
ATTN: James Schultz Chief Counsel
RE: Request for Interpretation FMVSS-121: Air Brake Systems 5.1.6, Antilock Warning Signal
Kelsey-Hayes Company, a domestic manufacturer of motor vehicle equipment including antilock systems, requests an interpretation of the above referenced section of Standard 121 as it relates to tag axles, liftable axles and removable dollies.
Interpretations on this section of the standard issued by your staff in the past, specifically one to Wagner Electric on May 26, 1972 and another to the Eaton Corporation dated December 26, 1974 state that the phrase "total electrical failure" means any electrical failure within the antilock electrical system circuitry which would cause loss of antilock control of every wheel on the vehicle.
Since an antilock system failure on one axle need not actuate the warning signal, we ask whether it is consistent with these interpretations to equip an axle on a vehicle, specifically a tag axle, liftable axle, or removable dolly, with an antilock system that does not have the capability to activate the warning signal in the event of an electrical failure. Electrical failures which would disable the antilock system on the wheels of the other axles would activate the warning signal such that the "total electrical failure" situation would be complied with. In other words, the total electrical failure situation cannot occur unless the antilock systems on the other axles are disabled and, if they are, the warning signal will be activated.
We have been advised by installers of tag axles that it is burdensome to match the antilock system of the tag axle with the same make as those originally equipped on the other axles of the vehicle and that making the necessary electrical connections to the failure detection circuits on the other axles creates unacceptable liability risks. The most practical means to mitigate this condition would be to use trailer type antilock system components, which do not have electrical failure detection capability, for tag axles and other axles customarily added by body builders, etc.
Your prompt attention to this request for interpretation will be appreciated.
John F. McCuer