Interpretation ID: aiam1809
Legal Counsel
Dana Corporation
P.O. Box 1000
Toledo
OH 43697;
Dear Mr. Shall: This responds to Dana Corporation's January 1, 1975, question whethe the exception found in S5.3.1(a) and S5.3.2(a) of Standard No. 121, *Air brake systems*, which permits 'controlled lockup of wheels allowed by an antilock system' is available in the case of a wheel which is not 'sensed' by the antilock system through a wheel sensor. You indicate that a statement in our November 1, 1974, letter to Dana Corporation appears to permit the exception for wheels that are not equipped with a antilock valve and logic system.; The answer to this question is no. The statement in our November 12 1974, letter states that 'the manufacturer may choose the number of wheel speed sensors and logic modules that he includes in his antilock system.' This statement simply recognizes that the standard does not require an antilock system, and that the number of wheel speed sensors and logic modules in any antilock system employed by a manufacturer is also not specified.; The standard does require 'no lockup' of any wheel above 10 mph excep 'controlled lockup allowed by an antilock system.' It is clear that a manufacturer who wishes to make use of this exception must provide an antilock system and that it must control lockup at the wheel in question. We pointed out in our November 12, letter that an antilock system would not control lockup of a wheel unless a sensor located on that wheel signaled the logic module of the lockup condition. It would not be sufficient that an antilock system controlled the air available to the wheels' brake system without the ability to sense that wheel's lockup.; To summarize, a manufacturer may choose whether to utilize an antiloc system, and whether to equip each wheel with a sensor. However, a manufacturer must equip each wheel with a sensor in order to take advantage of the controlled lockup exception at that wheel.; Yours truly, James C. Schultz, Acting Chief Counsel