Interpretation ID: nht73-2.48

DATE: 12/21/73

FROM: AUTHOR UNAVAILABLE; R. B. Dyson; NHTSA

TO: Blue Bird Body Company

TITLE: FMVSS INTERPRETATION

TEXT: This is in response to your November 8, 1973, request for an interpretation of the warning signal requirements of Standard No. 121, Air brake systems.

S5.1.5 of that standard states:

A signal, other than a pressure gauge, that gives a continuous warning to a person in the normal driving position when the ignition is in the "on" or "run" position and the air pressure in the service reservoir system is below 60 psi. The signal shall be either visible within the driver's forward field of view, or both audible and visible.

A warning that "the air pressure in the service reservior system is below 60 psi" is intended to mean that a warning device's sensor could be located in the system you described in your letter between the source of air pressure and the check valve(s) required by S5.2.1.5. As you pointed out, this location would sense pressure below 60 psi anywhere from the compressor through the entire service reservior system. A single warning installed before the check valves in a split service brake system would fulfill the requirements of S5.1.5 as long as it is positioned to sense pressure below 60 psi in any part of the split service reservior system.

P2 Your interpretation of the signal requirements is correct. The signal must be both audible and visible, or it must be visible within the driver's forward field of view. A simple audible signal is insufficient, as is a simple visible signal which is not within the driver's forward field of view.

Yours Truly,

November 8, 1973

Richard Dyson Assistant Chief Council NHTSA

Dear Mr. Dyson:

We have several questions relating to FMVSS 121, S5.1.5, warning signal.

Our interpretation of S5.1.4 and S5.1.5 is that a gauge and a signal are required on each service reservior system. Neither a gauge nor a signal are required on the "wet tank" which is between the service tanks and the compressor and is separated from the service tanks by check valves. It seems to us that a signal on the "wet tank" in lieu of on the service tanks might be more effective in warning the operator of the system malfunction. Our reasoning is as follows: If there is no signal on the "wet tank", a failure of any of the components shown in red on the attached sketch would not be known to the operator until the pressure in the service tanks dropped below 60 psi. With a signal on the "wet tank", the operator would immediately be aware of a failure in any of the components from the compressor through the service system.

Will a warning signal on the wet tank in lieu of a signal on each service tank meet the requirements of FMVSS 121? If not, would NHTSA consider such an amendment?

Also, a major brake manufacturer has supplied us with their written interpretation of FMVSS 121, S5.1.5. Their interpretation would allow warning signals to be visible or audible or both. This is in conflict with our understanding of S5.1.5 which says ". . . the signal shall be either visible. . . or both audible and visible." Which is correct?

Thank you for your early reply.

W. G. Milby Project Engineer

cc: Ben Newberry Dave Phelps

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