Interpretation ID: 21978ogm

Dear:

This is in response to your recent letter regarding a new product being produced by your company and the requirements of Federal Motor Vehicle Safety Standard No. 135, Passenger Car Brake Systems. Your letter describes your company's product as a supplemental pump/booster unit that is automatically activated to provide power-assist to a vehicle brake system if the primary power source for brake assist fails. You ask the agency to confirm that your system, which uses an electrically driven pump to provide vacuum boost to a conventional vacuum assisted brake booster, would meet the failed power-assist requirement of Standard No. 135.

After examining the information provided by your company, we agree that your company's design appears, in concept, to meet the failed power-assist requirements of Standard No. 135.

Paragraph S7.11.4 establishes the performance requirements for failed power-assist as follows:

The service brakes on a vehicle equipped with one or more brake power-assist units or brake power units, with one such unit inoperative and depleted of all reserve capability, shall stop the vehicle as specified in S7.11.4(a) or S7.11.4(b).

(a) Stopping distance from 100 km/h test speed: 168m (551 ft).

(b) Stopping distance for reduced test speed: S0.10V + 0.0158V.

In testing for this requirement, subparagraph S7.11.3(g) provides:

Disconnect the primary source of power for one brake power-assist unit or brake power unit, or one of the brake power unit or brake power-assist unit subsystems if two or more subsystems are provided.

Subparagraph S7.11.3(h) provides:

If the brake power unit or power-assist unit operates in conjunction with a backup system and the backup system is automatically activated in the event of a primary power service failure, the backup system is operative during this test.

In a vacuum-assisted brake system, the power or medium used to operate the brake power-assist system is vacuum, the primary source of which is the engine. Thus, the "primary source of power" of a vacuum-operated brake power-assist system is the engine intake manifold, which provides vacuum to the power-assist unit. Loss of vacuum generated by the engine constitutes a loss of the primary power source. The failed power-assist test of S7.11 seeks to replicate this loss and ensure that the driver will still be able to bring the vehicle to a stop in the required distance of 168 meters (551 feet) with the prescribed maximum brake pedal force of 500 Newtons. Therefore, in this test, the primary booster unit is disconnected and the system is depleted of all vacuum. The stopping tests are then conducted without reconnecting the brake power-assist unit to the vacuum source.

Under S7.11.3(h), a separate electric or vacuum accumulator that automatically activates in the event of failure of the primary power source would be a "backup system" that remained operative during the test. The system you describe in your letter, in which an electrically driven pump provides a source of vacuum, provides such a function. Thus, the electrically driven vacuum pump you described in your letter could be used to meet the failed power-assist requirement of Standard No. 135.

I hope this information is helpful to you. Should you have any further questions or need additional information, feel free to contact Otto Matheke of my staff at this address or at (202) 366-2992, fax (202) 366-3820.

Sincerely,

Frank Seales, Jr.
Chief Counsel

ref:135
d.11/17/00