Interpretation ID: 17539.wkm
Mr. C. Thomas Terry
Director, Safety Affairs and Regulations
General Motors Corporation
30200 Mound Road
Box 9010
Warren, MI 48090-9010
Dear Mr. Terry:
Please pardon the delay in responding to your letter to Mr. John Womack of this office in which you stated that General Motors (GM) is considering the use of dual vacuum hoses between the engine manifold and the booster unit to meet the failed power-assist requirement of Federal Motor Vehicle Safety Standard (Standard) No. 135, Passenger car brake systems.
You stated that the hoses would be connected to the manifold at two separate locations and to two separate non-removable check valves at the booster unit. Thus, failure of one hose would not affect system performance since the other hose would continue to provide vacuum to the booster unit. The check valves would prevent loss of vacuum from the booster in the event of failure of one of the hoses. You asked our agreement that in such a system, only one hose at a time need be disconnected when conducting the failed power-assist test of Standard No. 135, specifically subparagraphs S7.11.3(g) and (h). We have carefully considered your suggested interpretation but, for the reasons discussed below, we do not agree.
Subsection S7.11.4 establishes the applicable performance requirement 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: S0.10 + 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 (emphasis added).
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.
The agency does not consider the use of hoses from the engine manifold to the booster unit as the primary source of power for the brake power-assist 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 regardless of the number of vacuum hoses used to transmit the vacuum. Loss of vacuum generated by the engine constitutes a loss of the primary power source. That is what the failed power-assist test of S7.11 seeks to replicate, that is, the standard seeks to ensure that in the event of loss of power assist, 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 brake pedal force of 500 Newtons.
In this test, therefore, all hoses from the engine intake manifold to the booster unit are disconnected and the system is depleted of vacuum. The stopping tests are then conducted without reconnecting the brake power assist unit to the vacuum source. This is not to imply that the engine is turned off for this portion of the testing, but rather that the source of power, the engine manifold, is disconnected from the power assist unit. I note that with respect to S7.7, Stops with Engine Off, the difference is that residual vacuum remains in the system to be used to power the brake assist unit. For the inoperative brake power assist test, the system and any subsystems must be depleted.
A "backup" system as provided in S7.11.3(h) would be a separate electric or vacuum accumulator that would automatically activate in the event of failure of the primary power source. Thus, the dual hose system you described in your letter would not constitute a backup system. Rather, it would be no more than components of the primary power source.
I hope this information is helpful to you. Should you have any further questions or need additional information, feel free to contact Walter Myers of my staff at this address or at (202) 366-2992, fax (202) 366-3820.
Sincerely,
Frank Seales, Jr.
Chief Counsel
d.7/15/98
ref:105#135