Interpretation ID: 77-4.25
TYPE: INTERPRETATION-NHTSA
DATE: 10/31/77
FROM: AUTHOR UNAVAILABLE; F. Berndt; NHTSA
TO: Oshkosh Truck Corporation
TITLE: FMVSS INTERPRETATION
TEXT: This is in reply to your letter of August 29, 1977, asking for confirmation of your interpretation of S4.5.4 of Motor Vehicle Safety Standard No. 108.
Paragraph S4.5.4 requires that "The stop lamps on each vehicle shall be activated upon application of the service brakes." Oshkosh trucks are equipped with split air brake systems. This system incorporates a parking brake system on the rear axles. You indicated that there are three conditions under which the parking brakes will apply:
Condition 1. - Parking Application.
The spring brakes are driver applied through a hand operated parking control.
Condition 2. - Rear Service Brake System Failure.
The spring brakes can be driver applied through the service brake foot operated treadle valve control in the event of a failure in the rear service brake system.
Condition 3. - Spring Brake Cavity Pressure Loss.
A component failure which allows air pressure to exhaust from the spring cavity of the rear brake chambers will cause the spring brakes (parking brakes) to apply. This condition is not driver initiated."
You have interpreted Condition 2 as the only "service brake" application since it is the only one of the three that is driver-initiated through the service brake control, and that stop lamp activation under the other two Conditions is not required by Standard No. 108.
This will confirm your interpretation. With respect to Condition 1, we do not consider that driver application of the parking brake portion of the service brake system is "application of the service brakes" within the meaning of S4.5.4. Nor do we consider that activation of the parking brakes through component failure is "application of the service brakes", your Condition 3.
SINCERELY,
August 29, 1977
Chief Counsel NHTSA
Subject: FMVSS 108 Stop Lamp Circuitry Interpretation Request Oshkosh Truck Corporation equips its trucks and truck tractors with split air brake systems. The Bendix-Westinghouse Corporation has a publication which very concisely explains the operation of a split air brake system like the one adopted by Oshkosh Truck. A copy of this publication "Bendix-Westinghouse Dual Air Brake System" is attached.
As explained on page 5 of the Bendix publication, the spring brakes on the rear axles are the vehicle's parking brakes. There are, however, three conditions in which the spring brakes will apply.
Condition 1. - Parking Application.
The spring brakes are driver applied through a hand operated parking control. Refer to page 5 of the attachment.
Condition 2. - Rear Service Brake System Failure.
The spring brakes can be driver applied through the service brake foot operated treadle valve control in the event of a failure in the rear service brake system. Refer to page 6 of the attachment.
Condition 3. - Spring Brake Cavity Pressure Loss.
A component failure which allows air pressure to exhaust from the spring cavity of the rear brake chambers will cause the spring brakes (parking brakes) to apply. This condition is not driver initiated.
FMVSS 108 requires that the stop lamps be activated upon application of the service brakes (para. 4.5.4) but does not require that the stop lamps be activated upon application of the parking brakes. Although all three of the above conditions are possible with the vehicle in motion, Oshkosh Truck Corporation interprets condition 2 as the only "service brake" application of the three described in that this spring brake application is the only one of the three that is driver initiated through the service brake control. In its interpretation of FMVSS 108 Oshkosh Truck Corporation has determined that stop lamp activation is not mandatory when the spring brakes are applied by conditions 1 or 3 described above (regardless of the vehicle's FMVSS 121 status).
Oshkosh Truck requests written confirmation. Your immediate attention to this request will be greatly appreciated.
Stephen C. Nimmer Senior Supervising Engineer
[Publication Omitted.]