Mr. Richard C. Kempf
Manager Technical Legislation
2911 Meyer Road
Post Office Box 1109
Fort Wayne, IN 46801
Dear Mr. Kempf:
Please pardon the delay in responding to your letter to me stating that Navistar interprets Federal Motor Vehicle Safety Standard (Standard) No. 121, Air Brake Systems, to provide that the antilock brake systems (ABS) required by the standard need not be operational when the vehicle is switched by the driver to the all-wheel drive (AWD) mode. Your interpretation is not correct.
You stated that certain vehicles equipped with AWD normally operate in the two-wheel drive mode, but the AWD mode is selectable by the driver for severe service operation. You also correctly pointed out that paragraph S6.1.11 of Standard No. 121, when relating to special drive conditions, provides that such vehicles are tested with the AWD mode disengaged.
Paragraph S5.1.6(a) of Standard No. 121 provides:
Each single-unit vehicle manufactured on or after March 1, 1998, shall be equipped with an antilock brake system that directly controls the wheels of at least one front axle and the wheels of at least one rear axle of the vehicle. Wheels on other axles of the vehicle may be indirectly controlled by the antilock brake system.
This provision clearly requires that all single-unit vehicles equipped with air brake systems must also be equipped with ABS.
This is a general requirement that applies under all conditions.
There is no exception from this requirement for vehicles equipped with AWD, whether being operated in the 2WD or AWD mode.
The requirements of section S5 are tested in accordance with the conditions set forth in section S6. Paragraph S6.1.11 of that section provides, as one of the road test conditions:
A vehicle equipped with an interlocking axle system or a front wheel drive system that is engaged and disengaged by the driver is tested with the system disengaged (emphasis added).
Thus, an interlocking axle, or AWD, system controlled by the driver is disengaged for the road tests conducted under subsection S6.1. That does not mean, however, that ABS may be disengaged whenever the AWD system is in operation. The equipment requirements of S5 are not limited by the road test conditions of S6.
In summary, the ABS required by Standard 121 may not be disengaged or disabled when the vehicle is switched into AWD mode, but a vehicle in the AWD mode would not be required to meet the road test requirements of subsection S6.1. While the ABS may not be disabled, however, its operation could be modified to better suit off-road conditions, such as construction, logging, or mining operations, when AWD is selected.
I hope this information is helpful to you. Should you have any further questions or need additional information, please feel free to contact Walter Myers of my staff at this address or at (202) 366-2992.
Acting Chief Counsel