§393.51 Warning Devices and Gauges

Question 1: Is the low pressure warning device required to activate before the tractor protection valve?

Guidance: No. §393.51 does not explicitly require the warning device to operate before the protection valve. It is implied that if the operating pressure of the warning device is at least 1/2 of the governor cut-out pressure, and that pressure is not less than the pressure at which the protection valve (or similar device) activates, the requirements of §393.51 are satisfied.

Question 2: Is the vacuum portion of vacuum-assisted hydraulic brake systems required to have a warning device?

Guidance: No. Only the hydraulic portion of vacuum-assisted hydraulic brake systems is required to have a warning device. FMVSS No. 105 does not require a warning device for the vacuum portion of the vacuum-assisted hydraulic brake systems. It is the intention of the FHWA that §393.51 be consistent with FMVSS No. 105.

Question 3: Are vacuum gauges required on the vacuum portion of vacuum-assisted hydraulic brakes?

Guidance: No. §393.51(d)(2) requires only that CMVs with vacuum brakes (not hydraulic brakes applied or assisted by vacuum) be equipped with a vacuum gauge.

Question 4: Is a warning device required in a CMV with a single hydraulic brake system which uses the driveline parking brake as the emergency brake system?

Guidance: No. Warning devices are not required on such CMVs because the driver will be given ample warning of system failure by the movement and feel of the brake pedal.

Question 5: What difference, if any, is there between a warning device and a warning signal?

Guidance: For purposes of §393.51, the terms may be used interchangeably.