Interpretation ID: nht75-1.12

DATE: 07/08/75

FROM: AUTHOR UNAVAILABLE; R. B. Dyson; NHTSA

TO: Mercedes-Benz of North America, Inc.

TITLE: FMVSS INTERPRETATION

TEXT: This is in response to your letter of May 5, 1975, asking at what point the test voltage may be measured during testing for compliance with the Standard No. 103, Windshield Defogging and Defrosting Systems, requirement that the blower motor test voltage be 15% over nominal system rating at the blower motor or the supply end of the motor dropping resistor. Your question relates to the meaning of "the supply end of the motor dropping resistor."

Measurement of the blower motor test voltage should occur at the supply side of the motor where there is no resistor. Where the system contains a resistor, the voltage should be measured at the supply side of the resistor, not between the resistor and the motor. The reason for this is that the test voltage level specified in the standard is intended to relate only to the voltage as it is fed into the defrosting and defogging system. The purpose of the voltage level specification is to assure a system capability to handle voltage levels that will normally be encountered during operation of the defroster and defogger. This can be accomplished by measurement of the voltage before the current reaches the resistor.

Yours Truly,

MERCEDES-BENZ OF NORTH AMERICA. INC.

May 5, 1975

National Highway Traffic Safety Administration

Subject: Request for Interpretation; Federal Motor Vehicle Safety Standard 103 - Windshield Defrosting & Defogging Systems

FMVSS 103 through reference to SAE J902, requires that the blower motor test voltage be 15% over nominal system rating at the blower motor (for example 13.8 volts on the 12 volt system) or the supply end of motor dropping resistor.

The terminology "the supply end of motor dropping resistor" does not, in our opinion, provide specific direction on the test methods to be used for designing to this Standard. We would appreciate receiving clarification as to whether or not the test voltage may be measured at:

1. the motor,

2. the circuit between the dropping resistor and the motor, or

3. the dropping resistor on the side of the electrical power source.

We recommend that the requirements be interpreted to mean that the test voltage shall be measured at the motor or before the dropping resistor if included in the defroster motor circuit.

Permitting the use of a dropping resistor in the defroster motor circuit permits the designer to provide exact blower motor speeds for this function. Should, however, this resistor be excluded during compliance testing, the higher voltage at the motor would result in higher fan speeds, which might result in less than optimum deicing of the windshield.

Should you require any additional information concerning this request, do not hesitate in contacting this office.

Heinz W. Gerth

Assistant Vice President

Engineering