Warranty - Speedometer and Theory of Operation: Overview

REF. NO.: MBNA 54/39

TO: OUR MERCEDES-BENZ PASSENGER CAR DEALERS

DATE: October 1992

SUBJECT:
MODELS 107, 124, 126, 140 WITH ELECTRONIC SPEEDOMETERS CHECKING ELECTRONIC SPEEDOMETERS PRIOR TO REPLACEMENT

In a recent test of electronic speedometers returned under warranty, most were found to have no defect. Therefore, electronic speedometers should be tested according to the procedures described herein before replacing them. All electronic speedometers replaced under warranty are tested by MBNA. Any electronic speedometers found not defective will be returned to the dealer with the corresponding claim debited.





Theory of operation

A toothed wheel splined to the transmission output shaft rotates past the vehicle speed sensor (Figure 1), generating an AC signal with a frequency directly proportional to vehicle speed. This signal is a sine wave at approx. 1 V AC.

The sensor harness (2) conveys this signal to the instrument cluster (3). In the speedometer (4), an electronic circuit converts the frequency signal from the speed sensor to a voltage signal, which actually determines the speedometer indicator needle position (Figure 1). Since the speedometer contains a powered circuit (the frequency converter), it must have 12 V power to function properly.

Before replacing a speedometer or speed sensor, first verify the complaint, then test each part of the system to determine what is faulty.

If a test indicates a part is defective, replace only that part and then confirm proper operation of the speedometer system.