Download THE 6.2 LITER DIESEL ENGINE

5B. California Diesel Electronic Control System (DECS)
The speed of the vehicle is sensed by the speed
sensor. The speed sensor consists of a light em itting
diode and a phototransistor both of w hich are
enclosed in one connector. This connector is located
in the back of the speedometer cluster next to the
speedometer cable. A wiring harness connects the
sensor to the ECM (Figure 5-70).
The light em itting diode is lit any tim e the ignition is
turned on. The light given off is in the infrared area of
the light spectrum and is not visible to the human eye.
The diode directs its light toward the back of the
speedometer cup w hich is painted black, and the shiny
drive magnet which is part of the speedometer rotating
parts.
As each bar of the drive magnet passes the light beam
of the diode, the light beam is reflected back to the
photo transistor. The photo transistor generates an
electrical signal to the ECM. This signal is
representative of vehicle speed.
See Figure 5-71. The speed sensor is supplied w ith 12 volts. When a voltm eter is hooked from term inal A9 of the
ECM to ground and the speedometer cable turned, the voltage will swing between 8 volts and som ething under 1
volt rather than going up to 12 volts.
5-82