Astro Van M V6-262 4.3L VIN W CPI (1992)
Speedometer Head: Description and Operation
Without Electronic Cluster
The Speedometer/Odometer used with the standard cluster is an electronic speedometer. It requires no speedometer cable. The speedometer receives a
vehicle speed signal from the Vehicle Speed Sensor Buffer (DRAC). The DRAC receives an AC voltage signal from the transmission mounted Vehicle
Speed Sensor (VSS).
The VSS generates a signal that indicates the speed of the vehicle. The Signal is processed by the solid-state Vehicle Speed Sensor Buffer (DRAC) to
supply inputs to the Electronic Control Module (ECM), the Cruise Control Module and the Speedometer.
The VSS is mounted in the transmission. A toothed rotor rotates near a coil, producing voltage pulses in the coil.
The rotor that is attached to the transmission turns four times faster than a standard speedometer cable. The coil near the rotor generates 40 pulses per
revolution.
The frequency of the AC voltage coming from this coil depends on the vehicle speed. As the speed increases, so does the number of voltage pulses per
second.
The Vehicle Speed Sensor Buffer (DRAC) takes the voltage pulses from the sensor and uses them to close three solid-state output switches. Each
output terminal is switched to ground at a rate that is proportional to the speed of the vehicle. The Speedometer is switched by the primary rate of the
VSS. The ECM and the Cruise Control use a lower frequency. Their input switches are operated by a circuit that divides the sensor frequency by two.
Different Vehicle Speed Sensor Buffers (DRAC's) are used to match the vehicle final drive ratio to the components.
CAUTION:
THE DIAGNOSTIC TEST REQUIRES THAT YOU FREE-ROLL THE REAR WHEELS OF THE VEHICLE ON THE HOIST. BE SURE
THE WHEELS ARE FREE OF OBSTRUCTIONS AND THAT YOU AND OTHERS STAY CLEAR OF THE WHEELS AT ALL TIMES.
FAILURE TO DO SO MAY RESULT IN INJURY OR DEATH. DO NOT BRING WHEEL SPEED ABOVE 70 MPH.
