C 1500 Truck 2WD V8-350 5.7L (1989)
Vehicle Speed Sensor: Description and Operation
The Vehicle Speed Sensor generates a signal that indicates the speed of the vehicle. The signal is then processed in the Instrument Cluster, which
provides inputs to the Rear Wheel Antilock Brake Module, the Electronic Control Module, the Cruise Control Module, the Speedometer, and the
Odometer.
The Vehicle Speed Sensor is mounted in the transmission. A magnet rotating within a coil creates 40 voltage pulses for every revolution of the
Transmission Output Shaft (Propeller Shaft). The frequency of this AC signal is dependent on the vehicle's speed. As the speed increases, so does the
frequency.
The AC signal goes to the Instrument Cluster where the Input Buffer and Ratio Adapter convert the signal to 128,000 pulses per mile. This is done by
dividing the AC signal by the divide ratio. The divide ratio is determined by the pin configuration of the Programming Clip. The Programming Clip
configuration is determined by the tire radius and the axle drive ratio, and manually set.
The signal output from the Ratio Adapter is 128,000 pulses per mile. This signal is used to close a Solid State Switch every time a pulse occurs. The
switch grounds terminal D of the Antilock Brake Module 128,000 times a mile, giving an accurate speed signal.
The signal from the Ratio Adapter is also used to close switches to the Cruise Control Module and the ECM. Before the signal is sent to the Cruise
Control Switch, it is divided by 32, creating 4000 pulses per mile. From here it also goes to the Speedometer and Odometer. After being further divided
by 2, creating 2000 pulses - per mile, it is used to close a switch grounding terminal A1O of the ECM.
The Output Switches in the Instrument Cluster are Solid State Switches, not mechanical ones. Self-powered test lights or ohmmeters should not be used
to test them. Do not measure the resistance at the Outputs of the Instrument Cluster.
