Vehicle Speed Sensor (VSS)

On Grand Cherokee, VSS signal is sent through Controller Anti-Lock Brake (CAB) module by rear wheel speed sensor located on rear axle. On Cherokee and Wrangler, VSS is located on extension housing of transmission or transfer case. PCM uses VSS to determine vehicle speed and distance traveled.

VSS is an 8-pole switch which provides PCM with a pulse or switching rate, proportional to vehicle speed. By comparing number of pulses to time elapsed, PCM determines vehicle speed and distance traveled. VSS generates 8 pulses per sensor revolution. This signal, along with a closed throttle signal from TP sensor, indicates a closed throttle deceleration to PCM.

Under deceleration conditions, PCM adjusts IAC motor to maintain desired MAP value. During idle (vehicle stopped), PCM receives no signal from VSS. PCM adjusts IAC motor to maintain a desired engine speed.