Code 24 - VSS Circuit: Notes

The ECM applies and monitors 12 volts on circuit A10. The Vehicle Speed Sensor (VSS) alternately grounds and opens circuit from ECM terminal A10 when vehicle is in motion. This pulsing action takes place about 2000 times per mile. ECM will calculate vehicle speed based on time between pulses. With vehicle in motion, scan tester reading should closely match speedometer reading.

1. Code 24 will set if vehicle speed equals zero MPH when engine speed is greater than 1400 RPM, TPS is less than 5 percent (closed throttle), a low load condition (high vacuum) exists and all conditions are met for 5 seconds. These conditions are met during a load deceleration. Disregard Code 24 that sets when vehicle is not in motion.
2. If 8-12 volts is indicated at instrument panel connector, circuit from ECM terminal A10 is open between instrument panel connector and the VSS, or there is a faulty vehicle speed sensor. Less than one volt at the instrument panel connector indicates circuit from ECM terminal A10 is shorted to ground. After disconnecting circuit at the VSS, if voltage reads greater than 10 volts, VSS is defective. If voltage remains less than 8 volts, check circuit between speed buffer and ECM terminal A10 for short to ground. If circuit is not grounded, check for faulty connection at ECM or faulty ECM.