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Fault Code 15 - No Vehicle speed sensor signal
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Code 15 relates to a failure in the vehicle speed sensor circuit. The job of the vehicle speed sensor is to inform the computer about the speed of the vehicle so the computer can calculate engine load and prepare the engine idle speed control system while the vehicle is decelerating. On deceleration, the idle speed control system will allow additional air flow into the engine as the vehicle rolls to a stop. Usually code 15 will be accompanied by erratic speedometer readings (or no reading), erratic AT gear shifting, TCC lockup clutch instability, and intermittent or complete failure of the cruise control.
The VSS is a permanent magner variable reluctance sensor that is mounted to the transmission or transfer case tail shaft. The sensor output is a series of pulses, the frequency of which are proportional to vehicle speed.
The most likely causes of VSS problems are:
Remove the connector from the sensor and clean the contacts on the sensor and connector. Remove the sensor assembly from the transmission and check the drive gear. Reinstall the sensor (index it correctly) and reconnect the wiring. Disconnect the battery for 15 seconds to reset the fault, then drive for a while and see if the problem reappears. If it does not, you have fixed the problem.
To check the vehicle speed sensor, disconnect the electrical connector to the sensor. Using a voltmeter, check for reference voltage to the sensor. Connect the positive probe (+) to the orange wire terminal and the negative probe (-) to the black/light blue wire terminal. The reference voltage should be approximately 8.0 volts. Next, check the signal voltage from the PCM. Connect the positive probe (+) to the white/orange wire terminal and the negative probe (-) to the black/light blue wire terminal. The signal voltage should be approximately 5 volts. If there is no reference or signal voltage available, or the ground is missing, trace each back to the PCM and check the voltage there (8.0V is terminal 7, signal is pin 47, and ground is pin 4). Repair any wire or the connector if a voltage is present at the PCM but not at the connector. If the voltages are not present at the PCM, remove and reconnect the PCM connector and recheck - it is possible that the PCM connector has some corrosion on the terminals. If connector removal and replacement did not correct the problem, the PCM is probably bad - have the PCM diagnosed with a scan tool.
The best way to test the vehicle speed sensor circuit is with a scanner while driving the truck. If the scanner detects a signal from the vehicle speed sensor, it will display a reading other than zero and the speed signal should vary smoothly with the vehicle speed. If you have access to a scanner or oscilloscope, typical waveforms for good and bad sensors are shown below.
Last Update: August 8, 2001