DTC 72, Vehicle Speed Sensor Signal Loss: Notes

WARNING: This page does not describe the selected car, but rather 20 other vehicles, including the 1993 GMC Yukon, 1993 GMC Vandura Special, 1993 GMC Vandura, 1993 GMC Suburban, and 1993 GMC Rally Camper Special. However, it is still accessible from the selected car via links, so may be relevant.

The speed sensor circuit consists of a magnetic-induction sensor, Vehicle Speed Sensor (VSS) buffer, and wiring. Gear teeth on output shaft induce an alternating current in sensor. This signal is then transmitted to buffer. The buffer compensates for various axle ratios and converts signal into a square wave signal for use by the speedometer, cruise control, ABS and ECM.

The buffer sends 2 different signals to the ECM. Circuit No. 437 relays transmission output speed, which is used to control shift points, line pressure torque converter clutch and DTC 24 and 72. Circuit No. 1716 relays vehicle speed which is used to control engine operating conditions and DTC 16.

On 4WD models, the ECM uses both the transfer case output speed signal and the 4WD low switch to determine transmission output shaft speed. The ECM uses transmission output shaft speed for shift points, line pressure control, torque converter clutch control and DTC 24 and 72.

DTC 72 will set if either of the following conditions is met for 2 seconds:

Transmission output speed changes more than 1000 RPM (transmission not in Park or Neutral).
Transmission output speed changes more than 2050 RPM (transmission in Park or Neutral).
NOTE: Test numbers refer to numbers on diagnostic chart.

This tests vehicle speed sensor signal to ECM. Use Digital Volt-Ohmmeter (DVOM) on 20-volt DC scale.
This tests vehicle speed sensor signal to vehicle speed sensor buffer.
This tests vehicle speed sensor signal.