DTC 24, Vehicle Speed Signal Low (With 4L60-E A/T): Notes

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. When DTC 24 sets, only 2nd gear at maximum line pressure will occur. DTC 24 sets when the following conditions are met for 6.4 seconds:

Transmission is not in Park or Neutral.
Circuit No. 437 voltage is constant.
Engine speed is more than 3000 RPM.
Output speed less than 250 RPM.
Manifold absolute pressure 14.5-36.9 psi (100-255 kPa).
Throttle position 10-100 percent.
NOTE: Test numbers refer to numbers on diagnostic chart.

This test monitors voltage on circuit No. 437. With wheels turning, the pulsating action causes a varying voltage. Voltage variation will be greater at low wheel speeds and an average of 4-6 volts at about 20 MPH.
Less than one volt at ECM connector indicates circuit No. 437 is shorted to ground. Disconnect circuit No. 437 at VSS buffer. If voltage now reads more than 10 volts, the VSS buffer is faulty. If voltage remains less than 10 volts, then circuit No. 437 is grounded. If circuit No. 437 is not grounded, check for a faulty ECM connector or ECM.
A steady 8-12 volt reading at the ECM connector indicates that circuit No. 437 is open or VSS buffer is faulty.