With 4L60 Automatic Transmission

Circuit Description
The ECM applies and monitors 12 volts on CKT 437. CKT 437 connects to the vehicle speed sensor (VSS), which alternately grounds CKT 437 when drive wheels are turning. This pulsing action takes place about 2000 times per mile and the ECM will calculate vehicle speed based on the time between "pulses."

A "Scan" tool reading should closely match the speedometer reading with drive wheels turning.

Test Description
Number(s) below refer to circled number(s) on the diagnostic chart.
Code 24 will set if:
^ CKT 437 voltage is constant.
^ Engine speed is more than 2000 rpm.
^ Vehicle speed signal (voltage on terminal "A9" is less than 5 mph (8 km/h).
^ All conditions must be met for 40 seconds.
1. This test monitors the ECM voltage on CKT 437. With the wheels turning, the pulsing action will result in a varying voltage. The variation will be greater at low wheel speeds to an average of 4-6 volts at about 20 mph (32 km/h).
2. A voltage of less than 1 volt at the ECM connector indicates that the CKT 437 wire is shorted to ground. Disconnect CKT 437 at the vehicle speed sensor. If voltage now reads above 10 volts, the vehicle speed sensor is faulty. If voltage remains less than 10 volts, then CKT 437 wire is grounded. If 437 is not grounded, check for a faulty ECM connector or ECM.
3. A steady 8-12 volts at the ECM connector indicates CKT 437 is open or a faulty vehicle speed sensor.
4. This is normal voltage which indicates a possible intermittent condition.

Diagnostic Aids
1. "Scan" reading should closely match the speedometer reading, with drive wheels turning.