DTC 24

CIRCUIT DESCRIPTION
The ECM applies and monitors 12 volts on CKT 437, CKT 437 connects to the DRAC, which alternately grounds CKT 437, when receiving voltage pulses from Vehicle Speed Sensor (VSS) 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.
1. Code 24 will set if:
^ CKT 437 voltage is constant.
^ engine speed is more than 1200 rpm.
^ Vehicle speed signal indicates less than 2 mph (3 km/h) on Tech 1.
^ Not in park or neutral.
^ All conditions must be met for 5 seconds.
These conditions are met during a road load deceleration.
2. This test determines if the DRAC is receiving the A/C signal from the VSS.
3. This test monitors the DRAC 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).

DIAGNOSTIC AIDS
1. "Scan" reading should closely match speedometer reading, with drive wheels turning.
2. Check Park/Neutral (P/N) switch diagnosis chart.
3. If Park/Neutral (P/N) switch is OK, refer to "Intermittents" in "Symptoms." - Intermittent Malfunctions