DTC 24

Code 24 Chart:

Code 24 Wiring Diagram:

CIRCUIT DESCRIPTION:

The Vehicle Speed Sensor consist of a permanent magnent generator, buffer, speedometer, and ECM. The PM generator is a permanent magnent generator attached to the transmission. As the vehicle moves, the generator creates a sine wave electrical pulse, which is routed to the buffer. In the buffer the signal is changed from a sign wave to a square wave and amplified. The square wave is an "ON/OFF" signal. The length of time between pulses determines vehicle speed. The ECM sends a 12 volt signal out on circuit 437. The frequency by which the signal is pulsed low is used by the ECM to determine vehicle speed.

NOTE: Because of all the possible color code combinations used on electrical wiring diagrams, always refer to ECM CONNECTOR IDENTIFICATION under ELECTRICAL AND ELECTRONIC WIRING DIAGRAMS for correct color code identification of circuit.

TEST DESCRIPTION: Numbers below refer to circled numbers on the diagnostic chart.

1. Code 24 will set if:

^ TPS reading shows closed throttle.
^ Engine speed between 1100 and 4000 rpm.
^ Low load condition (low air flow).
^ Not in park or neutral.
^ All conditions must be met for 20 seconds.
^ These conditions are met during a road load deceleration.

2. This step checks to see if the fault is in CKT 437, including the ECM or the VSS/Buffer circuit. The ECM is the source of 12 volts via CKT 437 to the buffer in a normal working system.

NOTE: Disregard a code 24 that sets when the drive wheels are not turning.


DIAGNOSTIC AIDS:

^ An intermittent may be caused by a poor connection, rubbed through wire insulation, or a wire broken inside the insulation.

Check For:

^ Poor connection or damaged harness. Inspect ECM harness connectors for backed out terminal "A10" for improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.

^ Intermittent test. If connections and harness check OK, raise drive wheels, block other wheels, idle engine in low gear and "Scan vehicle speed, monitor while moving related connectors and wiring harness. If the failure is induced, the "Vehicle Speed" display will change. This may help to isolate the location of the malfunction.

^ If "Scan" displays vehicle speed, check park/neutral switch CHART C-lA on vehicle with auto trans. If switch is OK check for intermittent connections. An open or short to ground in CKT 437 will result in a Code 24. If the customer also complained about a loss of mph on the Instrument Panel (IP), check the Permanent Magnet (PM) generator circuit. Refer to TESTING PROCEDURES/DIAGNOSIS BY SYMPTOM/INTERMITTENTS.