DTC 79

Vehicle Speed Sensor Circuits:

CIRCUIT DESCRIPTION
Vehicle speed information is provided to the ECM by the vehicle speed sensor which uses a Permanent Magnet (PM) generator and is mounted in the transaxle. The PM generator produces a pulsing voltage whenever vehicle speed is over about 3 mph. The AC voltage level and the number of pulses increases with vehicle speed. The ECM then converts the pulsing voltage to mph which is used for calculations and the mph can be displayed with a scan tool. Output of the generator can also be seen by using a digital voltmeter on the AC scale while rotating the generator.

The function of VSS buffer used in past model years has been incorporated into the ECM. The ECM then supplies the necessary signal to the instrument panel (4000 pulses per mile) for operating the speedometer and the odometer. If the vehicle is equipped with cruise control, the ECM also provides a signal (2000 pulses per mile) to the cruise control module.

To prevent misdiagnosis, disregard a DTC 79 set when drive wheels are not turning.

TEST DESCRIPTION
If DTC 79 is current, CKTs 400 or 401 are shorted to voltage. If DTC 79 is history, than DTC 79 should be cleared and vehicle road tested. If DTC 79 resets, than CKTs 400 AND 401 should be checked for intermittent short to voltage.

The ECM is programmed with a VSS (high) diagnostic check. The diagnostic check will be enabled when the throttle position is greater than 31 percent and the vehicle speed is greater than 10 mph. The diagnostic checks for discrepancy in vehicle speed.


DIAGNOSTIC AIDS:

^ Tech 1 scan tool should indicate a vehicle speed whenever the drive wheels are turning greater than 3 mph.

^ A problem in CKT 389 will not affect the VSS input or the readings on a Tech 1 (or equivalent) scan tool.

^ Check CKT 400 and CKT 401 for proper connections to be sure they are clean and tight and the harness is routed correctly. Refer to "Intermittents" in "Symptoms".

^ (A/T) A faulty or misadjusted Park/Neutral Position (PNP) switch can result in a false DTC 79. Use a Tech 1 (or equivalent) scan tool and check for proper signal while in overdrive (4T60E-T4). Refer to CHART C-1A for PNP switch diagnosis check.Chart C-1A Park/Neutral Switch Diagnosis