Test B12 - VSS

NOTE: Perform this test when Service Code 29 is displayed during QUICK TESTS or when directed here by other test procedures.

Fig 1: Vehicle Speed Sensor Circuit

To prevent replacement of good components, be aware that the following circuit test is to be used only when Continuous Memory Code 29 is displayed. This test checks vehicle speed sensor, wiring harness, and ECA. Before testing the VSS, perform the following drive cycle procedure:

Automatic Transmission Equipped Vehicles. Record and clear CONTINUOUS SELF-TEST codes. Warm engine to normal operating temperature. In Low gear, accelerate moderately to 25 MPH and coast down to idle and stop. Shut off engine. Perform KOEO SELF-TEST. If Code 29 is present, go to step 1). If not, fault cannot be duplicated at this time. If any other codes are present, return to QUICK TEST. If codes are not present, test is completed.

Manual Transmission Equipped Vehicles. Record and clear CONTINUOUS SELF-TEST codes. Warm engine to normal operating temperature. Start in first gear, shift to second gear, and accelerate moderately to 40 MPH. Coast down to idle and stop. Shut engine off. Perform KOEO SELF-TEST. If Code 29 is present, go to step 1). If not, fault cannot be duplicated at this time. If any other codes are present, return to QUICK TEST. If codes are not present, test is completed.

Continuous Memory Code 29. Code 29 indicates there is insufficient input to the ECA from the VSS. Possible causes are: faulty VSS, open or shorted circuit or a faulty ECA. Perform drive cycle procedure. If Code 29 is displayed, go to next step. If not, fault cannot be duplicated at this time.
Checking VSS Sensor. Turn key off and wait 10 seconds. Disconnect vehicle speed sensor. Set DVOM on 200K-ohm scale. Measure resistance across vehicle speed sensor. If reading is between 190-240 ohms, go to next step. If reading is incorrect, replace vehicle speed sensor and repeat drive cycle procedure.
VSS Harness Continuity. Turn key off and wait 10 seconds. Remove ECA 60-pin connector and inspect for damaged pins, corrosion, or loose wires. Repair as necessary. Install breakout box. With ECA and VSS disconnected, set DVOM on 200-ohm scale. Measure resistance between test pin No. 3 at breakout box and VSS harness connector. Also measure resistance between test pin No. 6 at breakout box and VSS harness connector. If both readings are 5 ohms or less, go to next step. If readings are 5 ohms or more, service open circuit in VSS wiring harness. After repairs, repeat step 1).
Checking VSS Harness For Shorts To Power Or Ground. Turn key off. With ECA and VSS disconnected, set DVOM on 200K-ohm scale. Measure resistance between test pin No. 3 and test pins No. 37, 40 and 6 at breakout box. Also measure resistance between test pin No. 6 and test pin No. 37. If all readings are greater than 10K ohms, go to next step. If any reading is less than 10K ohms, repair short(s) in VSS wiring harness. After repairs, repeat step 1).
Test By Substitution. Substitute VSS with known good sensor and repeat drive cycle procedure. Connect ECA and VSS. Repeat step 1), and return to this step. If Code 29 is present, replace ECA and repeat step 1). If no code is present, replace original vehicle speed sensor.