Magazine for the auto service professional
Issue link: http://asp.epubxp.com/i/392413
36 | September/October 2014 same numbers that were being refected on the instrument cluster. A call was made to the dealer to see if they stocked a VSS. When they said that they had one the technician fgured, "If the dealer stocks it, they go bad all the time." The new VSS was plugged in, the problem did not occur, and the vehicle was consid- ered fxed. One day later, the customer went on a 160-mile drive and reported that it was "worse than before." The Lucerne was back and the frst thing the technician tested for was to see if the VSS was defective. However, the sensor itself tested at 1.926 kohms, which is within specifcations. Granted, checking resistance on an intermittently bad sensor does not prove anything, so it appeared wise to test out the circuit to see if we could narrow down things outside of pre- suming a sensor from the dealer was bad. Using a wiring diagram, it was discerned that the transmission control module (TCM) was a separate module on this vehicle and that via serial data, it communicated VSS data to the powertrain control module (PCM) and instrument cluster. So, the best place to test frst was the wiring to the TCM. The diagram showed that a purple wire was the VSS low signal and that the yellow/ grey wire was the circuit high (see Figure 2). The wires tested out fne, with a resistance of less than an ohm each, generally settling at 0.4 ohms (see Figures 3, 4 and 5). After setting up the vehicle to be test driven with a labscope, the labscope did not pick up any changes in the VSS signal when the instrument cluster was acting up. Even though the vehicle was a 2011, the only logical issue could be with the TCM giving Connections Figures 3, 4 and 5: After removing the air box, the transmission control module can be exposed for testing. When testing with a labscope, T-pins can be carefully backprobed into the back of the harness to the same wires that were used during the resistance testing. Then, the labscope's wires could be run up from under the hood, through the driver's side window, to a PC-based labscope. Figure 6: This is a simple way to do a soft ECU reset. Simply remove the battery cables, place a jumper cable between them, and walk away for 30 minutes. Just don't forget any ra- dio codes that may be needed later and make sure the idle and steering angle relearns will not be a problem beforehand.