Lorenzo Ortiz from Phillips Transmission had a 2005 Acura TSX with the MCTA five-speed transmission come into his shop for repairs. After it was repaired and installed into the vehicle, it had a new problem. The car would not start. The immobilizer lamp was flashing and the scan tool could not communicate with the engine control module, yet all other modules would communicate.

Since this was not an original problem the transmission installation work was inspected. The ECM/PCM power and ground circuits checked good. But upon inspection of the two 5-volt supply speed sensor circuits, the 5-volt supply circuit for the countershaft speed sensor (yellow wire with a blue tracer) was found to be shorted to ground (see wiring diagram in Figure 1). The other 5-volt supply circuit checked good.

The actual cause occurred when the transmission was removed. The R&R technician did not disconnect the countershaft speed sensor; instead, it was removed from the transmission and left it connected to the vehicle wire harness. When the transmission was installed back into the vehicle, the R&R technician did not realize that there is a 0.080” thick spacer that goes between the speed sensor and the transmission case (Figure 2), so it was installed. When an attempt was made to start the vehicle, the speed sensor was instantly damaged by the counter shaft gear (Figure 3). This damage shorted the 5-volt supply circuit for the countershaft speed sensor. It also caused the no-start condition and the loss of communication with the ECM. The immobilizer lamp was flashing because it could not communicate with the ECM. When the damaged speed sensor was disconnected, the engine started and ran, the immobilizer lamp stopped flashing and communication with the ECM returned.

Once a new speed sensor and 0.080” spacer was installed, all complaints were gone and the vehicle operated normally. As a side note, there were no communication codes set; however, there was a message in the multiplex module signaling a loss of communication with the ECM.

This incident serves as an example in some ways as to knowing that when a 5-volt sensor supply circuit is shorted, it negatively affects CANBUS communication.