Clean Tachometer Output (CTO)
WARNING: This page is about a different car, the 2007 Mercury Mariner and 2007 Ford Escape. However, it is still accessible from the selected car via links, so may be relevant.
The PCM uses a crankshaft position (CKP) sensor input to calculate the engine speed. The engine speed information is then output to the transaxle control module (TCM) through the CTO circuit, as a frequency signal. The PCM also broadcasts a redundant engine speed message to the TCM over the controller area network (CAN) communication link. When the broadcasted engine speed disagrees with the hardwired CTO signal, or when the CTO circuit concern condition is detected, the TCM stores an appropriate diagnostic trouble code (DTC).