Idle Speed Control Closed Throttle Determination

One of the fundamental criteria for entering RPM control is an indication of closed throttle. Throttle mode is always calculated to the lowest learned throttle position voltage seen since engine start. This lowest learned value is called "ratch", since the software acts like a one-way ratch. The ratch value (voltage) is displayed as TPREL PID. The ratch value is relearned after every engine start. Ratch will learn the lowest steady TP voltage seen after engine starts. In some cases, ratch can learn higher values of TP. The time to learn higher values is significantly longer than the time to learn lower values. The brakes must also be applied to learn the higher values.

All PCM functions are done using this ratch voltage, including idle speed control. PCM goes into closed throttle mode when TP voltage is at ratch (TPREL PID) value. Increase in TP voltage, normally less than 0.05 volt, will put PCM in part throttle mode. Throttle mode can be viewed by looking at TP MODE PID. With throttle closed, PID must read C/T (closed throttle). Slightly corrupt values of ratch can prevent PCM from entering closed throttle mode. An incorrect part throttle indication at idle will prevent entry into closed throttle RPM control, and could result in a high idle. Ratch can be corrupted by a Throttle Position (TP) sensor or circuit that "drops out" or is noisy, or by loose/worn throttle plates that close tight during a decel and spring back at a normal engine vacuum.