PWM Closed Loop

In closed loop mode, the ECU will use a PID algorithm to attain the target RPM looked from the CL Target RPM table (based on coolant temperature). It will also compensate for additional loads at idle (such as AC/lights) to try and maintain the idle. PID settings P, I and D, as well as the update interval determine how this function operates. For further information on PID tuning (which other areas of the ECU such as closed loop boost control use) please see the relevant section in the appendix. DROs such as target RPM, target Error and the P,I and D terms can be added to the current tab to aid its tuning.


The Closed Loop algorithm also features the ability to ‘flair’ the engine speed on starting, as well as smooth the engines ‘return’ to idle after driving. To do this you must first calibrate your OPEN LOOP duty table such that a steady, desired RPM is held for all coolant temperatures. Typically you would aim for 1,200rpm when cold and around 950 RPM when warm, though this of course varies with engines. Do this by setting the idle control to ‘Open Loop’ mode and tune the OPEN LOOP DUTY table to achieve these results.


The starting strategy in closed loop mode will also reference the Open Loop Duty table, so it is important it is set correct, as when in closed loop mode, and the engine is below cranking max RPM, the duty of the idle valve will be set to the Open Loops table setting, plus an addition of duty % defined by the ‘CL Start Inc(%)’. This increase causes the engines RPM to be higher on initial start.


Once the engine catches, the duty (and thus the engines RPM) will be held at this higher level for a time in seconds set by the ‘CL Start Delay(s)’ field. The duty will then taper to closed loop mode - this stops the RPM ‘dipping’ low once the engine has started. Typically you would add around 10-15% duty, and hold it for 5-6 seconds.


The other additional settings for the closed loop idle are under the ‘CL - RTI’ (Return-to-Idle). This dictates what the idle duty should be (and then again, held at) once returning from a driving state (ie higher RPM/higher TPS etc) such that the idle catches correctly. Typically a small addition of duty (3-5%) and a smaller hold time (1-3 seconds) will usually suffice to have the engine return to a steady closed loop idle from any running condition.


An example process of tuning PWM Closed loop is here.