The proper monitoring technique for idle control is to measure the current flow in milliamps. A proper system running around 650rpm will have a current flow around 600ma.
A lower current 500ma say will result in a lower rpm reading. If you add A/C it will jump the current 50ma.
One common problem is the Overvoltage Protection Relay. If this fails totally the idle current goes to zero and the speed is usually high when warm. If the relay connections fail to carry the load the current drops and the idle valve goes slow. Seems strange that zero current will give a higher rpm than a low current reading will.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician