Usualy only acceptable value is 0. signal isn't true pwming, but some kind of pseudo pwm - irregular and with interrupts.
Any diode should do the job.
May be some capacitor can help to mask rattling?
Settings for IAC speed are between 0 and 15.
0 = ~ 250 Hz
15 = ~50 Hz
A high frequency makes the solenoid lazy, reacts slowly to small changes. Too small frequency makes it oscillate and might hit the end positions (rattle noise).
I use 6 which is about 100 Hz and works very well with dual coil ICV on my BMW E30 (1980s 3-series). It is a bit quieter than the stock frequency which is lower, easily heard with ignition on, engine off. It is a 3 pin deal, with two solenoids working against each other and uses two outputs on the ECU (one inverted) and is probably designed this way to prevent dependancy on battery voltage.
Without diodes to control solenoid flyback voltage you won't get anywhere in adjusting the idle. I use the diodes that come with the rescue kits, 1N4007 which can handle 1A. Not sure if it's the perfect one for the job but a Schottky diode like SR806 that can handle 8A and is faster than the 1N4007.
- Anode to switched point of the solenoid (channel output, EC18-pin..)
- Cathode (marked with a line) to the other pad of the solenoid: the same where you connect the positive +12V (switched and fused) power