S5 NA ECU question on pin 1J.
 

On pin 1J of ECU ignition coil(trailing), IGs-T(select signal). The FSM shows approx 4.4V ignition switch on, approx 2.2V at idle.
Is it normal at idle for the voltage to rapidly swing between 1.7V to 2.4V? When back probing all my other values were steady.
I thought maybe since this was the select signal that the bounce was normal, but you think they would mention it in the remarks if it was.

First of all, a digital multimeter is too slow to accurately measure DC voltage swings (also known as alternating current) in real time, but can only do rms voltage.

The select wire determines which trailing coil should fire. If I remember correctly, it's a square wave signal on this wire with the low voltage portion signaling one coil and high voltage signaling the other. I bet if you measured this wire with you multimeter set to AC the rms voltage would be around 2.2v.

Thank you for the thought, I will give it a check when it is not raining. There is a comments section to the side of every entry in the FSM, and all other measurements are done DC, looks like they would mention to check this as AC.

the way the trailing FC ignition works is thus.

there is one called the IGT-L which fires every time the trailing ignition fires. there is the IGT-s, which is the select, when this is on the ignitor switches from coil 1 to coil 2. so this will go in half time to the IGT-L. then there is a confirmation signal back to the ecu, so it knows the ignition has fired.

I checked the voltage again and discovered a funny thing. At idle at 700-800 rpm, the voltage DC does the bouncing, measured AC it is 2.1V. Now if we bump the RPM up to 900-1000 rpm, guess what? The voltage DC measures 2.1 V also. So on my car 100 to 200 rpm at idle makes the signal bouncing or steady on DC with my multimeter. Anyway, looks like my ECU is checking out.