MAF signal wire reading at 18.7 v?

Hello, just to be clear, you are reading 18.7volts on the MAF Signal wire with the sensor unplugged and Key On Engine Off? And this is using battery Negative as your meter Ground? So the 18.7volts is not coming from the MAF sensor if it is unplugged. I have seen something cause high voltage like this one time before. And will explain but its not the easiest. (I will post the wiring diagrams for it as well). And this 18.7volts is a repeatable fault you can see?.. I would check other circuits without cutting the wires to test, use back probing pins, and be easy on the connectors.

https://www.2carpros.com/articles/how-to-use-a-voltmeter/

Here is the Ford oem wiring diagrams because it looks like the aftermarket diagram might have an error on it. First thing, is the charging system at normal voltage?

If it is then I wouldnt be surprised if you find this 18volts on other circuits, but the one time I have seen voltage that was reading over 19volts, was a very rare situation where the PCM was pulsing one of the transmission solenoids off and on very fast (under normal operation this would be called Pulse Width Modulated) meaning the PCM pulses a solenoid and varies on "On" to "Off" time to control how open the solenoid is.

An example is a fuel injector, when a fuel injector shuts off, there is a voltage spike. Its usually about 60-80volts or so, and this will be seen with a scope on the control wire. So if this is really 18volts and is not a faulty multimeter, which I would try another one anyway, start checking other circuits for the same high voltage reading like this, if the MAF is unplugged, its not coming from the sensor. And the P0102 is a MAF reading low code.
Something to note, the vehicle I saw with this high voltage was only with the key On, so it wasnt a charging system fault. Very strange though and have never seen it again.

Thank you..
I will look into your explanation and understand it.

I might have some more questions omce I proceed.

I appreciate you.