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.

I assume this is not a running condition, and therefore not the Alternator. Here is a picture of a fuel injector waveform, you can see when it shuts off there is a large spike in voltage, this happens with any inductive load that creates a large enough magnetic field. Once that magnetic field collapses, it induces a high voltage spike. The next waveform is how many things are controlled electronically in many vehicles. This is a pulse width modulated signal, the difference between the On time and Off time are what determines the duty cycle. So on the vehicle I saw, with the key just On, the PCM/ECM was pulsing a control solenoid for the transmission, I dont even think it was being fed 12v but I dont remember every detail, and since the multimeter can only sample a voltage reading at a certain speed, you get an average voltage reading, hence the reason for using a scope, it is fast enough to capture events that a meter cannot.

With the MAF unplugged still, see if you are getting the same voltage reading right at the PCM connector. Its also possible that voltage signal is being fed threw a corroded connector and coming from something else than the PCM. So you need to keep checking for that same high voltage until you determine where its actually coming from. If this is not a charging system fault, it would be a very interesting case study, due to the low voltage code.
Here are the PCM connector pinouts for C175E which is the PCM connector that has the MAF signal wire, pin 25, Yellow/Violet wire. See what you read there.