Do not sweat the 0.5 vs. 0.05 volts for this problem. It is too low; that is all that is important.
I probably did not explain the MAP sensor properly as the back-up strategy. On a lot of cars, the engine will not start or run with a failed MAF sensor. On some, the Engine Computer uses MAP readings as a back-up strategy to allow the engine to run well enough so you don't need a tow truck. The engine is not going to run without issues, but it will run.
You have a much better handle on this than most people would, but I would like to point out one thing you should keep in mind for the future. A digital voltmeter has an extremely high internal resistance, (impedance). For all practical purposes, no current flows through it for it to work. When you apply, or have a voltage on one end of a wire, then measure it on the other end, you aren't really testing that wire. Suppose there is a splice buried in the harness that is heavily-corroded and is introducing 1 meg ohms of resistance. The voltmeter will still see the full voltage at the other end of the wire, but you'd ever get enough current to flow through that resistance to do any usable work. You need to cause current to flow through the wire so the results of that resistance would show up. With current flow, voltage will be dropped across that undesirable resistance. In this sad story, that resistance is real high, so the voltage dropped across it will also be real high, as in close to 12 volts. That would leave you near 0 volts measured at the end of the wire. I should make that 3 volts since that's what you were using. 12 volts is what is normally used when talking about car circuits.
Think of a pressure gauge on a compressed air line. No air flows through the gauge for it to work. Suppose you have a hundred pounds of pressure at the start of a long pipe run. Halfway down that line the pipe is 99 percent restricted. You'd still read 100 pounds at the end of the line, until you tried to have some flow to run an air tool. Then the pressure would drop to almost 0 pounds. The only way to test for that restriction is to measure the pressure at the end of the line while trying to run an air tool.
Similarly, the only way to check a wire is to cause current to flow through it. One easy way to do that is to use a test light instead of a voltmeter. Test lights work by the current flowing through them, and you won't get enough current if there is resistance in the wire.
I got confused with the chain of events as to the various voltage readings, but I am getting the feeling there is a high resistance in a wire based on your two extreme voltages. One is maximum low and one is maximum high, and there is no in between. That is the kind of thing we'd see when measuring a wire that is open circuit at the end vs. One we are trying to draw current through an unwanted resistance. I wish I could be there to look over your shoulder. Instead of using a wimpy 3 volts, either apply 12 volts and use a test light, even a head light bulb, or measure the wire's resistance with an ohm meter.
Wednesday, October 26th, 2016 AT 10:02 PM