if your convinced your ground is okay, and you're definitely getting 5 volts input, AND 2 different sensors are giving you the same results, then the first thing I'd check is the connector plug! You could have a very corroded terminal or a broken or nearly broken wire inside the plug...
My corvette's right headlight door would open, but wouldn't close. Last weekend while tracing the wiring, I found a spot where the wiring made a sharp bend...it wasn't visible, but one wire had broken inside the insulation at that bend!
I have a couple of jury-rigged jumper wires to test things like this (I also do this with my voltmeter)...just take a decent piece of 12 or 14 gauge wire, about 6 inches long, and about 1/8" from each end, stab a straight pin thru the wire (USE PLIERS!!). Now, stick the pins thru the wire of your choice before and after the connector. Best thing would be to make 3 of these jumpers and test all three wires at once. (just don't let any of the pins touch each other!)
If you start getting correct voltage readings, you'll know the plug is the problem!
If the readings are still wrong, then at least you know it's not the connector!
