I also ordered some fancy meter probing accessories since my Fischer Price multimeter didn't come with anything useful.
Someone buy this man a proper multimeter!
Where we're at currently & what we know for sure: The transfer case position switch gets actuated when the T-case is shifted into low gear, which then connects one of the pins of the skid control ECU to (effectively) battery voltage. That's why you should get battery voltage (10-14v) when you measure voltage between that pin (with the ignition ON) and body ground with the T-case in low gear, per the FSM. When the T-case is in high gear, that switch is left 'open', so it's left unconnected (though, probably with an internal pull-down resistor I would assume). That means if you do a resistance check on the switch pins, when the switch is not pressed you should get infinite resistance, and when it's depressed the resistance should drop to (effectively) zero. I've confirmed that behavior with a functioning switch out a spare T-case that I have.
What's a little confusing to me is that if
@MongooseGA clears that code in Techstream, and then leaves the T-case in high, the computer shouldn't be able to tell that there's a faulty switch - so it shouldn't throw a code. Therefor, it would have no reason to lock out 5th gear (we're still working under the
assumption that it locks out 5th if that code is present).
He's not sure if he cleared that code when he had access to Techstream or not though, so I'm hoping a new switch and clearing of the codes will let the transmission function normally. If he wasn't more computer-adverse than my grandparents, he could test this pretty quick - but alas.
Side note: My guess the ECM or skid control computer can probably detect that there's an issue with this switch by doing a 'sanity check' by comparing engine RPMs, transmission gear, and wheel sensor speeds. It can probably guestimate whether or not it's in low gear by doing this.
If it's not getting a good 'hey, I'm in low gear!' signal from the transfer case position sensor, but yet the sanity check tells it that it is (high engine RPMs, high gear in the transmission, but low wheel speed sensor readings), it probably throws this code. That's why I wouldn't think that staying in high gear with a bad sensor would cause the code to pop - the sanity check would come back fine.
That's me pulling things out of thin air though, so take that with a healthy grain of salt.