Because you don't have the negative battery hooked up, some of the circuits/accessories that are on the constant (non-keyed) power side of the truck WILL conduct B+ voltage all the way through.

Since the volt meter draws VERY LITTLE current, you WILL see battery voltage regardless while doing a VOLTAGE DROP TEST (which is what you're doing; even if not on purpose) because the positive voltage doesn't just say, "Gee, I guess I'd be trespassing on Negative's property if I actually went through this circuit since he's not here.....".

Seriously, if your ASE mechanic buddies don't know anything about voltage drop tests (or theory), they better stick to tire rotations.


Edit:

I'm not trying to come across as a jerk, but why in the world are you chasing something that is not a problem?

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