With the truck running take the positive lead and put it on the battery positive terminal. Then take the negative lead and put it on the positive feed to the fuse box, or wherever you have been taking your reading. Do this with the meter on volts. You will get a voltage drop reading like this, it will show you how much voltage chose the meter as the path of least resistance vs through the wire like it should go. I would expect about 0.3V to 0.5v. That would be a good reading. Higher than that and you need to look at the positive cable from the battery to the fuse box for high resistance. If it reads normal then you need to look at the ground, disconnect the battery and take a resistance reading from the ground of the fuse box to the ground terminal. That should be like 0.2to 0.4 ohms

 

OBD port voltage reading is never accurate for whatever reason. I have voltage readers (for starter and aux) which are highly accurate close to the battery terminals for monitoring voltages and charging.

PS: I just traced the 12V from the ODB port back to the battery in the schematics. There is a lot of wiring, terminals and two fuses in between as well there are many other consumers on the same voltage rail as the ODB port. I think the voltage drop is not a defect, but by design (wire gauge, contact resistances). Since the consumers are all electronic devices which stabilize the voltage down to 5V and/or other lower voltages internally it isn't an issue. You can't simply measure the battery voltage on the ODB port precisely and it will vary with load (ignition off/on, engine running/not running). The only precise way to measure is at the battery terminals.

 

@Discorama Well in that case. I am going to call it a day my voltage is solid in the engine compartment