The truck hasn't been driven for 10 days and I checked the batteries which are reading 12.51-V. Anyhow, turned key on let it go through the wait to start etc and turned the key. It grunted once when the starter engaged and then everything shutdown. No dash lights, no lift pump no overhead console, no headlights etc.
Whenever you have an electrical problem such as this - DON"T mess with the electrical connections right away. Why? Because poor electrical connections can and will change their behavior, especially if twisted, tugged, pushed or pulled upon, or even attempted to clean. Just because a connection is tight and appears to be clean does not mean that it has good electrical continuity.
DO use your multimeter to perform a form of a voltage drop test. This will give you immediate and accurate information.
In your case, I would have a second person hold your multimeter (set on DC volts, appropriate scale) with one lead on the positive battery post and the other lead on the negative battery post of the driver side battery. The leads must be on the posts - NOT the wires clamped to it.
Attempt to crank the engine. If your symptoms are the same and the battery voltage remains the same (within a tenth of a volt or so of static voltage), then the batteries are not the issue. This strongly suggests that you have a poor connection somewhere in the circuit.
If your symptoms are the same and the battery voltage drops dramatically, then the batteries are very discharged or need to be replaced.
The multimeter will not lie.
I think
@DavidC was trying to get you to do something similar to this.
You have the best resource on your truck to perform a battery load test. Once your problem is repaired you can perform a voltage drop test using your starter for the electrical load. The starter draws approximately 400 amps while cranking.
* Remove the fuel injection pump relay (stops the engine from starting)
* Hook up multimeter to the driver side battery in the same manner as before.
* Crank over the engine for 15 seconds (the voltage should stay above 10.2 volts for the duration)
- John