11.8 volts at idle unhook battery goes to 14.5

Check the crossover positive cable. They go bad right where the cable goes into the battery lug at the drivers battery.

The ECM controls voltage output of the alternator based on what'ss read on that lug. Bad cable equals more voltage output from the alternator.

The passenger battery is overcharged ready to blow during this.

Ok well here's the best part is the passenger side battery did swell and blow caps off

Yep drivers battery lug under the insulation. Cut it back.

OK something was acting up yesterday morning. My gauge cluster works but doesn't light up and no dome light. I found that it could be the gauge cluster in the back. It blows fuse as soon as you put one in. Even tried a 20 for curiosity.

Sounds like a short if the crossover cable is good. Check voltages with a meter before doing anything. The gage on the cluster can be unreliable. Does this issue exist when the key is removed?

There are 3 failures in automotive wiring... Open, Short, & Grounds... Short is the term for two hot wires touching... and open is a wire that is broken inside.... and a ground is a wire going to ground...

Excessive resistance from corrosion, when the gases from the battery have backed up into the cable will increase the resistance... once this resistance increases your running on one battery... this is also what happens when you have a bad ground...

I agree either you have the cross over cable damaged.. or have some loose grounds...

Use a good digital volt meter and use a process called voltage drop where the voltmeter is used on either the positive or negative side of the connections.... you'll find it that way.. This has to be connections or cables...

Research "voltage drop" across a circuit on google. Perform a voltage drop test, that should expose the problem in your charging system. Good luck, and please do share what you find.

Well guys I do believe y'all helped me figure it out. I took terminals off cleaned them again nothing. So I then unhooked the driverside battery and it would just click so I tried starting it with just driverside and it would switched batteries and just clicked so I knew it was crossover cable. Took the bolts out of terminals and cleaned the ends of the crossover cable stuck it back together and would start with just passenger side hooked up. Right now its doing fine will drop down to 12ish about 4 times after start up but then stay around 15... I guess I need to make a longer test drive to make sure.

The "12ish about 4 times" would be your grid heater kicking in.

good to see you fixed it, BUT note unhooking the battery cable ends while the truck is running is a bad idea on many fronts. resulting arc combined with gasses vented from battery could result in exploding battery, voltage surge from hooking/removing likely to damage ECM/PCM or alternator.

this used to be backyard old school method to check to see if the alternator was working....definitely not safe practice