07 9900i voltage issue

Have you checked with a voltmeter it's actually dropping?
My first guess would be slipping belt. Could be voltage regulator or wire to the batteries. Just because a wire looks good on the outside,doesn't mean it's 100%.

 

Ok, this can be tricky. Chances are there is nothing wrong with the batteries, belts, alternator or main cables. The volt gauge in the dash is actually data feed from the engine ECM. The voltage the gauge shows is the same voltage the ECM is getting. Under high engine loads the ecm is commanding a higher pulse width from the electronic injectors which uses more current, that is why the volt gauge is dropping off under a heavy pull. Basically the ECM is starting to starve for power. The volts should not drop off like that. This is an indication of a bad or weak main ECM power supply. Your ECM will have its own dedicated power harness from the batteries. Follow this harness from the batteries to the engine, check all your connections, pins and fuses and pay close attention to any spots where the wires may be rubbed through and partially corroded. Make sure the actual battery it is connected to is properly connected to the other batteries and getting properly charged.

 

If you want to go a bit further, the truck harness connector at the ecm is where the power and grounds go in. Ultimately this is best place to check for proper power and grounds. The wiring should be "load" tested. This is using the circuit to power something that takes a lot of current, like a head light bulb. If you have a bad wire it will still show full battery voltage with a digital multimeter or basic 12v test light, because those do not take much amperage to work or read. A head light bulb takes a lot of amperage to be bright and so if the wiring is weak or bad somewhere it cannot supply the head light with the current it needs to be bright, so it will be dim instead. You would remove the ECM connector. The main battery positive terminals are 48, 52 and 53. The main grounds are 63, 65 and 67. If you connect a wire from one of the positive pins, to a head light, then from the head light back to one of the ground pins, the head light should be nice and bright. All powers and grounds should be checked. It is common for one main power lead or one main ground lead to split and power or ground multiple terminals. The splice connections can go bad and cause one of the power or ground leads to have high resistance. Key switch power comes in on pin 70 of the same connector.

 

wconner, I have the same truck/engine set up as you and I've been having the same issues since I bought the truck 4yrs ago. I agree with HeavyD, it's probably something in the feed for the gauge.

On my truck the gauge will show 13-14 volts under normal load, at no load it will jump to 14-14.5 volts and under a heavy load it will drop down to 12volts then jump back up. I hooked a volt meter up to the 12v feed on the dash and it reads a constant 13.3-13.8(depending if I have the lights on, fan on high,etc)even when the gauge is moving around.

Like you I replaced everything and even went as far as replacing the power board for the bank of gauges the volt gauge is on and nothing has made a difference. I got tired of messing with it and have just given up on it and live with the gauge moving around.