I've tried looking everywhere else but nobody has an answer for me. Whenever I start my truck, the voltage gauge goes up to a hair above 14, then will drop down to maybe about 12 and the lights will dim and the idle will change. then it will go back up to 14 and back to normal, then back down. It repeats this for a couple minutes until I actually start driving, then it's normal until I start the truck again. I tried changing the alternator thinking it was the regulator, but the problem still occured. If anyone has any sort of solution I would greatly appreciate it.

 

You replaced a perfectly good alternator. The lights dimming and voltage fluctuations are caused by the grid heaters cycling on and off. They stop when you reach a certain speed (I think it is around 17 mph) or the coolant temp reaches a certain temp (I can't remember the exact temp off the top of my head).

 

perfectly normal behavior. grid heaters (intake air heater directly under the intake horn) cycling on and off will drop your power to 12v putting a good 160-180A draw on the system. there is a reason your truck has dual batteries, and that reason is not to start the truck...its because of the grid heater. it puts such a massive electrical draw, it needs dual batteries to handle the amperage draw and still keep the ECM running @ 12v.

the ECM cycles the heaters on and off so it doesn't heat up the wires/alt/batteries too much. there are actually temp sensors below each battery build into the battery holders.

The heaters will run any time you start the truck when the temp is below 60*F outside.

they will cycle on and off for something like 5 minutes, or until you reach 20mph. whichever comes first.

the cummins engine has a grid heater instead of glow plugs to aid in cold starts by heating the intake air.

like said above...you replaced a perfectly good alternator.

btw they won't shut off based on coolant temp.

your truck can have 195* coolant in it and have been parked for 5 minutes shut off, and as soon as you start it again if it is still below 60* outside they will start cycling again... its on a timer but can be canceled if the vehicle reaches 20mph+

 

Not only that, but the voltage regulator is built into the ECM, not the alternator.

I suppose I could be wrong, but I was under the impression that the battery temp sensor was only under the driver's side battery - which is one of the reasons why they recommend replacing both batteries at the same time. I didn't so much as look at my passenger's side battery when I swapped them out.