1998 379 Peterbilt

Having an issue with charging system, engine is N14 celectplus 525, truck is 1998 Peterbilt 379, alternator puts out 14.2V checked starter cables it reads 14.2V then once you get to the batteries it's all over the place, if nothing is on they will usually read 13.8-13.9v, with all lights on usually drops to 13.6 if you use the AC or heat fan speed 1 will drop around 13.4 and if you kick on sleeper AC or heat around 13.2lowest I've seen is 13v but alternator and starter don't usually drop, the gauge in dash never goes below 13.5\13.6 my inverter up front connected to power outlet and inverter in back wired to batteries usually read below 13.5 with everything on like running at night etc,

Batteries are brand new, alternator year old. This issue didn't start till after I had new ECM installed also after it was installed I noticed the shop had arched one of my positive cables but the line had a two post fuse\relay link in it so I wouldn't it would of effected anything past the fuse, I've checked the link when hooked up and it's still getting power thru it in both sides so I assume it wasn't bad, when I start using lights AC etc when my volt gauge drops a little it causes my other dash gauges to read wrong like water and oil temp will show hotter the lower the volt gauge goes,

I'm using a multi meter to take readings on everything when I've been checking things manually

I'm not sure if these old Pete's had anything between the batteries and starter\ecm or if any fuses or relays would effect this or a battery cut off

Any help is appreciated

 

Yes the volt gauge will drop depending on what's being used, and the lower it goes the higher my water and oil temp gauges will read then it you turn off everything the volt gauge will pop back up to 14+V and the temp gauges will drop back to normal to read the actual temps

I know the volt gauge would probably drop a little the more things are on but wasn't sure how much is normal drop and why it would effect my engine temp gauges

Before the ECM swap the volt gauge wouldn't move much if at all when anything was being used

 

Thank you everyone for all the replys, turned out to be my starter was the issue, I had cleaned frame grounds and almost all the connections I could, this morning after unloading starter took a crap, swapped starters this afternoon and volts\charging system pop up to 13.8-14v and holds constantly no matter what's on or being used.