Hello all, driving my 2001 2500 5.9 magnum, up the highway today And my check gages light was on and my voltmeter was rock bottom on 8. Voltage system to low is the diagnostic code so where should I start?

Also, I did my plugs, wires, cap and rotor. Oil, oil filter, fuel filter and I swear my truck is slightly more gutless in 3rd and sort of mid range. I have a newly rebuilt tranny, less than 12,000km on it. Have had a few mechanics check it out and they think my engine sounds strong and tuned, but I just feel and hear slightly more reving and sluggish in mid range. My gut is telling me that my tranny is not strong enough for my truck or bands need tightening?? I have 4:10 gears. Dana 60 fr. 80 in back and 37" mud terrains.
I forgot to stick a magnet down my throttle body to see if my plenum had been done. I have 213,000km on it.

Any help would be awesome guys.

 

Need to solve your voltage issues before you worry about performance issues. If voltages aren't right, nothing else will be either. Have to alternator tested. (on the truck, if possible.) Sounds like either the alt, or the voltage regulator, has failed. (provided the wiring is good....) Unfortunately, voltage regulator is internal to the pcm on these trucks..... Find out what the problem is, take it from there.