Why do I show system low voltage?

Try removing both batteries and load testing them. I suspect one or the other may have a bad cell because the driver side cables route directly over to the passenger side battery. Both batteries should show the exact same voltage when running. Separate, charge and load testing both batteries lets you look for a week or dead cell and then fully clean ALL terminals and grounds with a wire brush and battery terminal cleaner. (The stuff that sprays on yellow and turns pink when it hits battery acid) Reassemble with the batteries in their opposite sides and see what ya got! I hope this helps. Dale

Thank you. There seams to be a bad ground on the battery on the left side. When you have the truck running and remove the battery on the right it continues to run, put it back and remove the one on the left the truck dies. We've removed the batteries charged them, cleaned all terminals and cables, swapped batteries and we still have 18volts on the left and 15 on the right. BATTERIES are being replaced today. Thanks for your help