Here is a ever growning list of tools that come in VERY handy when working on your cummins.
First and Foremost, Knipex 86 03 250 Parellel Jaw Plyers The key to these plyers is unlike channel lock plyers the jaws on these stay parellel all of the time.
They run right about $50, but we promise they are worth 3x that. Once you use a set of these they will become the single most loved tool in your toolbox. These plyers allow you to put even and steady clamping force on any size object.
A GOOD Multimeter is a absoulete requirement for working on a Cummins powered vehicle. You want a True RMS multimeter to check for things like AC noise. Most Flukes will do that. I personally use this with good results
EXTECH EX470 I like it because it has a built in IR temp gun. this is VERY handy for tracking things down like a dead cylinder
It runs about $100 on amazon