American M-Series military vehicles follow a common system of electrical circuit numbers. This simplifies maintenance procedures and training of military mechanics who have to work on many different vehicles. In some cases it allows common parts to be used for multiple applications.
M-3 tank engine wiring harness manufacture at Chrysler, World War II.
For a more complete listing of military standard circuit numbers than the table on this page, consult this file based on TACOM's Ordnance Drawing 7070301 [285k PDF download].
Every electrical line in
the vehicle will have small metal tags crimped around the wire near every
junction or connector. The metal tag will bear a number and that number
uniquely identifies what electrical circuit the wire is carrying. All wiring is
black so the metal tags are the only ID for you to use, unlike color-coded
commercial vehicle wiring.
Electrical components, such as switches, recepticles, and lights may
also have the same identifying numbers. If they come with pigtails, there will
probably be a tag on each wire. If they just have a socket, the circuit number
may be part of the housing or painted on. If you make any changes to your
vehicle wiring, try to preserve the wire tags and to properly tag anything you
add. That will help you or the next owner when the time comes to try to
understand what you did. If you can stamp military style metal tags, great. If
not, use other wire label methods but use the correct numbers.
In order to help you when you service your vehicle or make any changes
to its wiring, here is the list of numbers. The information on this list was
compiled from PS Magazine, #38 (1955) and TM 9-8000 Principles of Automotive
Vehicles (1956). No one vehicle has all of these circuits and most wheeled
vehicles use only a small subset of them.