This SAE Recommended Practice defines a recommended practice for implementing circuit identification for the circuits of the power and signal distribution systems of the Class 8 trucks and tractors. This document provides a description of a supplemental circuit identifier that shall be utilized in conjunction with the original equipment manufacturers primary circuit identification as used in wire harnesses but does not inlcude electrical or electronic devices which have pigtails. The supplemental circuit identifier is cross-referenced to a specified subsystem of the pwer and signal distribution system identified in Section 5. This document will provide a common method of identifying circuit function for commercially available Class 8 vehicles. It will guide the service technician in diagnosing vehicle electrical susbystems. The identification method will supplement component OEM circuit identification for each major subsystem