This SAE Recommended Practice supersedes SAE J1978 FEB1998, and is technically equivalent to ISO/DIS 15031-4:December 14, 2001. This document is intended to satisfy the requirements of an OBD scan tool as required by U.S. On-Board Diagnostic (OBD) regulations. The document specifies: a. A means of establishing communications between an OBD-equipped vehicle and external test equipment, b. A set of diagnostic services to be provided by the external test equipment in order to exercise the services defined in SAE J1979, c. Conformance criteria for the external test equipment. Differences from ISO Document The ISO 15031-4 document is intended to satisfy the requirements of OBD requirements in countries other than the U.S., and includes functionality not required or not allowed in the U.S. Notable exceptions are: a. Proposed U.S. OBD regulations will allow ISO 15765-4 as an allowable OBD serial data link at 500 kbps only. Only those provisions of ISO 15765-4 that pertain to 500 kbps are required to be supported by an SAE J1978 diagnostic scan tool. b. Proposed U.S. OBD regulations will not allow greater than 20 V at the SAE J1962 connector. Only the Type A connector as defined in SAE J1962 needs to be supported by an SAE J1978 diagnostic scan tool. Differences between the documents are highlighted in the technical requirements sections of this document. Deleted text is highlighted with struck through and new text is highlighted with bold italic. NOTE - To maintain equivalency of the documents, a comma is used as a decimal marker for numeric values in this document.
The J1978 document is overdue for a five year review. Recent changes to J1979 - E E Diagnostic Test Modes and to J1699-3 - OBD II Vehicle Conformance Test Cases will impact the document content of J1978 and revisions will be required. The ongoing development of J1699-2 - Test Cases for OBD II Scan tools and I/M Test Equipment - will also impact J1978. Since the J1978 Task Force has not met in a very long time (2002, I believe) I suggest that participants be solicited during the next E E Systems Diagnostic Standards Committee meeting on April 15, 2011.