A multiplex communication system has been developed for the Ford Alternative Fuel Qualified Vehicle Modifier (QVM) program, linking the vehicle's powertrain control module (PCM) to the alternative fuel (natural gas or propane) control module (AFCM). The system allows precalibrated diagnostic routines (OBD and non-OBD) to be selectively modified and disabled which otherwise would be affected by alternative fuel operation. The system enables the broadcasting of selected information from the PCM for use by the AFCM. An additional benefit is smoother transition between alternative fuel and gasoline operation when the gaseous fuel is depleted. Also, improved emissions on the alternative fuel have been noted due to the link. System development and function are detailed in this paper.