Diagnostic Aids

The "instant/average" data request line, circuit 477, is connected to ground by pushing the button on the Fuel Data panel.

1. Code .7.8 indicates that the ECM is not receiving a 0 voltage signal when the average button is depressed. To diagnose this condition, turn the ignition on and display Code .7.8. Remove the Fuel Data panel connector and momentarily jumper harness pin D to ground. This action simulates the operation of the "instant/average" button and, if Code .7.8 disappears, the ECM is operating properly. Check circuit 990, the ground dine, for an open. If circuit 990 is OK, check for a faulty Fuel Data connector or faulty Fuel Data panel.
2. If Code .7.8 remains after grounding harness pin D, then check circuit 477 for an open or short to ground. If circuit 477 is OK, check for a faulty ECM com1ector or faulty ECM.