Instantaneous Fuel Economy Always Reads 0 Miles/GAL Or 99 Miles/GAL

Ensure speedometer is operating properly. Verify continuity and absence of short circuits in circuit No. 305, between cluster connector "A" (terminal No. 11) at the instrument cluster connector and Data Output Line (DOL) circuit (terminal No. 34) at the EEC-IV connector. If there is no continuity or shorts are present, repair wiring of circuit No. 305 as necessary. See Figure .
If continuity is present and there are no short circuits, ensure the presence of a DOL signal from the EEC-IV module. With key off, connect breakout box to EEC-IV module or check for signal output at cluster connector "A" (terminal No. 11). Set DVOM on 20-volt scale. Start engine and measure voltage at test pin No. 34 (DOL circuit) at the breakout box. Voltage should be 10-12 volts.
NOTE: The EEC-IV module computes fuel usage and outputs this signal on the DOL circuit. The DOL signal is a digital pulse type. Duty-cycle on-time is approximately 5% at idle and increases with engine load and RPM.
If EEC-IV module signal on DOL circuit is present, replace EIC (fuel computer module). If signal is absent, replace EEC-IV module. Reconnect components and retest system.