Several things to check:
The DLC (Data Link Connctor) has to supply battery voltage on pin 16 (Key On) to the scan tool and grounds on pins 4 & 5 of the DLC. Check these first. Loss of power will cause a "no communication" message on most scanners. Pin 16 is supplied by either Fuse F24 or F28 in the underdash fusebox depending on date of manuafacture. On laeter productions, F24 also supplies power to brake light switch for the stop lamps. On early production, F28 supplies pin 16 and the power seat module, if present.
Assuming a return style fuel system, the fuel pump is supplied by the fuel pump relay in the underhood fuse box. High side coil power is supplied via the NO contacts of the PCM POWER relay. Low side is controlled by the PCM and may be jumpered to ground to test relay and pump power function when the PCM RELAY is energized.
Check IGN ground to battery ground at the TPS. Check Vref at the TPS.
Steve
