DTC 78 - FUEL PUMP CONTROL CIRCUIT
CAUTION: If ECM replacement is instructed in following testing,
always ensure ECM connectors and ground circuit are okay. If
either are suspect, repair and repeat testing to confirm ECM
malfunction.
Circuit Description
The 2-speed fuel pump is operated by the fuel pump ECU which
receives a signal from ECM to compensate fuel pressure for start,
light engine load or high engine load. DTC is set during the following
3 conditions:
1) Open or short in fuel pump circut is detected for 1 second
or more with engine speed of 1000 RPM or less.
2) Open in input circuit of fuel pump ECU with engine speed of
1000 RPM or less.
3) Open or short in fuel pump ECU diagnostic signal line with
engine speed of 1000 RPM or less.
Possible causes for all conditions are:
* Fuel pump ECU open or short circuit.
* Fuel pump ECU.
* ECM power supply circuit.
* Fuel pump.
* ECM.
Diagnosis & Repair
1) Turn ignition on. Using jumper wire, connect DLC1
terminals +B and FP. See Fig. 11. Check if fuel pressure can be felt
in hose to fuel filter. If pressure is felt, go to next step. If
pressure cannot be felt, go to step 3).
2) Check for open or short in +B and FP circuits between DLC1
and fuel pump ECU. Fuel pump ECU is located back of left rear
wheelwell, behind trim panel. See appropriate wiring diagram in L -
WIRING DIAGRAMS article. Repaire as necessary. If circuits are okay,
go to step 5).
3) Ensure ignition is on. Using DVOM, measure voltage between
ground and DLC1 terminal +B. If voltage is 9-14 volts, go to next
step. If voltage is not 9-14 volts, inspect ECM power supply circuit
and +B circuit between DLC1 connector and main relay. See appropriate
wiring diagram in L - WIRING DIAGRAMS article. Repair as necessary.
4) Inspect FP circuit for open or short between DLC1 connector, fuel pump and ground. Repair as necessary. If circuit is
okay, replace fuel pump.
5) Access fuel pump ECU. Fuel pump ECU is located back of
left rear wheelwell, behind trim panel. Disconnect fuel pump ECU
connector. Check voltage between terminals "E" and FPC at ECU wiring
harness connector. See Fig. 18. Turn ignition switch to START
position. If voltage is 4.5-5.5 volts, replace fuel pump ECU. If
voltage is not 4.5-5.5 volts, go to next step.
6) Inspect FPC circuit between ECM and fuel pump ECU for
open. See appropriate wiring diagram in L - WIRING DIAGRAMS article.
Repair as necessary. If circuit is okay, inspect "E" circuit between
fuel pump ECU and ground. Repair as necessary. If both circuits are
okay, go to next step.
7) Inspect DI circuit between ECM and fuel pump ECU for open
or short. See appropriate wiring diagram in L - WIRING DIAGRAMS
article. Repair as necessary. If circuit is okay, replace ECM and
retest