Lexus SC300 / Lexus SC400. Manual - part 938

NG

OK

NG

OK

OK

NG

OK

NG

4

Check for open and short in harness and connector between terminal FP of check connector,

fuel pump and body ground (See page 

IN–27

).

Repair or replace fuel pump.

Repair or replace harness or connector.

5

Check voltage between terminals FPC and E of fuel pump ECU connector.

C

OK

P

1.

Remove the LH quarter trim panel. (See page

FI–81

)

2.

Disconnect fuel pump ECU connector.

Measure voltage between terminals FPC and E of fuel
pump ECU connector when ignition switch is turned to
start.

Voltage:

4 – 6 V

Replace fuel pump ECU.

6

Check for open in harness and connector between terminal FPC of engine (& ECT) ECU and ter-

minal FPC of fuel pump ECU, terminal E of fuel pump ECU and body ground (See page 

IN–27

).

Repair or replace harness or connector.

7

Check for open and short in harness and connector between terminal DI of engine (& ECT) ECU

and terminal DI of fuel pump ECU (See page 

IN–27

).

Repair or replace harness or connector.

Check and replace engine (& ECT) ECU.

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–113

Diag. Code   78

Fuel Pump Control Circuit

CIRCUIT DESCRIPTION

The fuel pump speed is controlled at two steps (high speed, low speed) by the condition of the engine (starting,
light load, heavy load), when the engine starts (STA ON), the engine & ECT ECU sends a Hi signal to the fuel
pump ECU (FPC terminal).
The fuel pump ECU then outputs Hi voltage (battery voltage) to the fuel pump so that the fuel pump operates
at high speed.
After the engine starts, during idling or light loads, the engine & ECT ECU outputs a High Low signal to the fuel
pump ECU, the fuel pump ECU outputs Lo battery voltage (about 9V) to the fuel pump and causes the fuel pump
to operate at low speed.
If the intake air volume increases (high engine load), the engine & ECT ECU sends a Hi signal to the fuel pump
ECU and causes the fuel pump to operate at high speed.

Code No.

Diagnostic Code Detecting Condition

Trouble Area

(1)  Open or short in fuel pump circuit for 1 sec. 

or more with engine speed 1,000 rpm or less. 

  (2 trip detection logic)*

Open or short in fuel pump ECU circuit.

78

(2)  Open in input circuit of fuel pump ECU(FPC) with

engine speed 1,000 rpm or less. 

  (2 trip detection logic)*

Fuel pump ECU

Engine & ECT ECU power source circuit.

Fuel pump

(3)  Open or short in diagnostic signal line (DI) of fuel

pump ECU with engine speed 1,000 rpm or less.

  (2 trip detection logic)*

Fuel  um

Engine & ECT ECU

*: See page 

TR–21

.

WIRING DIAGRAM

TR–114

–

ENGINE TROUBLESHOOTING

Circuit Inspection

DIAGNOSTIC CHART

DIAGNOSTIC CHART

Check for open and short in harness and
connector between check connector and fuel
pump.

Check voltage of terminal FPC.

Check for open and short in harness and
connector between ECU and fuel pump
ECU. (DI)

Check and replace ECU.

Replace fuel pump ECU.

Repair or replace harness or connector.

Repair or replace harness or
connector.

Check for open and short in harness and
connector between ECU and fuel pump
ECU. (FPC)

Repair or replace harness or
connector.

Repair or replace fuel pump.

Check voltage of terminal +B of check
connector.

Repair or replace harness or connector.

Check for open and short in harness and
connector between check connector and fuel
pump ECU.

Check fuel pump operation.

Check for ECU power source circuit.

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–115

OK

NG

NG

OK

OK

NG

INSPECTION PROCEDURE

1

Check fuel pump operation.

C

OK

P

(2) Turn ignition switch ON.

(2) Using SST. connect terminals + B and FP of check

connector.

SST 09843–18020

Check that there is projection of the head of the pulsa-
tion damper screw.

Projection of the head of the pulsation damper
screw.

Go to step 3.

2

Check for open and short in harness and connector between terminals + B 

↔

  + B FP 

↔

 FP of

the check connector and fuel pump ECU (See page 

IN–27

).

Go to step 5.

Repair or replace harness or connector.

3

Check voltage or terminal + B of check connector.

C

OK

P

Turn ignition switch ON.

Measure voltage between terminal + B of check connec-
tor and body ground.

Voltage:

10 – 14 V

Check for ECU power source circuit 
(See page 

TR–124

), and check for open in harness and

connector between terminal + B of check connector and
main relay.

TR–116

–

ENGINE TROUBLESHOOTING

Circuit Inspection