Lexus SC300 / Lexus SC400. Manual - part 937

OK

NG

OK

NG

6

Check resistance of EGR gas temp. sensor.

C

OK

P

Disconnect EGR gas temp. sensor connector
(See page 

FI–103

).

Measure resistance between terminals of EGR gas
temp. sensor connector.

Resistance:

69 – 89 k

 at 50



C (122



F)

12 – 15 k

 at 100



C (212



F)

2 – 4 k

 at 150



C (302



F)

Replace EGR gas temp. sensor.

7

Check for open and short in harness and connector between EGR gas temp. sensor and engine
& ECT ECU (See 

IN–27

).

Repair or replace harness or connector.

Check and replace engine & ECT ECU.

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–113

WIRING DIAGRAM

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 termi-
nal).
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.

DTC No.

DTC Detecting Condition

Trouble Area

(1) Open or short in fuel pump circuit for 1 sec. or more with en-

gine 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)*



Engine (& ECT) ECU

*: See page 

TR–25

.

TR–110

–

ENGINE TROUBLESHOOTING

Circuit Inspection

DIAGNOSTIC CHART

Check fuel pump operation.

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

Repair or replace harness or connector.

Check voltage of terminal +B of check
connector.

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

Repair or replace harness or connector.

Check voltage of terminal FPC.

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

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

Check and replace ECU.

Check for ECU power source
circuit.

Repair or replace fuel pump.

Replace fuel pump ECU.

Repair or replace harness or
connector.

Repair or replace harness or
connector.

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–111

OK

NG

NG

OK

OK

NG

INSPECTION PROCEDURE

1

Check fuel pump operation.

C

OK

P

1.

Turn ignition switch ON.

2.

Using SST, connect terminals +B and FP of check
connector.

SST 09843–18020

Check that there is pressure in the hose from the fuel
filter.

Fuel pressure can be felt.

Go to step 

.

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 



.

Repair or replace harness or connector.

3

Check voltage of 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–120

),

and check for open in harness and connector between
terminal +B of check connector and main relay.

TR–112

–

ENGINE TROUBLESHOOTING

Circuit Inspection