Lexus SC300 / Lexus SC400. Manual - part 681

OK

NG

OK

NG

OK

NG

3

Check resistance of EGR step motor.

C

OK

P

Disconnect EGR step motor connector.

Measure resistance between terminals shown below.

Replace EGR step motor.

4

Check for open and short in harness and connector between EGR step motor and engine & ECT
ECU (See page 

IN–27

).

Repair or replace harness or connector.

Check and replace engine & ECT ECU.

5

Check operation for EGR step motor.

C

OK

P

Remove EGR step motor.

(2) Connect the battery positive lead to terminals B1

and B2, and the negative lead to terminals S1 – S2
– S3 – S4 in that order.

(2) Connect the battery positive lead to terminals B1

and B2, and the negative lead to terminals S4 – S3
– S2 – S1 in that order.

(2) The valve moves in the closing direction.

(2) The valve moves in the opening direction.

Replace EGR step motor.

TR–112

–

ENGINE TROUBLESHOOTING

Circuit Inspection

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

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