Lexus SC300 / Lexus SC400. Service manual - part 688

ISC Valve Circuit

CIRCUIT DESCRIPTION

The ISC valve is provided on the intake air chamber and in-
take air bypassing the throttle valve is directed to the ISC
valve through a hose.
A step motor is built into the ISC valve. It consists of four
coils, the magnetic rotor, valve shaft and valve. When cur-
rent flows to the coils due to signals from the ECU, the rotor
turns and moves the valve shaft forward or backward,
changing the clearance between the valve and the valve
seat.
In this way the intake air volume bypassing the throttle valve
is regulated, controlling the engine speed.
There are 125 possible positions to which the valve can be
opened.

DIAGNOSTIC CHART

DIAGNOSTIC CHART

Check operation for ISC valve.

Replace ISC valve.

Repair or replace harness or
connector.

Proceed to next circuit inspection shown on
matrix chart (See page 

TR–35

).

Check harness and connector between EFI
main relay, ISC valve and ECU.

TR–140

–

ENGINE TROUBLESHOOTING

Circuit Inspection

OK

NG

OK

NG

INSPECTION PROCEDURE

1

Check ISC value.

C

OK

P

P

OK

C

Disconnect ISC connector.

Measure resistance between terminals shown below.

Remove ISC Valve

(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 value moves in the opening direction.

Replace ISC valve.

2

Check for open and short in harness and connector between EFI main relay and ISC valve, ISC
valve and engine & ECT ECU (See page 

IN–27

).

Repair or replace harness or connector.

Proceed to next circuit inspection shown on ma-
trix chart (See page 

TR–35

).

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–141

Fuel Pressure Control VSV Circuit

CIRCUIT DESCRIPTION

The ECU turns on a VSV (Vacuum Switching Valve)
to draw the air into the diaphragm chamber of the
pressure regulator if it detects that the temperature of
the coolant is too high during engine starting.
The air drawn into the chamber increases the fuel
pressure to prevent fuel vapor lock at high engine
temperature in order to help the engine start when it
is warm.
Fuel pressure control ends approx. 100 secs. after
the engine is started.

DIAGNOSTIC CHART

DIAGNOSTIC CHART

Check operation for fuel pressure control
VSV.

Replace fuel pressure control VSV.

Repair or replace harness or
connector.

Proceed to next circuit inspection
shown on matrix chart (See page

TR–35

).

Check for open and short in harness and
connector between main relay and ECU.

Check voltage of VSV power source.

Check and replace ECU.

WIRING DIAGRAM

TR–142

–

ENGINE TROUBLESHOOTING

Circuit Inspection

OK

NG

INSPECTION PROCEDURE

1

Check fuel pressure control VSV.

C

OK

P

OK

C

(2) Remove fuel pressure control VSV.

(2) Disconnect fuel pressure control VSV connector.

(2) Measure resistance between terminals.

(2) Measure resistance between each terminal and the

body.

1.  Resistance:  30 – 50  at 20C (68F)

2.  Resistance:  1M or higher

Check operation of fuel pressure control VSV when bat-
tery voltage is applied to the terminals of fuel pressure
control VSV connector or not.

Battery voltage is applied:
The air from pipe E is flowing out through the air fil-
ter.

Battery voltage is not applied:
The air from pipe E is flowing out through pipe G.

Replace fuel pressure control VSV.

Go to step 2.

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–143