Lexus SC300 / Lexus SC400. Manual — part 687

Cold Start Injector Circuit

CIRCUIT DESCRIPTION

The cold start injector is used to maintain the engine startability when it is cold. The injection volume, i,e, the
length of time the injector is energized, is controlled by the ECU and the cold start injector time switch.
During a cold start, when the starter turns the contacts in the cold start injector time switch close. Thus current
flows to the cold start injector coil, injecting fuel. At the same time, a bimetal in the heat coil is energized and
heats up. This soon causes the contacts to open, cutting off the current flow to the injector coil and stopping
fuel injection.
The injection duration of the cold start injector is determined by the coolant temperature and the length of time
current flows to the heat coil. When the engine is warm, the contacts are opened by the bimetal and the cold
start injector does not operate.
When the engine is hard to start and the starter is operated continuously, heat coil (2) heats up the bimetal keep-
ing the contacts open to prevent spark plugs from becoming fouled, which is caused by the cold start injector
operation when the open contacts close again.
When the engine is started at a coolant temperature of 22

°

C (72

°

F) or lower, the cold start injector operation

time is controlled by the cold start injector time switch.
When the coolant temperature is in the normal temperature range 22

°

C (72

°

F) or higher, the contacts of the cold

start injector time switch are open and the time switch is off, instead, the ECU controls the operating time of the
cold start injector.
In this way, the CO and HC levels can be reduced while the engine is being started and the engine startability
is maintained. Control by the ECU ends when the coolant temperature reaches 60

°

C (140

°

F).

TR–136

–

ENGINE TROUBLESHOOTING

Circuit Inspection

DIAGNOSTIC CHART

DIAGNOSTIC CHART

Check voltage of cold start injector power
source.

Check cold start injector.

Check start injector time switch.

Replace start injector time switch.

Repair or replace harness or
connector.

Replace cold start injector.

Check for open and short in harness and
connector between cold start injector and
start injector time switch.

Check fuel injection of cold start injector.

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

TR–35

).

Check for open in harness and connector
between battery and ECU.

Replace cold start injector.

WIRING DIAGRAM

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–137

NG

OK

OK

NG

INSPECTION PROCEDURE

1

Check voltage between terminal STJ of engine & ECT ECU connector and body ground.

C

OK

Hint

P

Connect to the Check Harness A.
(See page

TR–30

)

Disconnect start injector time switch connector.

Disconnect engine & ECT ECU connector.

Measure voltage between terminal STJ of engine &
ECT ECU connector and body ground when ignition
switch is turned to STA.

10 – 14 V

Go to step 3.

2

Check resistance of cold start injector.

C

OK

Hint

P

Remove cold start injector (See page

FI–27

).

Measure resistance between terminals of cold start in-
jector.

Resistance:

2 – 4

at 20

C (68

F)

Replace cold start injector.

Check and repair harness and connector between
engine & ECT ECU and cold start injector, cold
start injector and battery.

TR–138

–

ENGINE TROUBLESHOOTING

Circuit Inspection

OK

NG

OK

NG

OK

NG

3

Check start injector time switch.

C

OK

P

Remove start injector time switch (See page FI–88).

Measure resistance between terminals shown below.

Replace start injector time switch.

4

Check harness and connector between cold start injector and start injector time switch (See
page

IN–27

).

Repair or replace harness or connector.

5

Check fuel injection of cold start injector.

C

OK

P

Remove cold start injector (See page

FI–27

).

(2) Check fuel injection of cold start injector

(See page

FI–33

).

(2) Check fuel leakage of cold start injector

(See page

FI–34

).

(2) Fuel is injected normally.

(2) Fuel leakage is less than one drop per minute.

Notice:

Perform this check within the shortest possible
time.

Repair or replace cold start injector.

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

TR–35

).

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–139