Lexus SC300 / Lexus SC400. Manual - part 915

MEMO –

ENGINE TROUBLESHOOTING

TR–53

Diag. Code   14, 15

Ignition Signal Circuit

CIRCUIT DESCRIPTION

The ECU determines the ignition timing, turns on Tr1 at a predetermined angle (

°

CA) before the desired

ignition timing and outputs an ignition signal (IGT) ”1” to the igniter.
Since the width of the IGT signal is constant, the dwell angle control circuit in the igniter determines the
time the control circuit starts primary current flow to the ignition coil based on the engine rpm and ignition
timing one revolution ago, that is, the time the Tr2 turns on.
When it reaches the ignition timing, the ECU turns Tr1 off and outputs the IGT signal ”O”.
This turns Tr2 off, interrupting the primary current flow and generating a high voltage in the secondary coil
which causes the spark plug to spark. Also, by the counter electromotive force generated when the primary
current is interrupted, the igniter sends an ignition confirmation signal (IGF) to the ECU.
The ECU stops fuel injection as a fail safe function when the IGF signal is not input to the ECU.

Code No.

Diagnostic Code Detecting Condition

Trouble Area

14

No IGF1 signal to ECU for 8–11 consecutive
IGT1 signal.

•

Open or short in IGF1 and IGT1 circuit from
No. 1 igniter – ECU.

•

No. 1 igniter

•

ECU

15

No IGF2 signal to ECU for 8–11 consecutive
IGT2 signal.

•

Open or short in IGF2 and IGT2 circuit from
No. 2 igniter – ECU.

•

No. 2 igniter

•

ECU

HINT: No. 1 igniter is for left bank and No. 2 igniter is for right bank.

WIRING DIAGRAM

TR–54

–

ENGINE TROUBLESHOOTING

Circuit Inspection

DIAGNOSTIC CHART

DIAGNOSTIC CHART

Refer to inspection procedure in details.
HINT: If diag. code 14 is displayed, check No. 1 igniter circuit.

  If diag. code 15 is displayed, check No. 2 igniter circuit.

Check for spark.

Check for open and short in IGF signal circuit
between igniter and ECU.

Check voltage of terminal IGF.

Check and replace ECU.

Check voltage of terminal IGT.

Check voltage of igniter power source.

Check for open and short in primary coil circuit.

Check ignition coil.

Replace igniter.

Check and repair igniter power source circuit.

Check voltage of terminal IGT
(Disconnect igniter connector).

Check and repair igniter power source circuit.

Repair or replace harness or
connector.

Replace igniter.

Repair or replace harness or
connector

Replace ignition relay.

Replace ignition relay.

Check for open and short in IGT signal circuit
between igniter and ECU.

Check and replace ECU.

Repair or replace harness or
connector

Replace igniter.

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–55

OK

NG

OK

NG

OK

NG

INSPECTION PROCEDURE

HINT: If diag. code 14 is displayed, check No. 1 igniter circuit.

  If diag. code 15 is displayed, check No. 2 igniter circuit.

1

Check for Spark.

C

OK

Hint

Disconnect the high–tension cord from the distributor,
hold its end about 12.5 mm (1/2”) from the ground, see
if spark occurs while the engine is being cranked.

Spark should be generated.

To prevent excessive fuel injected from the injectors dur-
ing the check. Don’t crank the engine for more than 1–2
seconds at a time.

Go to step [4].

2

Check for open and short in harness and connector in the IGF and IGT signal circuit between
engine & ECT ECU and igniter (See page 

IN–27

)

Repair or replace harness or connector.

#

Disconnect igniter sonnector and check voltage between terminals IGF1, 2 of engine & ECT
ECU connector and body ground.

C

OK

P

(2) Disconnect igniter connector.

(2) Connect the Check Harness A.

(See page 

TR–30

).

(2) Turn ignition switch on.

Measure Voltage between terminals IGF1, 2 or engine
& ECT ECU connector and body ground.

Voltage:

4 – 6 V

Replace igniter.*1

Check and replace engine & ECT ECU.

*1:

When dia. code 14 is displayed, replace the igniter with 5
wire harness. (extra wire is for tachometer).  When diag.
code 15 is displayed, replace the igniter with 4 wire har-
ness.

TR–56

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ENGINE TROUBLESHOOTING

Circuit Inspection