Lexus SC300 / Lexus SC400. Service manual - part 631

IGNITION SYSTEM

IG–1

 IGNITION SYSTEM

DESCRIPTION

The ECU is programmed with data for optimum ignition timing under any and all operating conditions. Using

data provided by sensors which monitor various engine functions (rpm, intake air volume, engine temperature,
etc.) the microcomputer (ECU) triggers the spark at precisely the right instant.

The ECU monitors the engine condition by signals from each sensor, calculates the ignition timing and sends

an ignition signal to the igniter. High voltage from the ignition is distributed to each spark plug in the appropriate
order to generate a spark between the electrodes, which ignites the air–fuel mixture.
IGNITERS

The igniter temporarily interrupts the primary current with the ignition signal (IGT signal) from the ECU and

generates sparks at the spark plug. Also, as a fail–safe measure, when ignition occurs, an ignition confirmation
signal (IGF signal) is sent to the ECU.
IGNITION COIL

The ignition coil uses a closed core coil with the primary coil wrapped around the core and the secondary

coil wrapped around the primary coil. This allows the generation of a high voltage sufficient to cause a spark
to jump across the spark plug gap.
DISTRIBUTORS

This correctly distributes high voltage to the spark plug of each cylinder in the specified ignition order.

ENGINE SPEED SENSOR

The engine speed sensor detects the crank angle.

CAM POSITION SENSORS

The RH and LH cam position sensors detect the cam angle.

IG–2

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 IGNITION SYSTEM

Description

PRECAUTIONS

1.  Do not leave the ignition switch on for more than 10

minutes if the engine will not start.

2. 

With a tachometer connected to the system, connect the
tester probe of the tachometer to terminal IG È of the
check (”DIAGNOSIS”) connector.
HINT:

•

Allow the engine to warm up to normal operation
temperature.

•

Set the tachometer to the 4–cylinder range.

3.  As some tachometers are not compatible with this

ignition system, we recommend that you confirm the
compatibility of your unit before use.

4. 

Never allow the tachometer terminal to touch ground as
this could damage the igniter and/or ignition coil.

5.  Do not disconnect the battery when the  engine is

running.

6. 

Check that the igniter is properly grounded to the body.

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 IGNITION SYSTEM

Precautions

IG–3

SYSTEM CIRCUIT

OPERATION

To maintain the most appropriate ignition timing, the ECU sends a control signal so that the igniter sends

current to the ignition coil and the spark plugs produce a spark.

IG–4

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 IGNITION SYSTEM

System Circuit, Operation