1991 LEXUS SC400 engine oil

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 CHARTDIAGNOSTIC 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
WhereEverybodyKnowsYourName

YESNO
NOYES
YES
NO
OKNG
BASIC INSPECTION
When the normal code is displayed in the diagnostic code check, troubleshoot\
ing should be performed in the
order for all possible circuits to be considered as the causes of the pr\
oblems.
In many cases, by carrying out the basic engine check shown in the follo\
wing flow chart, the location causing
the problem can be found quickly and ef ficiently. Therefore, use of this check is essential in engine troubleshoot-
ing.
1Is battery voltage 11 V or more when engine is stopped?
Charge or replace battery.
2Is engine cracnked?
Proceed to matrix to matrix chart of problem
symptoms on page TR±35.
3Does engine start?
Go to step [7]
4Check air filter
C
Hint
PRemove air filter
Visially check that the air cleaner element is not exces-
sively damaged or oily.
If necessary, clean element with compressed air. First
blow from inside thoroughly, then blow off outside of ele-
ment.
Repair or replace
Go to Step [5].
TR±24±
ENGINE TROUBLESHOOTING Basic Inspection
WhereEverybodyKnowsYourName

OKNG
OKNG
5Check idle speed.
C
OK
P(1) Warm up engine at normal operating temperature.
(2) Switched off all accessories.
(3) Switched off air conditioner.
(4) Shift transmission into ªNº range.
(5) Connect tachometer test probe to terminal IG
of
check connector, and set the tachometer to the
4±cylinder range.
Check idle speed.
Idle speed: 650 ~ 750 rpm
Caution:

NEVER allow tachometer test probe to touch
ground as it could result in damage to igniter
and/or coil.

As some tachometers are not compatible with
this ignition system, we recommended that you
confirm compatibility of your unit before use.
Proceed to matrix chart of problem symptoms on page
TR±35 .
6Check ignition timing.
C
OK
P(1) Warm up engine at normal operating temperture.
(2) Shift transmission into ªNº range.
(3) Keep the engine speed at idle.
(4) Using SST, connect terminals TE1 and E1 of check
connector.
SST 09843±18020
(5) Using a timing light. connect the tester to No. 6 high±tension cord.
Check ignition timing.
Ignition Timing: 8±12  BTDC at idle
Proceed to page IG±28 and continue to troubleshoot.
Proceed to matix chart of problem symptoms on
page TR±35 .
±
ENGINE TROUBLESHOOTING Basic InspectionTR±25
WhereEverybodyKnowsYourName

CIRCUIT INSPECTIONDiag. Code 12
RPM Signal Circuit (No. 1)
CIRCUIT DESCRIPTION
The distributor in the Engine Control System contains three pick±up coil\
s (G1, G2 and NE).
The G1, G2 signals inform the ECU of the standard crankshaft angle.
The NE signals inform the ECU of the crankshaft angle and the engine spe\
ed.
 Code No. Diagnostic Code Detecting Condition Trouble Area
  
NoºNEºorºG1ºandºG2ºsignal to ECU within 2
 

Open or short in NE G circuit  No ºNEº or ºG1º and ºG2º signal to ECU within 2
sec after cranking 
Open or short in NE, G circuit.

Distributor
 12 sec. after cranking. 
Distributor

Open or short in STA circuit.
ECU  Open in ºGº
circuit. 
Oen or short in STA circuit.

ECU
TR±46±
ENGINE TROUBLESHOOTING Circuit Inspection
WhereEverybodyKnowsYourName

DIAGNOSTIC CHART
WIRING DIAGRAM
Check resistance of each pickup coils in
distributor
Check for open and short in harness and
connector between ECU and distributor.
Check air gap.
Check and replace ECU.
Replace distributor.
Repair or rep lace harness or connector.
Replace distributor
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±47
WhereEverybodyKnowsYourName

Disconnect distributor connector.
Measure resistance between each terminal shown in
table below.
Check resistance of each picup coils in distributor.

During cranking or idling, check between terminals G1,
G2, NE and G
of engine (&ECT) ECU.
HINT: The correct waveforms appears as shown in the illustration on the left.
Replace distributor.
Check for open and short in harness and connector between engine
(&ECT) ECU and distributor (See page IN±27
Repair or replace harness or connector.
INSPECTION PROCEDURE
TR±48±
ENGINE TROUBLESHOOTING Circuit Inspection
WhereEverybodyKnowsYourName

Remove distributor cap & rotor.
Using SST (G1 and G2 pickups) and a thikness gauge
(NE pickup). measure the air gap between the signal
rotor projection and pickup coil.
SST 09240±00020 fro G1 and G2 pickups
Air gap: 0.2±0.4 mm (0.008±0.016 in.)
Replace distributor.
Check and replace engine (& ECT) ECU.
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±49
WhereEverybodyKnowsYourName

CIRCUIT INSPECTIONDiag. Code 12
RPM Signal Circuit (No.1)
CIRCUIT DESCRIPTION
Cam position sensors (G1 and G2 signals) and engine speed sensor (NE sig\
nal) consist of a signal plate
and a pick up coil.
The G1, G2 signal plates have one tooth each on its outer circumference and\
are mounted on the left and
right bank camshafts.
When the camshafts rotate, the protrusion on the signal plate and the air gap on the pick up coil change,
causing fluctuations in the magnetic field and generating an electromotive forc\
e in the pick up coil.
The NE signal plate has 12 teeth and is mounted on the crankshaft. The NE s\
ignal sensor generates 12
NE signals per engine revolution. The ECU detects the standard crankshaf\
t angle based on the G1, G2
signals, and the actual crankshaft angle and the engine speed by the NE \
signals.
Code No.Diagnostic Code Detecting ConditionTrouble Area
wOpen or short in engine speed sensor, No. 1,w Oen or short in engine s eed sensor, No. 1,
No. 2 cam position sensor circuit
12No ºNEº or ºG1º and ºG2º signal to ECU
No. 2 cam osition sensor circuit
w Engine speed sensor12No NE or G1 and G2 signal to ECU
within 2 sec. after cranking.
wEngine s eed sensor
wNo. 1, No. 2 cam position sensorgNo. 1, No. 2 cam osition sensor
wStarter
wECU
TR±48±
ENGINE TROUBLESHOOTING Circuit Inspection
WhereEverybodyKnowsYourName