ECU LEXUS SC400 1991 Service Repair Manual

WIRING DIAGRAM
DTC25, 26, 27TRAC Sub±Throttle Valve
CIRCUIT DESCRIPTION
DTC No.DTC Detecting ConditionTrouble Area
25
The difference between the sub±throttle sensor signal
voltage and the voltage required by the step count is 1
V or more.

Sub±throttle actuator.

Throttle body (sub±throttle valve is truck or operation is
faulty).

Sub±throttle position sensor.

Open or short in sub±throttle position sensor circuit.

ECM

TRAC ECU
26
Even when sub±throttle valve is driven to the fully open
position, input voltage at terminal VSH does not come
within range of the specifications 3 ~ 4.5 V.

Sub±throttle actuator.

Throttle body (sub±throttle valve is truck or operation is
faulty).

Sub±throttle position sensor.

Open or short in sub±throttle position sensor circuit.

ECM

TRAC ECU
27When input voltage at ECU terminal VSH is less than
the specification by about 2.5 V for 0.2 sec. or more.

Sub±throttle actuator (return spring is disconnected).

Sub±throttle position sensor.

Open or short in sub±throttle position sensor circuit.

ECM

TRAC ECU
C
OK
Hint
PRemove air cleaner and duct.
Open and close the sub±throttle valve manually and
check the condition during operation.
The valve should operate smoothly without catch-
ing.
If sub±throttle actuator, sub±throttle valve, sub±throttle
position sensor and harness are free from any defect,
ECM may be defective. If ECM is free from any defect,
TRAC ECU may be defective.
±
BRAKE SYSTEM TRACTION CONTROL SYSTEM (TRAC)BR±189
WhereEverybodyKnowsYourName

The speed sensor detects the wheel speed and sends the ap-
propriate signals to the ECU. These signals are used for control
of both the ABS and TRAC control systems. The front and rear
rotors each have 48 serrations. When the rotors rotate, the mag-
netic field emitted by the permanent magnet in the speed sensor
generates an AC voltage. Since the frequency of this AC voltage
changes in proportion to the speed of the rotors (wheels), the
frequency is used by the ECU to detect the speed of each wheel.
BR3583
BR3582
DTC31, 32,
33, 34Speed Sensor Circuit
CIRCUIT DESCRIPTION
DTC No.DTC Detecting ConditionTrouble Area
31, 32,
33, 34
Detection of any of conditions (1) through (3):
(1) At vehicle speed of 10 km/h (6 mph) or more, pulses are not input for 5 sec.
(2) Momentary interruption of the vehicle speed sen- sor signal occurs at least 7 times in the time be-
tween switching the ignition switch ON and switch-
ing it OFF.
(3) Abnormal fluctuation of speed sensor signals with the vehicle speed 20 km/h (12 mph) or more.

Right front, left front, right rear and left rear
speed sensor.

Open or short in each speed sensor circuit.

ECU
Hint: DTC No. 31 is for the right front wheel speed sensor.
DTC No. 32 is for the left front wheel speed sensor.
DTC No. 33 is for the right rear wheel speed sensor.
DTC No. 34 is for the left rear wheel speed sensor.
Fail safe function: If trouble occurs in the speed sensor circuit, the E\
CU prohibits TRAC control.
BR±190
±
BRAKE SYSTEM TRACTION CONTROL SYSTEM (TRAC)
WhereEverybodyKnowsYourName

IN±33).
WIRING DIAGRAM
DTC41IG Power Source Circuit
CIRCUIT DESCRIPTION
This is the power source for the ECU and becomes power source for the CPU an\
d actuators.
DTC No.DTC Detecting ConditionTrouble Area
41
When either of the following (1) or (2) is detected:
(1) Voltage at ECU terminal IG is less than 9.5 V for more than 10 sec. while engine speed is 500 rpm
or higher.
(2) Voltage at ECU terminal IG is more than 17 V for more than 5 sec.

Battery

IC regulator

Open or short in power source circuit

ECU
Fail safe function: If trouble occurs in the power source circuit, the E\
CU prohibits TRAC control.
DIAGNOSTIC CHARTDIAGNOSTIC CHART
BR±192±
BRAKE SYSTEM TRACTION CONTROL SYSTEM (TRAC)
WhereEverybodyKnowsYourName

OKNG
NGOK
OKNG
INSPECTION PROCEDURE
1Check battery positive voltage.
Voltage: 10 ± 14 VCOK
Check and repair the charging system.
2Check voltage between terminals IG and GND of TRAC ECU connector.
C
OK
P1. Remove TRAC ECU with connectors still con-nected.
2. Turn ignition switch on.
Measure voltage between terminals IG and GND of
TRAC ECU connector.
Voltage: 10 ± 14 V
Check and replace TRAC ECU.
3Check continuity between terminals GND of TRAC ECU connector and body gr\
ound.
C
OK
Measure resistance between terminal GND or TRAC
ECU connector and body ground.
Resistance: 1
or less
Repair or replace harness or connector.
±
BRAKE SYSTEM TRACTION CONTROL SYSTEM (TRAC)BR±193
WhereEverybodyKnowsYourName

OKNG
4Check ECU±IG fuse.
C
OK
PRemove ECU±IG fuse from J/B No. 1.
Check continuity of ECU±IG fuse.
Continuity
Check for short in all the harness and components con-
nected to ECU±IG fuse (See attached wiring diagram).
Check for open in harness and connector between
TRAC ECU and battery (See page IN±33).
BR±194±
BRAKE SYSTEM TRACTION CONTROL SYSTEM (TRAC)
WhereEverybodyKnowsYourName

WIRING DIAGRAM
DTC43ABS & TRAC ECU Malfunction
CIRCUIT DESCRIPTION
DTC No.DTC Detecting ConditionTrouble Area
43ABS control system or TRAC (BRAKE) control system
or communication circuit is abnormal.
ABS & TRAC ECU

Open or short in BRFA circuit
DIAGNOSTIC CHARTDIAGNOSTIC CHART
BR±196±
BRAKE SYSTEM TRACTION CONTROL SYSTEM (TRAC)
WhereEverybodyKnowsYourName

NGOK
NGOK
OKNG
INSPECTION PROCEDURE
1Check voltage between terminal BRFA of TRAC ECU and body ground.
C
OK
P1. Remove TRAC ECU with connectors still con-nected.
2. Turn ignition switch ON.
Measure voltage between terminal BRFA of TRAC ECU
and body ground.
Voltage: About 6 V (Pulse)
Check and replace TRAC ECU.
2Check voltage between terminal BRFA of TRAC ECU and body ground with TRAC ECU connec-
tor is disconnected.
C
OK
Hint
PDisconnect TRAC ECU connector.
Measure voltage between terminal BRFA of TRAC ECU
connector and body ground.
Measure voltage within 8 seconds from the time TRAC
ECU connector is disconnected.
Voltage: See step 1.
Check and replace TRAC ECU.
3Check for open and short in harness and connector between ABS & TRAC ECU and TRAC ECU
(See page IN±33).
Repair or replace harness or connector.
Check and replace ABS & TRAC ECU.
±
BRAKE SYSTEM TRACTION CONTROL SYSTEM (TRAC)BR±197
WhereEverybodyKnowsYourName

WIRING DIAGRAM
DTC44NE Signal Circuit
CIRCUIT DESCRIPTION
The TRAC ECU receives engine speed signals (NE signals) from the ECM.
DTC No.DTC Detecting ConditionTrouble Area
44No signal is input to terminal NEO 0.24 sec. after trac-
tion control is initiated.
Open or short in NEO circuit

ECM

TRAC ECU
DIAGNOSTIC CHARTDIAGNOSTIC CHART
BR±198±
BRAKE SYSTEM TRACTION CONTROL SYSTEM (TRAC)
WhereEverybodyKnowsYourName

OKNG
OKNG
INSPECTION PROCEDURES
1Check for open and short in harness and connector between terminal NEO of TRA\
C ECU and
terminal NEO of ECM (See page IN±33).
Repair or replace harness or connector.
2Check voltage between terminal NEO of TRAC ECU and body ground.
C
OK
P1. Remove TRAC ECU with connectors still con-
nected.
2. Turn ignition switch on.
Measure voltage between terminal NEO of TRAC ECU
and body ground for the engine conditions below.
Check and replace TRAC ECU or ECM.
If the same code is still output after the diagnostic code is deleted, che\
ck the contact condition of each
connection.
±
BRAKE SYSTEM TRACTION CONTROL SYSTEM (TRAC)BR±199
WhereEverybodyKnowsYourName

DTC45, 46Main Throttle Position Sensor Circuit
CIRCUIT DESCRIPTION
This circuit is not directly related to the TRAC control, but as it has an i\
nfluence on TRAC control when trouble
occurs in this circuit, it is used to switch off the TRAC system as a fail safe function.
DTC No.DTC Detecting ConditionTrouble Area
45
Either of the following (1) and (2) continues for 0.12
sec.:
(1) IDL switch of main throttle position sensoris ON.
(2) Input voltage of TRAC ECU terminal VTH:1.45 V or
more.
Main throttle position sensor

Open in VTH circuit

Short in IDL1 circuit

ECM

TRAC ECU
46
Either of the following (1) or (2) continues for0.12 sec.:
(1) Input voltage of TRAC ECU terminal VTH:4.3 V or
more.
(2) Input voltage of TRAC ECU terminal VTHwhile
IDL1 switch is OFF: 0.2 V or less
Main throttle position sensor

Short in VTH circuit

ECM

TRAC ECU
BR±200±
BRAKE SYSTEM TRACTION CONTROL SYSTEM (TRAC)
WhereEverybodyKnowsYourName