1991 LEXUS SC400 heater

NGOK
OKNG
INSPECTION PROCEDURE
1Check voltage between terminal HT of engine (&ECT) ECU connector and body g\
round.
C
OK
P1. Connect the Check Harness A. (See page TR±34).
2. Turn ignition switch on.
Measure voltage between terminal HT of engine (&
ECT) ECU connector and body ground.
Voltage: 10 ± 14 V
Go to step
.
2Check sub±oxygen sensor heater.
C
OK
PDisconnect sub±oxygen sensor connector.
(See page FI±78).
Measure resistance between terminals 1 and 2 of
sub±oxygen sensor connector.
Resistance: 5.1 ± 6.3
20
°C (68°F)
Replace sub±oxygen sensor.
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±79
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Diag. Code 27, 29Sub±Oxygen Sensor Circuit
CIRCUIT DESCRIPTION
The sub±oxygen sensor is installed on the exhaust pipe. Its construction\
and operation is the same as the main
oxygen sensor on page TR±62.
Code No.Diagnostic Code Detecting ConditionTrouble Area
(1) Open or short in heater circuit of sub±oxygen
sensor for 0.5 sec. or more.

Open or short in heater circuit of sub±oxygen
sensor

Sub±oxygen sensor heater.

ECU
27


29(2) Main oxygen sensor signal is 0.45 V or more and sub±oxygen sensor signal is 0.45 V or less
under sonditions (a) and (b).
(2 trip detection logic) *
(a) Coolant temp.: 80 5C (176 5F) or more.
(b) Accel. pedal: Fully depressed for 2 sec. or more.

Open or short in sub±oxygen sensor circuit.

Sub±oxygen sensor

ECU
*: See page TR±21.
HINT: Diag. code 27 is for the left bank sub±oxygen sensor circuit.
Diag. code 29 is for the right bank sub±oxygen sensor circuit.
TR±82
±
ENGINE TROUBLESHOOTING Circuit Inspection
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DIAGNOSTIC CHART
DIAGNOSTIC
CHART
HINT:
wWhen other codes are output in addition to 27 and 29 at the same time, check the ci\
rcuits for other
codes first.
w If diag. code 27 is displayed, check left bank sub±oxygen sensor circ\
uit, and if 29 is displayed,
check right bank sub±oxygen sensor circuit.
WIRING DIAGRAM
Check voltage of terminals HTL2, HTR2.
Check resistance of sub±oxygen sensor heater.
Check and repair sub±oxygen sensor heater
circuit.
Check operation of sub±oxygen sensor heater.
Check and replace ECU. Replace main oxygen sensor.
*: In this case, oxygen sensor can be deteriorated.
Replace main oxygen sensor.*
TR±84±
ENGINE TROUBLESHOOTING Circuit Inspection
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NGOK
OKNG
INSPECTION PROCEDURE
1Disconnect the engine & ECT ECU connector.
C
OK
P(2) Connect the Check Harness A.(See page TR±30)
(2) Disconnect engine & ECT ECU connector.
(2) Turn ignition switch on.
Measure voltage between terminals HTL2, HTR2 of en-
gine & ECT ECU connector and body ground.
Voltage: 10 ± 14 V
Go to step [3].
2Check sub±oxygen sensor heater.
C
OK
PDisconnect sub±oxygen sensor connector.
Measure resistance between terminals 1 and 2 of sub±
oxygen sensor connector.
Resistance: 5.1 ± 6.3
20 C (68 F)
Replace sub±oxygen sensor.
Check and repair harness or connector between
main relay and sub±oxygen sensor, sub±oxygen
sensor and engine & ECT ECU.
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±85
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Page 3 of 11
AIR CONDITIONING EVAPORATOR ODOR ± AC001±97 September 12, 1997
ES 3001. Removal of parts.
('92±'96) (a) Pull down the carpet from the center
console as indicated by red arrow in the
illustration.
(b) Remove the plate on the side of the heater unit using steps and
,
indicated with red arrows to show
direction, in the illustration.
CAUTION:
Do not bend the cable.
NOTE:
The Plate will be reused.
(c) Remove the filter.
(d) Remove the blower controller.
2. Clean the Evaporator. Follow the general procedures given
on page 2.
NOTE:
Location for insertion of spray nozzle
is indicated by the red arrow in the
illustration.
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AIR CONDITIONING EVAPORATOR ± AC001±97 September 12, 1997
ES 3003. Reinstallation of parts.
('92±'96)
(Continued) (a) Insert the filter (up side) into the
heater unit following the numbered
steps shown in the illustration.
(b) Insert the filter (down side) into the heater unit.
(c) Reinstall the plate on the side of the heater unit using steps and
,
indicated with red arrows to show
direction, in the illustration.
CAUTION:
Confirm that the plate is secure.
(d) Restore the carpet to its original position taking care not to bend
the cable.
Page 4 of 11
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226
AUTOMATIC AIR CONDITIONER
1. HEATER BLOWER MOTOR OPERATION
CURRENT IS APPLIED AT ALL TIMES THROUGH HEATER FUSE (60A) TO TERMINAL 5 OF HEATER RELAY. WHEN THE IGNITION SW IS
TURNED ON, CURRENT FLOWS THROUGH HEATER FUSE (10A) TO TERMINAL 1 OF HEATER RELAY " TERMINAL 3 " TERMINAL HR
OF A/C CONTROL ASSEMBLY. AT THE SAME TIME, CURRENT ALSO FLOWS FROM HEATER FUSE (10A) TO TERMINAL IG+ OF A/C
CONTROL ASSEMBLY AND TERMINAL 2 OF WATER VALVE VSV " TERMINAL 1 " TERMINAL WV OF A/C CONTROL ASSEMBLY.
* LOW SPEED OPERATION
WHEN THE BLOWER SW (A/C CONTROL ASSEMBLY) IS PUSHED TO LOW SPEED POSITION, THE CURRENT TO TERMINAL HR OF A/C
CONTROL ASSEMBLY FLOWS TO TERMINAL E OF A/C CONTROL ASSEMBLY " GROUND AND TURNS THE HEATER RELAY. AS A
RESULT, THE CURRENT TO TERMINAL 5 OF HEATER RELAY FLOWS TO TERMINAL 4 OF RELAY " TERMINAL 2 OF BLOWER
RESISTOR " TERMINAL 1 " GROUND AND CAUSES THE BLOWER MOTOR TO ROTATE AT LOW SPEED.
* MEDIUM (M1, M2, M3) AND HIGH SPEED OPERATION
WHEN THE BLOWER SW (A/C CONTROL ASSEMBLY) IS SELECTED TO MEDIUM (M1, M2, M3) OR HIGH POSITION, THE CURRENT TO
TERMINAL HR OF A/C CONTROL ASSEMBLY FLOWS TO TERMINAL E " GROUND AND TURNS THE HEATER RELAY ON. THEN, THE
CURRENT TO TERMINAL IG+ OF A/C CONTROL ASSEMBLY FLOWS TO TERMINAL BLR " TERMINAL (B)2 OF POWER TRANSISTOR
" TERMINAL (A)1 " GROUND. AS A RESULT, THE CURRENT TO TERMINAL 5 OF HEATER RELAY FLOWS TO TERMINAL 4 "
TERMINAL 2 OF BLOWER MOTOR " TERMINAL 1 " TERMINAL (A)2 OF POWER TRANSISTOR " TERMINAL (A)1 " GROUND AND
BLOWER MOTOR IS ROTATED AT SELECTED SPEED BY THE A/C CONTROL ASSEMBLY CONTROLLING THE CURRENT FLOW FROM
TERMINAL (B)2 OF POWER TRANSISTOR TO TERMINAL (B)1.
* AUTO FUNCTION
WHEN THE AUTO SW IN HEATER CONTROL SW (A/C CONTROL ASSEMBLY) IS SELECTED, THE CURRENT FLOW IS THE SAME AS
ABOVE MENTIONS, BUT THE A/C CONTROL ASSEMBLY DECIDES THE APPROPRIATE AIR FLOW VOLUME ACCORDING TO SET
TEMPERATURE AND THE INPUT SIGNALS FROM EACH SENSOR. BY CONTROLLING THE CURRENT FLOW\
FROM TERMINAL BLR OF
THE A/C CONTROL ASSEMBLY TO TERMINAL (B)2 OF POWER TRANSISTOR " TERMINAL (A)1 " GROUND , THE A/C CONTROL
ASSEMBLY CONTROLS THE BLOWER MOTOR STEPLESSLY.
2. OPERATION OF AIR INLET CONTROL SERVO MOTOR
(SWITCHING FROM FRESH TO RECIRC)
WITH IGNITION SW TURNED ON, THE CURRENT FLOWS FROM HEATER FUSE (10A) TO TERMINAL IG+ OF A/C CONTROL ASSEMBLY
" TERMINAL MREC " TERMINAL 6 OF AIR INLET CONTROL SERVO MOTOR " TERMINAL 2 " TERMINAL MFRS OF A/C CONTROL
ASSEMBLY " TERMINAL E " GROUND , THE MOTOR ROTATES AND THE DAMPER MOVES TO THE RECIRC SIDE. WHEN THE
DAMPER OPERATES WITH THE A/C SW AT RECIRC POSITION, THE DAMPER POSITION SIGNAL IS INPUT FROM TERMINAL 5 OF THE
SERVO MOTOR TO TERMINAL TPI OF THE ECU (BUILT INTO THE A/C CONTROL ASSEMBLY). AS A RESULT, CURRENT TO THE SERVO
MOTOR CIRCUIT IS CUT OFF BY THE ECU, SO THE DAMPER STOPS AT THAT POSITION.
(SWITCHING FROM RECIRC TO FRESH)
WITH IGNITION SW TURNED ON, WHEN THE RECIRC/FRESH SW IS SWITCHED TO THE FRESH SIDE, THE CURRENT FLOWS FROM
TERMINAL IG+ OF A/C CONTROL ASSEMBLY " TERMINAL MFRS " TERMINAL 2 OF AIR INLET CONTROL SERVO MOTOR "
TERMINAL 6 " TERMINAL MREC OF A/C CONTROL ASSEMBLY " TERMINAL E " GROUND , THE MOTOR ROTATES AND THE
DAMPER STOPS AT THAT POSITION.
3. OPERATION OF AIR VENT MODE CONTROL SERVO MOTOR
WITH IGNITION SW TURNED ON, THE CURRENT FLOWS FROM HEATER FUSE (10A) TO TERMINAL IG+ OF A/C CONTROL ASSEMBLY.
(SWITCHING FROM DEF TO FACE)
THE CURRENT FLOWS FROM TERMINAL MFACE OF A/C CONTROL ASSEMBLY " TERMINAL 4 OF AIR VENT CONTROL SERVO
MOTOR " TERMINAL 5 " TERMINAL MDEF OF A/C CONTROL ASSEMBLY " TERMINAL E " GROUND . THE MOTOR ROTATES AND
THE DAMPER MOVES TO THE FACE SIDE. WHEN THE DAMPER OPERATES WITH THE A/C SW AT FACE POSITION, THE DAMPER
POSITION SIGNAL IS INPUT FROM TERMINAL 1 OF THE SERVO MOTOR TO THE TERMINAL TPM OF THE ECU (BUILT INTO THE A/C
CONTROL ASSEMBLY). AS A RESULT, CURRENT TO THE SERVO MOTOR CIRCUIT IS CUT OFF BY THE ECU, SO THE DAMPER STOPS
AT THAT POSITION.
(SWITCHING FROM FACE TO DEF)
THE CURRENT FLOWS FROM TERMINAL DEF OF A/C CONTROL ASSEMBLY " TERMINAL 5 OF AIR VENT CONTROL SERVO MOTOR
" TERMINAL 4 " TERMINAL MFACE OF A/C CONTROL ASSEMBLY " TERMINAL E " GROUND , THE MOTOR ROTATES AND THE
DAMPER STOPS AT THAT POSITION.
SYSTEM OUTLINE
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227
4. OPERATION OF AIR MIX CONTROL SERVO MOTOR
WHEN THE TEMPERATURE SW IS TURNED TO THE ªCOOLº SIDE, THE CURRENT FLOWS FROM TERMINAL MC OF A/C CONTROL
ASSEMBLY " TERMINAL 4 OF AIR MIX CONTROL SERVO MOTOR " MOTOR " TERMINAL 5 " TERMINAL MH OF A/C CONTROL
ASSEMBLY " GROUND AND THE MOTOR ROTATES. THE DAMPER OPENING ANGLE AT THIS TIME IS INPUT FROM TERMINAL 1 OF
SERVO MOTOR TO TERMINAL TP OF A/C CONTROL ASSEMBLY, THIS IS USED TO DETERMINE THE DAMPER STOP POSITION AND
MAINTAIN THE SET TEMPERATURE.
WHEN THE TEMPERATURE CONTROL SW IS TURNED TO THE ªHOTº SIDE, THE CURRENT FLOWS FROM SERVO MOTOR "
TERMINAL MH OF A/C CONTROL ASSEMBLY " TERMINAL 5 OF AIR MIX CONTROL SERVO MOTOR " MOTOR " TERMINAL 1 "
TERMINAL MC OF A/C CONTROL ASSEMBLY, ROTATING THE MOTOR IN REVERSE AND SWITCHING THE DAMPER FROM ªCOOLº TO
ªHOTº SIDE.
5. AIR CONDITIONER OPERATION
THE A/C CONTROL ASSEMBLY RECEIVES VARIOUS SIGNALS, I.E., THE ENGINE RPM FROM THE IGNITER, OUTLET TEMPERATURE
SIGNAL FROM THE A/C THERMISTOR, COOLANT TEMPERATURE FROM THE WATER TEMP. SENSOR AND THE LOCK SIGNAL FROM
THE A/C COMPRESSOR, ETC.
WHEN THE ENGINE IS STARTED AND THE A/C SW (A/C CONTROL ASSEMBLY) IS ON, A SIGNAL IS INPUT TO THE ECU. (BUILT IN THE
A/C CONTROL ASSEMBLY).
AS A RESULT, THE GROUND CIRCUIT IN A/C CONTROL ASSEMBLY IS CLOSED AND CURRENT FLOWS FROM HEATER FUSE (10A) TO
TERMINAL 2 OF MAGNETIC CLUTCH RELAY " TERMINAL 3 " TERMINAL ACMG OF ENGINE AND ECT ECU " TERMINAL E OF A/C
CONTROL ASSEMBLY " GROUND , TURNING THE MAGNETIC CLUTCH RELAY ON, SO THAT THE MAGNETIC CLUTCH IS ON AND THE
A/C COMPRESSOR OPERATES.
AT THE SAME TIME, THE ENGINE AND ECT ECU DETECTS THE MAGNETIC CLUTCH IS O\
N AND THE A/C COMPRESSOR OPERATES.
OPEN DIRECTION TO AVOID LOWERING THE ENGINE RPM DURING A/C OPERATING.
WHEN ANY OF THE FOLLOWING SIGNALS ARE INPUT TO THE A/C CONTROL ASSEMBLY, THE CONTROL ASSEMBLY OPERATES TO
TURN OFF THE AIR CONDITIONING.
* ENGINE HIGH TEMP. SIGNAL IS HIGH.
* COOLANT HIGH TEMP. SIGNAL IS HIGH.
* A SIGNAL THAT THE TEMPERATURE AT THE AIR OUTLET IS LOW.
* A SIGNAL THAT THERE IS A LARGE DIFFERENCE BETWEEN ENGINE RPM AND COMPRESSOR RPM.
* A SIGNAL THAT THE REFRIGERANT PRESSURE IS ABNORMALLY HIGH OR LOW.
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