Lexus SC300 / Lexus SC400. Manual — part 876

DIAGNOSTIC CHART

DIAGNOSTIC CHART

WIRING DIAGRAM

Check and replace ECU.

Replace air flow meter.

Check and replace ECU.

Check voltage of air flow meter power source.

Check voltage terminal KS of ECU.

Check for open and short in harness and
connector between air flow meter and ECU.

Check voltage of terminal KS.

Repair or repolace harness or
connector

When diag. code 31 is displayed,
check and replace ECU.

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–89

NG

OK

OK

NG

INSPECTION PROCEDURE

1

Check voltage between terminals KS and E1 of engine & ECT ECU connector.

C

OK

P

Connect the check harness A.
(See page

TR–30

)

Measure voltage between terminals KS and E1 of en-
gine & ECT ECU connector while engine is cranked.

Voltage:

2.0 – 4.0 V
(Neither 0 V nor 5 V)

When diag. code 31 is displayed, check and replace en-
gine & ECT ECU.

2

Check for open and short in harness and connector between engine & ECT ECU and air flow
meter (See page

IN–27

).

Repair or replace harness or connector.

TR–90

–

ENGINE TROUBLESHOOTING

Circuit Inspection

OK

NG

NG

OK

3

Disconnect air flow meter connector and check voltage between terminals KS and E1 of engine
& ECT ECU.

C

OK

P

Diconnect the air flow meter connector.

Turn ignition switch on.

Measure voltage between terminals KS and E1 of en-
gine & ECT ECU.

Voltage:

4 – 6 V

Check and replace ECU.

4

Diconnect air flow meter connector and check voltage between terminals VC and E1 of engine
& ECT ECU.

C

OK

P

Diconnect the air flow meter connector.

Turn ignition switch on.

Measure voltage between terminals VC and E1 of en-
gine & ECT ECU.

Voltage:

4 – 6 V

Replace air flow meter.

Check and replace engine & ECT ECU.

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–91

Diag. Code 35

High Altitude Compensator Sensor
(HAC Sensor) Circuit

CIRCUIT DESCRIPTION

The HAC sensor is built into the ECU. This is a semiconductor pressure sensor with properties which cause its
electrical resistance to change when stress is applied to the sensor’s crystal (silicon) (piezoelectric effect). This
sensor is used to detect the atmospheric (absolute) pressure and outputs corresponding electrical signals. Fluc-
tuations in the air pressure cause changes in the intake air density, which can cause deviations in the air–fuel
ratio. The signals from HAC sensor are used to make corrections for these fluctuations. If the ECU records diag-
nostic code ”35”, the fail safe function operates and the atmospheric pressure is set at a constant 760 mmHg.

Code No.

Diagnostic Code Detecting Condition

Trouble Area

35

Open or short detected in HAC sensor circuit for 0.5

sec. or more.

ECU

DIAGNOSTIC CHART

DIAGNOSTIC CHART

Are there any other codes (besides Code 16)
being output?

Replace engine & ECT ECU.

Go to relevant diag. code chart.

TR–92

–

ENGINE TROUBLESHOOTING

Circuit Inspection