Lexus SC300 / Lexus SC400. Manual - part 928

NG

OK

3

Check voltage between terminals HTL2, HTR2 of engine & ECT ECU connector and body
ground.

C

OK

Hint

P

(2) Connect engine & ECT ECU connector.

(2) Warm up engine to normal operating temperature.

Measure voltage between terminals HTL2, HTR2 of en-
gine & ECT ECU connector and body ground, when en-
gine is idling and racing at 4,000 rpm.

In the 4,000 rpm racing check , continue engine racing
at 4,000 prm for approx. 20 seconds or more.

Replace sub–oxygen sensor.*

*: In this case, oxygen sensor can be deteriorated.

Check and replace engine & ECT ECU.

TR–86

–

ENGINE TROUBLESHOOTING

Circuit Inspection

–MEMO–

ENGINE TROUBLESHOOTING

TR–87

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) (piezo-
electric effect). This sensor is used to detect the atmospheric (absolute) pressure and outputs corre-
sponding electrical signals. Fluctuations 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 correc-
tions for these fluctuations. If the ECU records diagnostic code ”35”, the fail safe function operates and
the atmospheric pressure is set at a constant 101.3 kPa (760 mmHg, 29.92 in.Hg).

Code No.

Diagnostic Code Detecting Condition

Trouble Area

35

Open or short in HAC sensor circuit for
0.5 sec. or more.

 ECU

DIAGNOSTIC CHART

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

Replace engine (& ECT) ECU.

Go to relevant diag. code chart.

TR–86

–

ENGINE TROUBLESHOOTING

Circuit Inspection

Diag. Code   31

Air Flow Meter Circuit

CIRCUIT DESCRIPTION

As shown in the figure at right, when a pillar (Vortex
generating body) is placed in the path of a uniform flow,
vortices called Karman–Vortex are generated down-
stream of the object. Using this principle, a vortex gen-
erator is placed inside the air flow meter. By measuring
the frequency of the vortices generated, the ECU can
determine the volume of air flowing through the air flow
meter. The vortices are detected by their exerting pres-
sure on thin metal foil (mirror) surfaces and a light emit-
ting element and light receptor (LED and photo transis-
tor) positioned opposite the mirror which senses the
vibrations in the mirror optically. The ECU uses these
signals mainly for calculation of the basic injection vol-
ume and the basic ignition advance angle.

If the ECU records diagnostic code ”31”, it enters fail safe mode (See page 

TR–22

).

TR–88

–

ENGINE TROUBLESHOOTING

Circuit Inspection