Lexus SC300 / Lexus SC400. Manual - part 29

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Back–up Power Source Circuit

CIRCUIT DESCRIPTION

This is the backup power source for the air conditioner control assembly. Power is supplied even when
the ignition switch is off and is used for diagnostic code memory, etc.

DIAGNOSTIC CHART

WIRING DIAGRAM

Check voltage between terminal B of air
conditioner control assembly connector
and body ground.

Check DOME fuse.

Proceed to next circuit inspection

shown on matrix chart (See page

AC-36

).

Check and repair harness and connector
between air conditioner control assembly
and battery.

Check for short all in the harness 
and components connecteds to 
the DOME fuse (See attached wiring
diagram)

AC–74

–

AIR CONDITIONING SYSTEM

Troubleshooting

Check  voltage between terminals B of air conditioner control assembly

connector and body ground.

(1)

Remove console upper panel.(See page
 

BO-111

).

(2)

Remove A/C control assembly with connec–
tors still connected.

Measure voltage between terminals B of air  condi-
tioner control assembly connector and body 
ground.

Voltage:

10 – 14 V

Proceed to next circuit inspection shown on matrix
chart (See page 

AC-36

).

Check DOME fuse.

Remove DOME fuse from R/B No. 2.

Check continuity of DOME fuse.

Continuity.

Check for short in all the harness and 
components connected to the DOME fuse
(See attached wiring diagram).

Check and repair harness and connector between  air conditioner control assembly and battery.

INSPECTION PROCEDURE

–

AIR CONDITIONING SYSTEM

Troubleshooting

AC–75

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Heater Main Relay Circuit

CIRCUIT DESCRIPTION

The heater main relay is switched on by signals from the air conditioner control assembly.
It supplies power to the blower motor.

DIAGNOSTIC CHART

Check voltage between terminals HR of  air

conditioner control assembly connector

and body ground.

Check heater main relay.

Check HTR fuse.

Check and repair  harness  and connector

between air conditioner control assembly

and battery.

Proceed to next circuit inspection
shown on matrix chart (See page

AC-36

).

Check for short in all the harness
and components connected to
the HTR fuse (See 
attached wiring diagram).

Replace heater main relay.

WIRING DIAGRAM

AC–76

–

AIR CONDITIONING SYSTEM

Troubleshooting

Check  voltage between terminals HR of air conditioner control assem-
bly  connector and body ground.

(1)

Remove console upper panel.(See page 

BO-111

).

(2)

Remove A/C control assembly with connec–
tors still connected.

Proceed to next circuit inspection shown on ma-
trix chart (See page 

AC-36

).

Check heater main relay.

Check continuity between each pair of terminals of
heater main relay shown below.

Measure voltage between terminals HR of air condi-
tioner control assembly and body ground when
ignition switch is on and off.

(1)

Apply battery voltage between terminals 1
and 3

(2)

Check continuity between each pair of ter–
minal shown below.

Replace  heater main relay.

Remove HTR fuse from J/B No. 1.

Check continuity of HTR fuse

Continuity

Check for short in all the harness and 
components connected to the HTR fuse
See attached wiring diagram).

Check and repair harness and connector between  air conditioner control assembly and battery.

INSPECTION PROCEDURE

–

AIR CONDITIONING SYSTEM

Troubleshooting

AC–77