Isuzu engine C22NE / 22LE / 20LE. Service manual — part 57

DRIVEABILITY AND EMISSIONS 6E1-57

On-Board Diagnostic (OBD) System Check (Cont'd)

Step

Action

Value(s)

Yes

No

9

1. Refer to Engine Control Module (ECM) in On-Vehicle

Service.

2. Attempt to display ECM data with the Scan Tool.

Does the Scan Tool display ECM engine data?

-

Go to Step 2

Go to Step 10

10

Replace the EPROM or ECM.

Note) Refer to 6E1-243

-

Verify Repair

-

6E1-58 DRIVEABILITY AND EMISSIONS

DRIVEABILITY AND EMISSIONS 6E1-59

A/C CLUTCH CONTROL CIRCUIT DIAGNOSIS

Circuit Description

When air conditioning and blower fan are selected,

and if the system has a sufficient refrigerant charge, a

12-volt signal is supplied to the A/C request input of

the engine control module (ECM). The A/C request

signal may be temporarily canceled during system

operation by the electronic thermostat in the

evaporator case. The electronic thermostat may

intermittently remove the control circuit ground for the

A/C thermostat relay to prevent the evaporator from

forming ice. When the A/C request signal is received

by the ECM, the ECM supplies a ground from the

compressor clutch relay if the engine operating

conditions are within acceptable ranges. With the A/C

compressor relay energized, battery voltage is

supplied to the compressor clutch coil.

The ECM will enable the compressor clutch to engage

whenever A/C has been selected with the engine

running, unless any of the following conditions are

present:
• The throttle is greater than 95%.
• The engine speed is greater than 6000 RPM.
• The engine coolant temperature (ECT) is greater

than 122°C(252°F).

• Fan switch is OFF.
• A/C reguest switch is OFF.

6E1-60 DRIVEABILITY AND EMISSIONS

• Thermo switch (FRT) is OFF.
• Pressure switch is OFF.
• Ignition switch is OFF.

Diagnostic Aids

To diagnose an intermittent fault, check for the

following conditions:
• Poor connection at the ECM-Inspect harness

connections for backed-out terminals, improper

mating, broken locks, improperly formed or

damaged terminals, and poor terminal-to-wire

connection.

• Damaged harness-Inspect the wiring harness for

damage; shorts to Ground, shorts to Battery

Voltage, and Open circuits. If the harness appears

to be OK, observe the A/C clutch while moving

connectors and wiring harnesses related to the A/C.

A sudden clutch malfunction will indicate the source

of the intermittent fault.

A/C Clutch Diagnosis

This chart should be used for diagnosing the electrical

portion of the A/C compressor clutch circuit. A Scan

Tool will be used in diagnosing the system. The Scan

Tool has the ability to read the A/C request input to the

ECM. The Scan Tool can display when the ECM has

commanded the A/C clutch ON. The Scan Tool should

have the ability to override the A/C request signal and

energize the A/C compressor relay.

A/C Clutch Control Circuit Diagnosis

Step

Action

Value(s)

Yes

No

1

Was the “On-Board Diagnostic (OBD) System Check”

performed?

-

Go to Step 2

Go to OBD

System Check

2

1. Ignition ON, Engine OFF.

2. Review and record the customer complaint.

3. Operate the vehicle within the costomer complaint

conditions as noted.

4. Using the Scan Tool, monitor “DTC” info for DTC 87 or

88.

-

Refer to

Diagnostic Aids

Go to step 3

3

1. Ignition OFF.

2. Remove the A/C Compressor Clutch Relay from the

Underhood Electrical Center.

3. Ignition ON, Engine OFF.

4. Using a Digital Voltmeter (DVM), check for voltage on

the Fused pins of the A/C Compressor Clutch Relay

connector.

Does the DVM read the following value?

12 Volts

Go to Step 5

Go to Step 4

4

Check the suspect circuit(s) between the A/C Compressor

Clutch Relay connector and the Fuse for the following

conditions:
• A short to ground
• An open circuit
• A short to voltage
Was the problem found?

-

Verify Repair

-