Isuzu KB P190. Manual - part 319

6E-242  Engine Control System (4JH1) 

DTC A/C Compressor System Check 

Description 
When air conditioning (A/C) 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 to prevent the evaporator icing. Also, it is 
cancelled when the pressure switch detected abnormal
pressure in the line. When the A/C request signal is 
received by the ECM, the ECM supplies a ground from
the A/C compressor relay if the engine operating
conditions are within acceptable ranges. With the A/C
compressor relay energized, 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 and enable conditions
are met. 

 

Condition for Running the A/C Compressor: 

•  The engine is running. 
•  The A/C switch is ON. 
•  The engine coolant temperature (ECT) is less than 

105

°C(221°F). 

 

A/C Compressor System Check 
Schematic Reference: Engine Controls Schematics 
Connector End View Reference: Engine Controls
Connector End Views or Engine Control Module (ECM)
Connector End Views 

 
Chart 1 of 2 

Step Action 

Value(s) 

Yes 

No 

1 

Did you perform the Diagnostic System Check-
Engine Controls? 

 

Go to Step 2 

Go to Diagnostic 

System Check – 

Engine Controls 

2 

1.  Install the scan tool. 
2.  Turn OFF the ignition for 30 seconds. 
3.  Turn ON the ignition, with the engine OFF. 
4. Monitor the Diagnostic Trouble Code (DTC) 

Information of the engine control system with the 
scan tool and check whether following DTC(s) is 
set: 

•  P0115 (Symptom Code 1 & 2) 
•  P0645 (Symptom Code 4 & 8) 

Are any of the above DTC(s) set? 

 

Refer to Applicable 

DTC 

Go to Step 3 

3 

1.  Turn ON the ignition, with the engine OFF. 
2.  Turn ON the blower motor switch. 
Does the blower motor turn ON and operate 
correctly? 

 

Go to Step 4 

Refer to Applicable 
Diagnostic Chart in 

Heating & Air 

Conditioning 

Section 

4 

1.  Turn OFF the blower motor switch. 
2.  Keep the A/C switch OFF. 
Does the A/C Request Signal parameter indicate 
OFF? 

 

Go to Step 5 

Go to Chart 2 of 2 

with heater Step 1

 OR 

Go to Chart 2 of 2 

without heater Step 

1 

5 

1.  Start the engine and let the engine idle. 
2.  Keep the blower motor switch OFF. 
3.  Keep the A/C switch OFF. 
Does the A/C compressor clutch keep OFF 
(disengage)? 

 

Go to Step 6 

Go to Step 11  

6 

1.  Turn ON the blower motor switch. 
2.  Keep the A/C switch OFF. 
Does the A/C compressor clutch keep OFF 
(disengage)? 

 

Go to Step 7 

Go to Chart 2 of 2 

with heater Step 7

 OR 

Go to Chart 2 of 2 

without heater Step 

4 

Engine Control System (4JH1)  6E-243 

Step Action 

Value(s) 

Yes 

No 

7 

1.  Turn OFF the blower motor switch. 
2.  Turn ON the A/C switch. 
Does the A/C compressor clutch keep OFF 
(disengage)? 

 

Go to Step 8 

Go to Chart 2 of 2 

with heater Step 8

 OR 

Go to Chart 2 of 2 

without heater Step 

10  

8 

1.  Turn ON the blower motor switch. 
2.  Keep the A/C switch ON. 
Does the A/C compressor clutch ON and OFF 
(engage and disengage) periodically within the 
Condition for Running the A/C Compressor? 

 

Go to Step 9 

Go to Step 10 

9 

Does the air flow cool enough. 

 

System OK 

Refer to Applicable 
Diagnostic Chart in 

Heating & Air 

Conditioning 

Section 

10 

Does the A/C Relay Command parameter indicate 
ON at previous step? 

 

Go to Step 14 

Go to Chart 2 of 2 

with heater Step 9

 OR 

Go to Chart 2 of 2 

without heater Step 

11 

11 

1.  Turn OFF the ignition. 
2.  Disconnect the A/C compressor clutch harness 

connector (E-3). 

3.  Start the engine. 
Does the A/C compressor clutch ON (engage)? 

 

Go to Step 22 

Go to Step 12 

12 

1.  Turn OFF the ignition. 
2. Remove the A/C compressor relay from the 

engine room relay block. 

3. Connect a test lamp between the A/C 

compressor clutch voltage supply circuit (pin 2 of 
X-14 connector) and a known good ground. 

4.  Turn ON the ignition, with the engine OFF. 
Does the test lamp illuminate?  

 

Go to Step 13 

Go to Step 21 

13 

Repair the short to battery or ignition voltage on the 
A/C compressor voltage feed circuit between the 
A/C compressor relay (pin 2 of X-14 connector) and 
the A/C compressor (E-3). 
Did you complete the repair? 

 

Go to Step 23 

 

14 

1.  Turn OFF the ignition. 
2. Remove the A/C compressor relay from the 

engine room fuel block. 

3.  Connect a test lamp between the voltage feed 

circuit of the relay switch side terminal (pin 1 of 
X-14 connector) and known good ground. 

4.  Turn ON the ignition, with the engine OFF. 
Does the test lamp illuminate? 

 

Go to Step 15 

Go to Step 17 

15 

1.  Keep the ignition ON, with the engine OFF. 
2.  Disconnect the test lamp. 
3.  Connect a 3-amp fused jumper wire between 

both switch side terminals of the A/C compressor 
relay (pins 1 and 2 of X-14 connector). 

Does the A/C compressor clutch click (engage)?  

 

Go to Step 19 

Go to Step 16 

6E-244  Engine Control System (4JH1) 

Step Action 

Value(s) 

Yes 

No 

16 

1.  Keep the ignition ON, with the engine OFF. 
2.  Keep 3-amp fused jumper wire with connected. 
3. Disconnect A/C compressor clutch harness 

connector. 

4.  Connect a test lamp between A/C compressor 

voltage feed circuit (E-3) and a known good 
ground. 

Does the test lamp illuminate? 

 

Go to Step 20 

Go to Step 18  

17 

Repair the open circuit or high resistance on the 
relay switch side voltage feed circuit between the 
A/C compressor relay (pin 1 of X-14 connector) and 
the A/C (10A) fuse (EB-13). 
Did you complete the repair? 

 

Go to Step 23 

 

18 

Repair the open circuit or high resistance on the 
relay switched voltage supply circuit between the 
A/C compressor relay (pin 2 of X-14 connector) and 
the A/C compressor clutch (pin 1 of E-3 connector). 
Did you complete the repair? 

 

Go to Step 23 

 

19 

1.  Turn OFF the ignition. 
2. Inspect for an intermittent and for poor 

connections at the A/C compressor relay 
terminals (pins 3 and 5 of X-14 connector). 

3.  Repair the connection(s) as necessary. 
Did you find and correct the condition? 

 

Go to Step 23 

Go to Step 21 

20 

1.  Turn OFF the ignition. 
2. Inspect for an intermittent and for a poor 

connection at the harness connector of the A/C 
compressor clutch (E-3). 

3.  Repair the connection(s) as necessary. 
Did you find and correct the condition? 

 

Go to Step 23 

Go to Step 22 

21 

Replace the A/C compressor relay. 
Did you complete the replacement? 

 

Go to Step 23 

 

22 

Replace the A/C compressor. Refer to Heating and 
Air Conditioning section. 
Did you complete the replacement? 

 

Go to Step 23 

 

23  1. Reconnect all previously disconnected fuse, 

relay or harness connector(s). 

2.  Turn OFF the ignition for 30 seconds. 
3.  Turn ON the ignition, with the engine OFF. 
4.  Turn ON the blower motor switch. 
Does the blower motor turn ON and operate 
correctly? 

 

Go to Step 4 

Refer to Applicable 
Diagnostic Chart in 

Heating & Air 

Conditioning 

Section 

 
Chart 2 of 2 with heater 

Step Action 

Value(s) 

Yes 

No 

1 

1.  Turn OFF the ignition. 
2.  Replace the thermo relay with the horn relay or 

replace with a known good relay. 

3.  Turn OFF the blower motor switch. 
4.  Turn OFF the A/C switch. 
5.  Turn ON the ignition, with the engine OFF. 
Does the A/C Request Signal parameter indicate 
OFF? 

 

Go to Step 33 

Go to Step 2 

Engine Control System (4JH1)  6E-245 

Step Action 

Value(s) 

Yes 

No 

2 

1.  Turn OFF the ignition. 
2.  Remove the thermo relay from the engine room 

relay block. 

3.  Turn ON the ignition, with the engine OFF. 
Does the A/C Request Signal parameter indicate 
OFF?  

 

Go to Step 3 

Go to Step 4 

3 

1.  Turn OFF the ignition. 
2.  Reinstall the thermo relay. 
3.  Remove the radiator grille in order to access the 

pressure switch. 

4. 

Disconnect the pressure switch harness 
connector (C-24). 

5.  Turn OFF the blower motor switch. 
6.  Turn OFF the A/C switch. 
7.  Turn ON the ignition, with the engine OFF. 
Does the A/C Request Signal parameter indicate 
OFF? 

 

Go to Step 6 

Go to Step 5 

4 

Repair the short to battery or ignition voltage on the 
thermo relay signal input circuit between the thermo 
relay (pin 1 of X-15 connector) and the engine 
control module (ECM) (pin 33 of C-56 connector). 
Did you complete the repair? 

 

Go to Step 39 

 

5 

Repair the short to ground between the thermo relay 
(pin 5 of X-15 connector) and pressure switch (pin 1 
of C-24 connector). 
Did you complete the repair? 

 

Go to Step 39 

 

6 

Repair the short to ground between the pressure 
switch (pin 2 of C-24 connector) and the electronic 
thermostat (pin 2 of C-55 connector), then electronic 
thermostat (pin 1 of C-56 connector) and the A/C 
switch (pin 5 of B-57 connector). 
Did you complete the repair? 

 

Go to Step 39 

 

7 

1.  Test the A/C switch circuits shorted each other. 

(pins 1 and 5 of B-57 connector). 

2.  Repair the circuits as necessary. 
Did you find and correct the condition? 

 

Go to Step 39 

Go to Step 36 

8 

1.  Test the circuit between the blower motor switch 

(pin 1 of B-13 connector) and the A/C switch (pin 
1 of B-57 connector) for a short to ground. 

2.  Repair the circuits as necessary. 
Did you find and correct the condition? 

 

Go to Step 39 

Go to Step 37 

9 

Inspect the A/C (10A) fuse (EB-13) in the engine 
room fuse block. 
Is the A/C (10A) fuse (EB-13) open? 

 

Go to Step 10 

Go to Step 11 

10 

1.  Replace the A/C (10A) fuse (EB-13). If the fuse 

continues to open, repair the short to ground on 
a circuit fed by the fuse or check for a shorted 
attached component. 

2. Repair the short to ground or replace the 

component as necessary. 

Did you complete the repair? 

 

Go to Step 39 

