Isuzu KB P190. Manual - part 320

6E-246  Engine Control System (4JH1) 

Step Action 

Value(s) 

Yes 

No 

11 

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

relay block. 

3.  Connect a test lamp between the voltage feed 

circuit (pins 1 and 3 of X-15 connector) and a 
known good ground. 

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

 

Go to Step 12 

Go to Step 20 

12 

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.  Connect a 3-amp fused jumper wire between the 

pressure switch harness (pin 1 of C-24 
connector) and a known good ground. 

6.  Turn ON the ignition, with the engine OFF. 
Does the thermo relay click (engage)? 

 

Go to Step 13 

Go to Step 15 

13 

Does the A/C Request Signal Request parameter 
indicate ON? 

 

Go to Step 16 

Go to Step 14 

14 

1.  Turn OFF the ignition. 
2.  Keep the 3-amp fused wire with connected. 
3.  Disconnect the ECM C-56 harness connector. 
4.  Connect a test lamp between the thermo relay 

input circuit (pin 33 of C-56 connector) and a 
known good ground. 

5.  Turn ON the ignition, with the engine OFF. DO 

NOT START the engine. 

Does the test lamp illuminate? 

 

Go to Step 32 

Go to Step 21  

15 

1.  Turn OFF the ignition. 
2.  Keep the 3-amp fused wire with connected. 
3.  Replace the thermo relay with the horn relay or 

replace with a known good relay. 

4.  Connect a 3-amp fused jumper wire between the 

pressure switch harness (pin 1 of C-24 
connector) and a known good ground. 

5.  Turn ON the ignition, with the engine OFF. 
Does the thermo relay click (engage)? 

 

Go to Step 27 

Go to Step 22 

16 

1.  Turn OFF the ignition. 
2.  Connect the pressure switch harness connector. 
3. Remove the grove box in order to access 

electronic thermostat. 

4. Disconnect the electronic thermostat harness 

connector (C-55). 

5.  Connect a 3-amp fused jumper wire between the 

electronic thermostat voltage feed circuit (pin 2 of 
C-55 connector) and a known good ground. 

6.  Turn ON the ignition, with the engine OFF. 
Does the thermo relay click (engage)? 

 

Go to Step 17  

Go to Step 23 

17 

1.  Turn OFF the ignition. 
2. Connect a test lamp between the electronic 

thermostat terminal (pin 1 of C-55 connector) and 
battery voltage. 

3.  Turn ON the blower motor switch. 
4.  Turn ON the A/C switch. 
Does the test lamp illuminate? 

 

Go to Step 18 

Go to Step 19 

Engine Control System (4JH1)  6E-247 

Step Action 

Value(s) 

Yes 

No 

18 

1.  Turn OFF the ignition. 
2. Connect a test lamp between the electronic 

thermostat voltage feed circuit (pin 3 of C-55 
connector) and a known good ground. 

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

 

Go to Step 29 

Go to Step 25 

19 

1.  Turn OFF the ignition. 
2. Remove the center cluster bezel in order to 

remove HVAC control lever. 

3.  Disconnect the A/C switch harness connector (B-

57). 

4.  Connect a test lamp between the A/C switch 

ground harness (pin 1 of B-57 connector) and 
battery voltage. 

5.  Turn ON the blower motor switch. 
Does the test lamp illuminate? 

 

Go to Step 24 

Go to Step 26 

20 

Repair the open circuit or high resistance on the 
thermo relay voltage feed circuits between the 
thermo relay (pin 2 of X-15 connector) and ECM 
(10A) fuse (EB-4), then thermo relay (pin 3 of X-15 
connector) and A/C (10A) fuse (EB-13). 
Did you complete the repair? 

 

Go to Step 39 

 

21 

Repair the open circuit or high resistance on thermo 
relay input circuit between the ECM (pin 33 of C-56 
connector) and the thermo relay (pin 1 of X-15 
connector). 
Did you complete the repair? 

 

Go to Step 39 

 

22 

Repair the open circuit or high resistance on the 
pressure switch voltage feed circuit between the 
thermo relay (pin 4 of X-15 connector) and pressure 
switch (pin 1 of C-24) connector. 
Did you complete the repair? 

 

Go to Step 39 

 

23  1. Test the voltage feed circuit between the 

pressure switch (pin 2 of C-24 harness 
connector) and electronic thermostat (pin 2 of C-
55 connector) for an open or high resistance. 

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

 

Go to Step 39 

Go to Step 28 

24 

1.  Test the electronic thermostat switching circuit 

between the electronic thermostat (pin 1 of C-55 
connector) and the A/C switch (pin 5 of B-57 
connector) for an open or high resistance. 

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

 

Go to Step 39 

Go to Step 30 

25 

Repair the open or high resistance on the voltage 
feed circuit on the electronic thermostat between the 
A/C (10A) fuse (EB-13) and the electronic 
thermostat (pin 3 of C-55 connector). 
Did you complete the repair? 

 

Go to Step 39 

 

26 

1.  Test the A/C switch ground circuit between the 

A/C switch (pin 1 of B-57 connector) and blower 
motor switch (pin 1 of B-13 connector) for an 
open or high resistance. 

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

 

Go to Step 39 

Go to Step 31 

6E-248  Engine Control System (4JH1) 

Step Action 

Value(s) 

Yes 

No 

27 

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

connections at the thermo relay terminals (pins 
1, 2, 3 and 4 of X-15 connector). 

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

 

Go to Step 39 

Go to Step 33 

28 

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

connections at the harness connector of the 
pressure switch (pins 1 and 2 of C-24 connector).

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

 

Go to Step 39 

Go to Step 34 

29 

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

connections at the harness connector of the 
electronic thermostat (pins 1, 2 and 3 of C-55 
connector). 

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

 

Go to Step 39 

Go to Step 35 

30 

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

connections at the harness connector of the A/C 
switch (pins 1 and 5 of B-57 connector). 

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

 

Go to Step 39 

Go to Step 36 

31 

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

connection at the harness connector of the 
blower motor switch (pin 1 of B-13 connector). 

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

 

Go to Step 39 

 Go to Step 37 

32 

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

connection at the harness connector of the ECM 
(pin 33 of C-56 connector). 

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

 

Go to Step 39 

Go to Step 38 

33 

Replace the thermo relay. 
Did you complete the replacement? 

 

Go to Step 39 

 

34 

Replace the pressure switch. Refer to Heating and 
Air Conditioning section. 
Did you complete the replacement? 

 

Go to Step 39 

 

35 

Replace the electronic thermostat. Refer to Heating 
and Air Conditioning section. 
Did you complete the replacement? 

 

Go to Step 39 

 

36 

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

 

Go to Step 39 

 

37 

Replace the blower motor switch. Refer to Heating 
and Air Conditioning section. 
Did you complete the replacement? 

 

Go to Step 39 

 

38 

Important: Replacement ECM must be 
programmed. 
Replace the ECM. Refer to Engine Control Module 
(ECM) Replacement in this section. 
Did you complete the replacement? 

 

Go to Step 39 

 

Engine Control System (4JH1)  6E-249 

Step Action 

Value(s) 

Yes 

No 

39  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 Char 1 of 2 

Step 4 

Refer to Applicable 
Diagnostic Chart in 

Heating & Air 

Conditioning 

Section 

 
Chart 2 of 2 without 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 37 

Go to Step 2 

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 5 

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 7 

Go to Step 6 

4 

1.  Turn OFF the ignition. 
2.  Remove the glove box in order to access the 

electronic thermostat. 

3.  Back probe a test lamp between the electronic 

thermostat harness (pin 2 of C-55 connector) and 
battery voltage. 

4.  Turn ON the blower motor switch. 
5.  Turn OFF the A/C switch. 
6.  Keep the ignition OFF. 
Does the test lamp illuminate?  

 

Go to Step 8 

Go to Step 9 

5 

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 43 

 

6 

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 43 

