Isuzu N-Series. Manual - part 1148

8-304 CAB AND CHASSIS ELECTRICAL

Inspection of waveform by oscilloscope

1. Connect a resistance of 1.3 to 5k ohm (1.4W or more) between the harness side connectors 8 B-51 and 1 B-55

of the meter.

2. Install a speedometer tester.
3. Turn on the starter SW.
4. Check the waveform at the time when the vehicle speed is at 60 Km (37 mph).

1 2 3 4 5 6 7 8

9 10 11 12 13 14

1 2 3 4 5

6 7 8 9 10 11 12

B-51

B-55

N8A5562E

Normal waveform

0V

23.6 m sec. 

2V or less

Approximate battery voltage

N8A5367E

CAB AND CHASSIS ELECTRICAL 8-305

2. Tachometer
2-1. Tachometer Does Not Function

2-2. Tachometer Needle Fluctuates (May Be Wide Fluctuation)

2-3. Tachometer Needle Jumps Erratically

N8A5368E

N8A5368E

• IS THE PULSE INPUT NORMAL?

YES

NO

REPAIR A POOR CONNECTION AT THE
CONNECTORS IN THE CIRCUIT OR A FAILURE IN
THE HARNESS, OR REPLACE ENGINE SPEED
SENSOR

REPLACE THE TACHOMETER

N8A0307E

8-306 CAB AND CHASSIS ELECTRICAL

3. Coolant Temperature Gauge
3-1. Coolant Temperature Gauge Needle Does Not Move

• IS THE CONNECTOR OF THE THERMO UNIT

CONNECTED SECURELY?

• DISCONNECT THE CONNECTOR OF THE THERMO

UNIT

• CONNECT A 3.4W BULB BETWEEN THE HARNESS

SIDE CONNECTOR 1  E-7  AND THE GROUND OF THE
THERMO UNIT

• WHEN THE STARTER SW IS TURNED ON, DOES THE

TEMPERATURE GAUGE NEEDLE MOVE ABOUT 10
SECONDS AFTER THAT?

• IS THE THERMO UNIT NORMAL?

YES

NO

REPAIR A POOR
CONNECTION AT THE
CONNECTOR

REPLACE THE
THERMO UNIT

YES

YES

NO

YES

• IS THERE A BROKEN

WIRE IN THE PRINTED
CIRCUIT OF THE
METER ASSEMBLY?

YES

NO

REPLACE THE PRINTED
CIRCUIT BOARD

CHECK THE COOLANT
TEMPERATURE GAUGE
AND REPLACE IT WITH A
CORRECT ONE IF
NECESSARY

• REPAIR A POOR

CONNECTION AT THE
CONNECTORS OR AN
OPEN CIRCUIT IN THE
CIRCUIT

NO

• IS THE CIRCUIT BETWEEN THE TEMPERATURE

GAUGE AND THE THERMO UNIT NORMAL?

NO

CONNECT THE CONNECTOR SECURELY

N8A0308E

CAB AND CHASSIS ELECTRICAL 8-307

3-2. Coolant Temperature Gauge Reading Is Too Low (or High)

• DOES THE THERMOSTAT OPERATE NORMALLY?

• TURN THE STARTER SWITCH ON

• WHEN INSERTING AND PULLING OUT THE

THERMOSTAT CONNECTOR 1  E-7  SEVERAL
TIMES, DOES THE INDICATION OF THE COOLANT
TEMPERATURE GAUGE VARY?

• IS THE THERMO UNIT NORMAL?

REPAIR A POOR CONNECTION AT THE CONNECTOR
OF THE THERMO UNIT

• IS THERE AN OPEN CIRCUIT IN THE METER

ASSEMBLY PRINTED CIRCUIT BOARD?

REPLACE THE THERMO UNIT

YES

YES

NO

YES

NO

YES

NO

REPLACE THE PRINTED CIRCUIT BOARD

CHECK THE COOLANT TEMPERATURE GAUGE AND
REPLACE IT IF FOUND DEFECTIVE

NO

REPLACE THE THERMOSTAT

N8A0309E