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
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