Isuzu N-Series. Manual - part 538

Engine Electrical   6D-19

Starting System

General Description 

Circuit Diagram

Legend

1. Engine Control Switch
2. Generator (Integral Regulator)
3. Charge Relay
4. To Heater and A/C Relay
5. Low Charge Indicator Light
6. Glow Plug Relay
7. Glow Plugs

8. Glow Plug Indicator Light
9. ECM

10. Starter Cut Relay

11. To Exhaust Brake Solenoid Valve

12. Starter Relay
13. Starter
14. ECM Main Relay

N6A6586E

6D-20   Engine Electrical

Starter and Glow Plug Relays 

The starter is a 4.5 kilowatt, reduction drive model.
The gear housing and armature end bearing housings
are aluminum. The brush assembly has replaceable
brushes. The commutator bars are mica insulated and
are undercut.
The starter circuit starts at the batteries. The battery
cable goes to the large terminal on the starter.
From that terminal wires lead to the engine control
switch and the starter relay.
From the engine control switch the starter circuit goes
through Fuse #3 and the transmission neutral switch to
the starter relay. When the engine control switch is
turned to start and the transmission neutral switch is
closed, the starter relay closes to complete the circuit
from the batteries to the starter solenoid. The starter
circuit is also connected to the Engine Control Module
(ECM). The Engine Control Module (ECM) operates
when the engine control switch is turned to the “ON”
position.
The starter and glow plug relays are located on the
frame at the left rear side of the cab.

Legend

1. Starter Relay
2. Glow Plug Relay

Maintenance 

Keep the starter’s exterior clean. Remove corrosion
from the terminals, leads, and connectors.
Tighten the starter to engine mounting bolts and the
electrical cable retaining nuts.

Diagnosis of Starting System

1. No Cranking, No Sound

• Discharged batteries. Turn on the headlights and

check the brightness. If the headlights are dim,
charge the batteries. Check for the cause of the
discharged batteries.

• Sulfated battery terminals. Turn on the headlights.

The headlights will be bright. Attempt to start the
engine. The headlights will be very dim or go out.
Clean all the battery terminals and cable ends.

• A starter that draws too much current.
• Starter relay won’t work. Attempt to start and listen

for a “click” at the starter relay. If there is no “click,”
check the starter relay control terminals with a test
light. If the test light shows that power is getting to
the starter relay, check the ground circuit with the
test light. If the ground circuit is OK, replace the
starter relay.

• Engine control switch start circuit won’t close.

With the engine control switch turned to start,
check the starter relay with a test light. If the test
light doesn’t come on, check the starter circuit at
the engine control switch with the test light. If the
test light comes on, check the harness for an open
and repair. If the test light does not come on,
replace the engine control switch.

• Starter relay power circuit isn’t closing. Attempt to

start. The starter relay “clicks.” Check the
secondary terminals with a test light. If the test light
lights on the battery side only, replace the starter
relay. If the test light lights on both terminals, check
for an open at the starter.

• Starter clicks when start is attempted. Check all

power connections for corrosion. Clean
connections if corrosion is present. Terminal on
starter should have at least 18 volts on it when
cranking the engine. If voltage is OK, replace the
starter.

• Starter cut relay

If the problem found, repair the following circuit as
necessary.

- Starter cut relay ground circuit. (Between starter

cut relay and ECM.)

- ECM power supply circuit. (Between fuse 10 A

and starter cut relay circuit.)

• Neutral switch or inhibitor switch 

If the problem found, repair the following circuit as
necessary.

- Neutral switch or inhibitor switch ground circuit.

(Between Frame-LH and neutral switch.)

- Starter relay power supply circuit. (Between

fuse 10 A and starter relay.)

- Starter relay ground circuit. (Between starter cut

relay and diode.)

2. Slow Cranking, Solenoid Clicks or Chatters

1

2

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Engine Electrical   6D-21

• Discharged batteries. Turn on the headlights and

check the brightness. If the headlights are dim,
charge the batteries. Check for cause of
discharged batteries.

• Corrosion on the battery terminals. Check for

corrosion. Hint: Test for warm terminals. Clean all
the battery terminals and connectors.

• Loose or dirty connections. Measure the cranking

voltage at the battery terminals. If the voltage is
less than 18 volts, load test the battery. If the
battery is bad, replace the battery. If the battery is
good, repair the starter.

If the voltage is 18 volts or more, measure the voltage
from the battery NEGATIVE terminal to the engine
block. If the voltage is 0.5 volt or more, repair the
ground cable and connections.
If the voltage is less than 0.5 volt, measure the voltage
at the battery cable terminal on the starter. If the
voltage is 18 volts or more, repair the starter. If the
voltage is less than 18 volts, clean and tighten the
positive cable connections. If the voltage is still less
than 18 volts, replace the positive cable.

On-vehicle Service: Starter System 

Maintenance

Keep starter terminals and all other terminals in the
electrical system clean and tight. A loose or corroded
connection or terminal will cause excessive resistance
in the system that will result in hard starting.
At regular intervals, inspect the starting system to
locate and correct potential causes of trouble before
the system performance is affected.
Starting motors do not require lubrication except during
overhaul.

6D-22   Engine Electrical

Starter

Components

Starting Motor

Legend

1. Magnetic Switch 
2. Gear Case 
3. Torsion Spring 
4. Clutch
5. Pinion
6. Dust Cover

7. Rear Cover
8. Armature 
9. Yoke

10. Brush

11. Through Bolt 

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