Isuzu N-Series. Manual - part 636

ENGINE ELECTRICAL 6D-37

QOSII System Troubleshooting

1. Problems when the water temperature of the engine is 0

°C below.

Notice:
Circuit number is shown in the previous pre-heating chart.

Condition

Cause

Circuit

Glow plug relay will 
not be lit.

Bad connection of Fusible link wire between battery and starter.

3

Bad connection or disconnection of starter SW circuit

Bad connection or disconnection of Fuse No. SBF3
Malfunction of other circuits occurred simultaneously.

2

Bad starter switch.

Bad connection of glow plug relay terminal.

1

Disconnection of glow plug relay excitation coil.

Disconnection of circuit between glow plug relay and timer.

6

Bad glow relay. (The main contact does not pass electricity.)

Bad connection of timer.

Bad timer.

Bad earth circuit of timer.

7

Bad thermoswitch. (It remains ON even in water temperature below 0

°C.)

Earth short-circuiting of thermo switch.

7

Through glow plug 
relay turns ON, pre-
heating is not done.

Bad connection of Fusible link wire between battery and glow relay.

1

Bad connection or disconnection in preheating circuit of glow relay main
contact terminal connector.

Bad connection between plug connector and preheating circuit.

9

Glow plug relay 
turns ON, but does 
not go OFF.

Bad timer.

Bad timer. Short circuiting in circuit or earth between 5 terminal and glow
relay.

6

Bad glow relay.

Indication lamp is 
not lit.

Bad timer.

Burning-out of light bulb. 8

8

N6A3577E

6D-38 ENGINE ELECTRICAL

2. Problems when the time water temperature of the engine is 0

°C or above.

Notice:
Circuit number is shown in the previous pre-heating chart.

3. Burning-out of glow plug

When only one line is burned out, it has no effect on the start up of the engine. Even if one line is burned out,
judging from the characteristics of the glow plug, the impressed voltage will change only slightly.
Therefore, no judgment can be made by normal testing while the glow plug is installed. In order to check for
disconnections and burn-outs, the glow plugs will have to be removed from the connectors and checked one by
one for continuity.

Condition

Cause

Circuit

Indicator lamp is not 
lit

Bad timer

Burning-out of bulb. 8

8

Turn glow relay ON. Bad thermoswitch or disconnection of thermo SW circuit.

(Indicator will light up for 3.5 seconds.)

4

Bad timer.

Bad timer. Short-circuiting in circuit or earth between terminals 5 and glow
relay.

ENGINE ELECTRICAL 6D-39

QOSIII PREHEATING SYSTEM (4JG2 ONLY)

General Description

QOS III preheating system features a quick-on glow plug with thermometer control of the glowing time and the after-
glow time function.
The system consists of a controller, indicator lamp, thermosensor, vehicle speedsensor, dropping resistor, relay (2
pcs), and temperature self-control type glow plug (4 pcs)
With the employment of the thermoswitch, the glow time changes according to the engine coolant temperature, thus
allowing optimum starting conditions to be obtained.
The afterglow time function is controlled by thermosensor, vehicle speed sensor, and the engine runstall sensor
(charge relay).

System Diagram

N6A3720E

6D-40 ENGINE ELECTRICAL

QOS II Timing Chart

N6A3721E