Glow Plug System: Description and Operation
FUNCTION OF PREGLOW SYSTEM
GENERAL
On a diesel engine, combustion occurs when the fuel is injected into the highly compressed and thus greatly heated combustion air and self- ignites.
When the engine is cold, the self-ignition temperature is not reached by compression alone. It is therefore necessary to provide a preglow system to raise the temperature of the compressed air, enabling the cold engine to start by igniting fuel particles on the glow plugs.
The duration of preglow depends on the ambient temperature.
DESIGN OF THE QUICK-START PENCIL-TYPE GLOW PLUGS
The pencil-type glow plugs consist essentially of a housing with a M12 x 1.25 external thread and a heating element press-fitted into the housing.
The single-pin connecting pin is screwed into the housing by a non-detachable brass round nut. The pencil-type glow plugs are designed for a voltage of 11.5 Volts and are connected in parallel.
The heating element consists of a series connected heating and control winding.
1 Control winding
2 Heating winding
When the glow system is switched on, a current of approx. 30 amperes flows to each glow plug. The heating winding heats the glow plug very rapidly. The control wiring increases its resistance as the temperature rises and limits the current to approximately 8-15 amperes. The glow plug is thus protected against overload.
Curvent curve of quick-start pencil-type Glow plug
After a glow period of 9 seconds, a heating element temperature of 900 °C is reached, the maximum temperature of 1180 °C being reached after 30 seconds.
Temperature curve at quick-start pencil-type glow plug.
TABLE OF ASSIGNMENT OF PREGLOW TIME RELAY - ENGINE - MODEL
LOCATION OF PREGLOW TIME RELAY
The preglow time relay is housed in the engine compartment on the left wheelhouse. The electrical connections are accessible after removing the protective cap.
On Model 140 the preglow time relay must be taken out in order to be able to remove the protective cap.
Shown on Model 201
FUNCTIONS OF PREGLOW TIME RELAY
The preglow time relay has the following functons:
- Switching on the glow current
- Indicating readiness to start
- Safety cutout
- Fault indication
PREGLOW WITHOUT AFTERGLOW MODEL 124 ENGINE 603.96 EXCEPT 1988
T = Ambient temperature of preglow time relay in °C
t = Preglow time in seconds
PREGLOW WITH AFTERGLOW
Preglow Time
T = Coolant temperature in °C
t = Preglow time in seconds
Afterglow Time
To improve warming-up properties, the glow plugs continue to glow when the engine is running dependent on the coolant temperature.
a) Version:
Model 124 Engine 603.96 1988
Model 201 Engine 602.961 Model Year 1988
T = Coolant temperature in °C
t = Afterglow time in seconds. Afterglow time up to max. 17.5 s (refer to diagram)
b) Version:
Models 124, 201 Engines 602.961/962,
Model 124 Engine 603.96 from Model Year 1989
T = Coolant temperature in °C
t = Afterglow time in seconds. Afterglow time up to max. 60 s (refer to diagram)
SAFETY CUTOUT
If the engine is not started within 20-25 seconds after the readiness to start indication being given, the glow current is interrupted by the safety cutout. If the engine is then started, the qlow system is switched on again for the duration of the starting operation. The duration of safely cutout is not fixed. It results from the time up to readiness to start (preglow indicator lamp going out) plus 20-35 seconds.
b Safety cutout
ACTUATING THE GLOW CURRENT
When the key is moved into position "2" (preglow, drive), the preglow time relay is switched on (voltage to terminal 15). The preglow time relay closes the circuit from terminal 30 (positive) to the glow plugs (R9).
If the key is turned into position "3,, (start), the preglow time relay remains activated and remains picked up through temimal 50. The glow process is continued until the key is turned back to position "2".
READINESS TO START INDICATION
a) Model 124 Engine 603.96 except 1988.
When the glow system is switched on, the preglow indicator lamp in the instrument cluster lights up. The glow duration is determined by a temperature sensor installed in the preglow time relay. Once the required glow time has been reached, depending on the ambient temperature of the preglow time relay, the preglow indicator lamp goes out, thus indicating that the engine is ready for starting.
b) Models 124, 126 Engine 603.960 1988, Model 201 Engine 602.961, Model 124 Engine 602.962, Engine 603.96 Effective 1989.
When the glow system is switched on, the preglow indicator lamp in the instrument cluster lights up. The glow duration is determined by the temperature sensor in the coolant. Once the required glow time has been reached, the preglow time reley switches off the preglow indicator lamp, indicating that the engine is ready for starting.
FAULT INDICATION
Model 124 Engine 603.96 Except 1988.
A fault in the preglow system is indicated by the preglow indicator lamp failing to light up when the key is moved into position "2".
The following faults are detected:
- Interrupt in the cable to connection terminal 30.
- 80 ampere fuse faulty.
- Fault in power relay of preglow time relay.
- Interrupt in one or several cables to the glow plugs.
Interrupt in one or several glow plugs.
FAULT INDICATION
Models 124, 201 Engine 602.961, 602.962, 603.96 1988 603.96 as of 1989.
A fault in the preglow system is indicated by the preglow indicator lamp failing to light up when the key is turned into position "2". In addition, the preglow indicator lights up for approx. 1 minute when the engine is running.
The following faults are detected:
- Interrupt in the cable to connection terminal 30.
- Preglow time relay faulty.
- Interrupt in one or several cables to the glow plugs.
- Interrupt in one or several glow plugs.
- Short-circuit at one or several glow plugs or in the cable.