Function of Preglow System - ra15006020705x(0705)

Engine 602.96, 603.96/97 TURBO (Model 463.3: for deviations see introductory notes) 

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.

Fig 1: Design Of Quick-Start Pencil-Type Glow Plugs

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

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.

Fig 2: Identifying Control Winding And Heating Winding

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

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 winding increases its resistance as the temperature rises and limits the current to approximately 8-15 amperes. The glow plug is thus protected against overload.

Current curve of quick-start pencil-type glow plug

Fig 3: Current Curve Of Quick-Start Pencil-Type Glow Plug

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

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 of quick-start pencil-type glow plug

Fig 4: Temperature Curve Of Quick-Start Pencil-Type Glow Plug

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Fig 5: Preglow System Wiring Diagram - Engine 602.961, Model 201.128

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Wiring Diagram Engine 602.961, Model 201.128

Fig 6: Preglow System Wiring Diagram - Engine 602.962, Model 124.128

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Wiring Diagram Engine 602.962, Model 124.128

Fig 7: Preglow System Wiring Diagram - Engine 603.96 Except 1988, Models 124, 126

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Wiring Diagram Engine 603.96 except 1988, Models 124, 126

Fig 8: Electric Wiring Diagram - Engine 603.96, 1988, 603.96 From Model Year 1989, Model 124

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Electric Wiring Diagram Engine 603.96 1988, 603.96 from Model year 1989, Model 124

Fig 9: Preglow System Wiring Diagram - Engine 603.971 Model Year 1992, Model 140

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Wiring Diagram Engine 603.971 Model Year 1992, Model 140

Engine	Model	Control unit Part no.	Version of injection system/preglow system	Remarks
602.96	124	007 545 99 32	Oblique injection, precombustion chamber 5°/180°, 60 s afterglow up to max. 25°C coolant temperature	as of 02/89 1990
	201	008 545 01 32
	124	001 545 98 32 002 545 06 32	Vertical injection without afterglow	up to 01/89
	201	003 545 10 32 003 545 60 32
	201	006 545 22 32	Oblique injection, precombustion chamber 5°, 15 s afterglow up to max. 60°C coolant temperature	Standard and
603.96	124	007 545 16 32	Oblique injection, precombustion chamber 5°/180°, 60 s afterglow up to max. 25°C coolant temperature	as of 02/89
		005 545 45 32	Vertical injection without afterglow	up to 01/89
		006 545 91 32	Oblique injection, precombustion chamber 5°, 15 s afterglow up to max. 60°C coolant temperature	1998 1988
	124	005 545 45 32	Vertical injection
	126		without afterglow	1986/87
	126	007 545 16 32	Oblique injection, precombustion chamber 5°/180°, 15s afterglow up to max. 60°C coolant temperature	1990

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.

Fig 10: Identifying Location Of Preglow Time Relay - Shown On Model 201

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Shown on Model 201

Functions of Preglow Time Relay 

The preglow time relay has the following functions:

• 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

Fig 11: Ambient Temperature Graph Of Preglow Time Relay - Preglow Without Afterglow

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Preglow with Afterglow Preglow Time 

T Coolant temperature in °C

t Preglow time in seconds

Fig 12: Coolant Temperature Graph - Preglow With Afterglow Preglow Time

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Afterglow Time 

To improve warming-up properties, the glow plugs continue to glow when the engine is running dependent on the coolant temperature.

1. 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)

   Fig 13: Coolant Temperature Graph - Afterglow Time

   Courtesy of MERCEDES-BENZ OF NORTH AMERICA.
2. 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)

   Fig 14: Coolant Temperature Graph - Afterglow Time

   Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

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 glow system is switched on again for the duration of the starting operation.

The duration of safety cutout is not fixed. It results from the time up to readiness to start (preglow indicator lamp going out) plus 20-35 seconds.

Fig 15: Safety Cutout Graph

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

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 terminal 50. The glow process is continued until the key is turned back to position "2".

Readiness to Start Indication 

1. 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.

2. b) Models 124, 126 Engine 603.96 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 relay 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 cables.