Operation of Preglow System
OPERATION OF PREGLOW SYSTEMC. Preglow system engine 617.95 standard version and USA starting 1981
General information
The preglow system corresponds in principle to system of model year 1980.
The preglow time relay has been modified. In addition, the temperature sensor in coolant circuit is no longer installed.
Preglow time relay
The preglow time relay is located in engine compartment at left on wheel house.
After removing plugged-on protective cap, the electrical connections as well as the fuse strip are accessible.
The temperature sensor installed up to now in coolant circuit is no longer installed. Instead, the relay or relay ambient temperature is obtained by means of an NTC resistor in preglow time relay.
Functions of preglow time relay
The preglow time relay has the following functions:
- Switching of glow current
- Ready-to-start indicator
- Safety shutoff
- Fault indicator
Switching of glow current
When turning key into position "2" (preglowing, driving) the preglow time relay (current on terminal 15) is switched on. The power relay (a) closes the circuit of terminal 30 (plus) via fuse for pencil element glow plugs G 1 - G 5.
When the key is moved into position "3" (start) the power relay (a) remains in energized condition under influence of terminal 50. Glowing continues until key is again turned into position "2".
Ready-to-start indicator
A temperature sensor installed in preglow time relay determines the duration of the glow period.
When the glow system is switched on, the preglow indicator lamp in instrument cluster lights up.
As soon as the required glow period, depending on ambient temperature of preglow time relay, has been attained, the preglow indicator lamp will go out and thereby indicate ready-to-start condition.
Safety shutoff
If, upon indication of ready-to-start condition, there is no start within 20-35 seconds, the glow current is interrupted by the safety shutoff. For a subsequent start, the glow system is again switched on for the duration of the starting procedure.
The safety shutoff is no longer fixed, but is the result of the time up to ready-to-start condition (preglow indicator lamp goes out) plus 20 to 35 seconds.
Fault indicator
A fault in preglow system is indicated by the preglow indicator lamp not lighting up when key is actuated in position "2".
The following faults are recognized:
- Interruption of line to connection terminal 30.
- Fuse 80 Amps. defective.
- Power relay in preglow time relay defective.
- Interruption of one or several lines to pencil element glow plugs.
- Interruption of one or several pencil element glow plugs.
Note: In the event of unfavorable tolerances of pencil element glow plugs or of Reed relay (d), response of fault indicator only after two pencil element glow plugs are defective is allowed.
Faults (monitoring of pencil element glow plugs) are indicated by comparing the current of the pencil element glow plug G 1 with current of remaining pencil element glow plugs G 2 to G 5 connected in parallel.
The currents of the two lines to pencil element glow plugs G 1 and G 2 to G 5 are flowing through two oppositely oriented Reed relay windings having a different number of turns in windings.
If the current flow in both windings is the same, the magnetic fields will cancel each other and the Reed contact will not respond.
If the balance of the magnetic fields is interrupted by the failure of one or several pencil element glow plugs, the Reed contact will close and the electronic unit (b) will be activated.
The preglow indicator lamp will switch off immediately and will therefore not light up when preglowing starts.