Glow Plug System: Testing and Inspection
CHECKING PREGLOW SYSTEMConventional tools
Voltmeter (measuring range 0-3 volts/0-30 volts)
Note: Battery charged min. 60 % (acid density approx. 1.22 g/cc).
B. Engine 617.95 standard and USA version starting 1981 (quick-start system)
Testing glow bulb and its line
In the event of an interruption:
Preglow indicator lamp not lighting up when preglow system is switched on in spite of ready-to-start condition.
Pull 4-point coupler from preglow time relay, turn key into position "2", bridge jack 1 and 3 of coupler.
If preglow indicator lamp is not lighting up, test glow bulb and renew, if required.
If glow bulb is in order, test black line from coupler jack 3 of preglow time relay to preglow indicator lamp for interruption, remove interruption.
If preglow indicator lamp lights up, preglow time relay is defective, replace preglow time relay.
Installation not concerning preglow time relay
If a preglow time relay has been stored in spare parts sector for a considerable period (refer to production date), the specified preglow time may be below specification after the relay has been installed, so that the engine will not start.
By preglowing several times one upon the other (5 10 times) the capacitor in preglow time relay is again activated and the specified preglow time is attained.
Testing main circuit of preglow system for interruption
In the event of an interruption:
Preglow indicator lamp not lighting up, engine cannot be started.
Test voltage at terminal 30 of preglow time relay against ground by means of voltmeter.
If no voltage (approx. 12 volts) is indicated, test red line for interruption and remove interruption, if required.
On model 126 from point of support terminal 30 to preglow time relay connection terminal 30.
On model 123 from cable connector engine harness terminal 30 to preglow time relay connection terminal 30.
If voltage is indicated, check 80-amps fuse for tight seat or interruption and replace, if required.
If no fault has been found up to now, test voltage on jack 1 of 4-point plug of preglow time relay against ground.
If no voltage (approx. 12 volts) is indicated with preglow system switched on, test red/black line from fuse box terminal 15 to plug jack 1 of preglow time relay for interruption and remove interruption, if required.
On model 126 from input fuse No.14 to plug jack 1 of preglow time relay.
On model 123 from input fuse No.12 to plug jack 1 of preglow time relay.
If voltage is indicated, connect voltmeter to jack 1 (terminal 15) and jack 4 (terminal 31) and test voltage.
If no voltage (approx. 12 volts) is indicated, test brown line from jack 4 to ground for interruption and remove interruption, if required.
If no fault has been found up to now, the preglow time relay is defective. Replace preglow time relay.
Testing pencil element glow plugs and their lines
In the event of an interruption:
Preglow indicator lamp not lighting up, engine fires poorly, an interruption of one or several pencil element glow plugs or lines to pencil element glow plugs may be indicated.
In the event of an interruption:
Preglow indicator lamp lights up, engine fires poorly after attaining ready-to-start condition, an interruption of a pencil element glow plug or a line to pencil element glow plugs of cylinder 2 to 5 or wrong current input of pencil element glow plugs in cylinder 1 to 5 may be indicated.
Measure power input of pencil element glow plugs with DC clip-on probe.
For this purpose, place probe over individual lines on pencil element glow plugs.
Move key in steering lock to position "2", the power input of each glow plug should then be 8-15 Amps. after 10-20 seconds.
If the value is higher than 15 Amps., renew glow plug.
At a value below 8 Amps., test electric line or glow plug with ohmmeter for interruption.
Test for interruption.
Pull 6-point coupler from preglow time relay.
Measure resistance against ground (engine block) with ohmmeter one after the other at
jack 1 of coupler = pencil element glow plug cylinder 1
jack 2 of coupler = pencil element glow plug cylinder 2
jack 3 of coupler = pencil element glow plug cylinder 3
jack 4 of coupler = pencil element glow plug cylinder 4
jack 5 of coupler = pencil element glow plug cylinder 5.
If resistance is measured, an interruption of the respective pencil element glow plug or supply line or connection is indicated.
If a smaller resistance (e.g. at 20 °C/68 °F < 1 ~) is measured, the supply line and the pencil element glow plug are in order.
Note: It is possible that an indicator lamp (as the result of unfavorable tolerances) will indicate a fault only after a failure of 2 pencil element glow plugs in cylinders 2 to 5.
To make sure that the fault indication in preglow time relay is not defective, disconnect 2 pencil element glow plugs of cylinders 2 to 5 in such a case and repeat preglow operation.
If the indicator lamp is now indicating a fault (not lighting up) the preglow time relay is in order.