E. With Key Starting System (Model 123.1)
Checking Preglow SystemConventional tools
Voltmeter (measuring range 0-3 volts/0-16 volts)
Ammeter (measuring range 0-60 amps)
Note
Battery charged mm 60 % (acid density approx. 1.22 g/cc).
E. With key starting system engine 615, 616, 617 in model 123.1
Preglow system in general
Glow plugs are connected in series. The current flows across glow plugs from cylinder (5-) 4-3-2-1 to ground. Each glow plug is designed for a voltage of 0.9 volt at a max. current of 63 amps.
For this reason, the glow plugs are connected to resistance conductors, on which the remaining voltage is consumed (drops). The resistance conductors will also become red hot during preglowing.
The extent of the preglow current is determined by the total resistance in the preglow system (glow plugs plus resistance conductors).
Consequently, the cores for a burnt-out glow plug or a defective fuse under influence of an excessively high current flow can never be the preglow time relay. In such cases there is a reduction of total resistance in preglow system (e.g. short circuit to ground of one or several glow plugs by coking or by contact with prechamber.
A ground short on glow plug results in varying effects in preglow system. A glow plug ground short of the 4th or 5th cylinders, for example, will immediately make the fuse defective, while in the event of a glow plug ground short of the 3rd cylinder, the glow plug of the 4th cylinder will burn out (perhaps after several preglowing operations, as the case may be). Therefore, in the event of a burnt-out glow plug, always test the prior glow plug for a ground short and replace, if required.
a) Preglow system not preglowing
This complaint is caused by an interruption in current circuit of preglow system.
For accurate determination of fault source (whether glow bulbs or other components), connect voltmeter (measuring range 0-30 volts) to current input of glow plug of 4th or 5th cylinder and to vehicle ground and read voltage (preglow system engaged).
Readout approx. 12 volts interruption at glow plugs including connections (but not at preglow time relay).
Readout 0 volt = fuse defective preglow time relay or activation of relay defective.
1. Check glow plugs for interruption
Connect negative cable of voltmeter to ground. Check voltage by means of positive cable of voltmeter connected to current input and output of 4 or 5 glow plugs (while disconnecting ground of glow plugs on 1st cylinder).
If 12 volts are measured at current input of a glow plug and 0 volt at output, this glow plug is interrupted and must be replaced.
Attention!
Never short connections of glow plugs or resistance conductors against ground (e.g. screw driver against engine block); this may result in failure of preglow time relay!
2. Testing preglow time relay and connections
Pull coupler from preglow time relay. Connect negative cable of voltmeter to vehicle ground. Check voltage at jack 5,4 and 3 one after the other by means of positive cable, at jack 3, the starter should be actuated.
Then connect negative cable of voltmeter to jack 1 and positive cable to jack 5 and measure voltage.
Nominal values
Jack 5 = approx. 12 volts (terminal 30, continuous voltage)
Jack 4 = approx. 12 volts (terminal 15, preglow system engaged~
Jack 3 = approx. 12 volts (terminal 50, while starting)
Jack 1 = approx. 12 volts (terminal 31, negative line)
If 0 volt is measured at one of the jacks, test respective line for interruption or repair interruption.
If 12 volts are measured on all jacks, the preglow time relay is defective, replace preglow time relay.
Wiring Diagram:
b) Preglowing system glowing too slowly or too fast
Nominal time at: + 20 0C168 0F approx. 12 seconds
- 5 °C/23 °F approx. 30 seconds
Preglowing system preglows too slowly because not enough current can flow through transition resistances in glow plugs or on connections.
Preglowing system preglows too fast because too much current flows through one or several glow plugs due to ground short.
These faults can be found by measuring voltage drop of each glow plug. Measure voltage on glow plugs at current input and output with test points of voltrn~er (measuring range 0-3 volts).
Attention!
Measure as close as possible to glow plug, since otherwise measuring faults (higher voltage drop) will show up by including the resistance conductors into measuring.
When measuring, pay attention to risk of short circuit!
c) Preglowing system preglows, preglow indicator not lighting up or not going out
If the preglow indicator lamp is not lighting up, pull coupler from temperature sensor - indicator lamp should now light up. In such a case, replace temperature sensor.
If the indicator lamp is not lighting up, pull coupler from preglow time relay and bridge jack 5 (terminal 30) with jack 7. If the indicator lamp is now lighting up, replace preglow time relay. If indicator lamp is not lighting up, check bulb or renew, or repair line interruption
If the preglow indicator lamp is not going out after attaining ready-for-starting condition, connect coupler of temperature sensor to ground If indicator lamp is now going out, the temperature sensor is interrupted and should be replaced. If the indicator lamp is not going out, the preglow time relay is defective or the line to relay is interrupted. Replace preglow time relay or repair line interruption.
Note: The temperature sensor can be tested with an ohmmeter.
Resistance values according to coolant temperature:
0°C/0°F = approx. 8500 ohms
+25°C/77°F = approx. 2500 ohms
+80°C/176°F = approx. 300 ohms
At readout of 0 ohm, the temperature sensor has a short circuit, at readout ohm, the temperature sensor is interrupted and should be replaced.