1977-1979 Mercedes-Benz 240D 300D 300CD 300TD W123 Glow Plug Tests Service Manual

Glow Plug Tests Vehicle Manual for 1977, 1978 and 1979 Mercedes-Benz W123. Model types specification:

240D – 65 PS (48 kW; 64 hp) @ 4200 from 02/79 72 PS (53 kW; 71 hp) @ 4400
300D – 80 PS (59 kW; 79 hp) @ 4000 from 09/79 88 PS (65 kW; 87 hp) @ 4400
300CD – 80 PS (59 kW; 79 hp) @ 4000 from 09/79 88 PS (65 kW; 87 hp) @ 4400
300TD – 80 PS (59 kW; 79 hp) @ 4000 from 09/79 88 PS (65 kW; 87 hp) @ 4400

Contents

Series Key Operated (2nd Version)*

COMPLAINT: NO START/HARD STARTING

Test Procedure:
1. Turn ignition sWitch to “PREGLOW” position.
2. Measure voltage at input of 50 amp fuse.
Results:
Approximately
10.5 volts
Glow plug circuit okay. CHECK FUEL SYSTEM, PUMP TIMING, AND ENGINE COMPRESSION.
Approximately
12 volts
Open circuit between rear glow plug and ground (at front of engine). CONNECl’ NEGATIVE LEAD OF VOLTMETER TO GROUND AND CHECK VOLTAGE AT INPUT AND OUTPUT CONNECTIONS OF EACH GLOW PLUG. When VOltmeter reads zero, the open is at that point
Zero volts Open circuit between rear glow plug and input from battery positive. CHECK FUSE. INSTALL NEW PREGLOW TIME RELAY AND RETEST. If reading is still zero volts. CHECK FOR OPEN WIRE. (Refer to Electrical Troubleshooting Manuai)
Other Readings Circuit resistance incorrect CHECK ALL CONNECTIONS FOR LOOSENESS OR OR CORROSION. REMOVE GLOW PLUGS AND CHECK CONDITION. CLEAN CARBON FROM GLOW PLUG HOLE WITH SPECIAL REAMER.
Series Key Operated (1st Version)

COMPLAINT: ENGINE STARTS, INDICATOR LAMP DOES NOT LIGHT

Test Procedure:
1. Unplug and ground lamp wire at thermo-time switch.
2. Turn ignition switch to “PREGLOW” position.
Results:
Lamp On DEFECTIVE THERMO-TIME SWITCH.
Lamp Off DEFECTIVE BULB OR OPEN WIRE.