Mercedes-Benz W123 diesel glow system and glow plugs
How do you check the glow plugs on a W123 diesel?
Disconnect the plugs' wiring so that the plug being measured is the only one connected. With the multimeter's continuity beep function, put one probe on the body and the other on the plug's terminal: if it does not beep, the plug is broken. First measure whether current reaches the glow plugs in the glow position.
Why does the glow light on a W123 diesel not come on?
The cause can be a faulty glow relay, for example one damaged by moisture. A healthy relay clicks twice: when the current is switched on and when the glow time ends. The light does not necessarily reveal faults in an individual plug, because a broken plug can still let current through.
Is there a difference in the W123 diesel glow system between older and newer models?
In older cars the glow plugs are in series without a relay, so when one burns out the rest do not glow. In newer ones the plugs are in parallel and there is a relay. From about 8/80, small glow plugs were used, so it is worth checking that the plugs and relay suit the car's model year.