Mercedes-Benz W124 diesel: glow plugs and glow light
Why does a Mercedes-Benz W124 diesel start badly when cold and the glow light only stays on briefly?
The most likely cause is one or more broken glow plugs. The glow relay monitors the plug current and shortens the glow time according to the number of broken plugs. A faulty glow relay can also prevent glowing completely; the glow light does not come on and the car will not start.
How do you test the glow plugs of a Mercedes-Benz W124 diesel?
Disconnect the connector of the glow relay wiring harness and measure the plugs from the connector pins, with the other meter lead on the body. A healthy plug gives about 0.7–1.0 ohms, while a broken one reads many times that. The connector pins are numbered, so the number tells the cylinder. The readings of healthy plugs differ by at most 0.1 ohm.
The glow light does not come on in a Mercedes-Benz W124 even though the glow plugs and relay are fine?
The cause can be the light's ground. The ground wire behind the instrument cluster may be broken or poorly attached; a new ground wire straight from the body to the cluster has restored the glow light. Also check that a single burnt-out plug does not switch the light off: when the light does not work at all, the first glow plug has often burnt out.
What should be noted when replacing glow plugs in a Mercedes-Benz W124 diesel?
On the OM601, check the plug type and voltage before buying, as the same engine has two different ratings. A plug that is too long breaks when tightened. On the wide-head OM606 engine, the plugs snap off easily; a broken plug is best left to a shop that uses special tools to remove it.