If the indicator lamp does not light and the engine starts, check the lamp and replace it. Otherwise, you need to perform the following test with a "probe":
- disconnect the relay connector and turn on the ignition;
- connect the "test probe" to both contacts of the connector block. If the lamp does not light, check the supply wiring. The "test" lamp must be known to be in good working order;
- if the engine does not start and the indicator lamp in the instrument cluster does not light, the following checks must be carried out:
- 1) check the integrity of the fuse;
- 2) turn on the ignition and check for voltage at terminal "15". To do this, check the wire of the small plug connection;
- 3) disconnect the large plug of the wire going to the spark plugs from the relay;
- 4) connect one end of the "test probe" to the "ground", and with the other end - connecting in turn - check all the relay contacts. The glow plug control device operates after the ignition is turned on, and voltage appears at each contact. Remember to turn the ignition off and on from time to time to keep the heating device in working order;
- 5) if the test results are unsatisfactory, replace the relay.
Article based on data from the website: MercedesMan
A faulty glow plug can be determined by the engine starting, but not all cylinders are working. The non-working cylinder starts to work with a delay, since diesel fuel is prone to self-ignition.