Mercedes ML Diesel Preheating Lamp Issue

I had a similar issue with my ML. The engine preheat system gave me fits for a while. The preheating lamp stayed on, and the engine was sluggish to start sometimes. Have you checked the electrical connections to the heating module itself? Specifically, look for any corrosion or loose wires. That was my problem; a corroded connector was causing intermittent issues. I also got the smoke on acceleration. Did you check the glow plugs?

Thanks for the quick reply! I appreciate the help. When you say 'heating module', are you referring to the preheating control unit? And where exactly is that located? I've visually inspected the accessible wiring, but I haven't disassembled anything yet to get a really close look. Also, how did you diagnose the glow plugs? I don't have a diagnostic tool myself.

Yes, the preheating control unit and heating module are the same thing. In my case, the issue was more complex than a simple visual check could reveal. Considering the smoke, and the fact that the preheating lamp is constantly on, it might be best to take it to a workshop. They can properly diagnose the glow plugs and check the preheating unit thoroughly. It could save you money in the long run by preventing further damage.

Okay, I understand. Thanks for confirming that the heating module and preheating control unit are the same. Given your experience and the potential for further issues, I'll take it to a workshop for a proper diagnosis. They confirmed the preheating control unit was defective and replaced it, including cleaning up the corroded connector. The smoke is gone, and the lights are off. Thanks again for your help!