W639 perplexing glow plug issues

[alpaco45]

Greetings everyone!
My glow plug light is acting strangely, you know? I hope the knowledge base here will be the answer I've been looking for because I've done a lot of searching, both on this site and elsewhere on the web, but nothing has worked so far.


For some time now, whenever I turn on the engine, the glow plug light returns and remains on for around 30 seconds before turning off again. It won't turn back on after it goes out, but if I leave the car parked for long enough on cold days, the procedure will happen again.

The Bosch Duraterm plugs are only around six months old, therefore it was shocking to see a code for glow plug number four not functioning properly when I scanned the codes. After changing the plug, clearing the codes, and rescanning, I encountered a new code indicating that the light output stage was not working correctly. It appeared, after some investigation, that this was probably the glow plug relay; I removed it to find, to my amazement, that the internal fuse for glow plug circuit number four had blown.

I received error messages for each of the four glow plugs after installing the new module that I had purchased from eBay (yes, I am aware of that).
Assuming the module was simply defective, I contacted the vendor, who sent me a replacement. After installing it, I got a fault code for glow plugs 3 and 4, but no code for the relay.

I decided to acquire one of the newer BERU type relays without the internal fuses instead because I have some experience with Chinese parts and it is not uncommon or impossible to have two defective things.

I thought that would solve the problem, but now I get codes for glow plugs three and four after installation 🤎 🙄.

I whipped out the multimeter and got to work, because it seemed highly improbable that three modules in a row, of different kinds and from separate vendors, would all be defective.

There is good continuity on all four of the wires when testing the multi-plug connection to the glow plug connections. While the engine is running, I get 12 V from the glow plug module's power source and 12 V from the relay's four connections; but, when the engine is off, I get no voltage at all. From what I can tell, the three-pin connector that goes to the relay is the canbus signal, but the readings on it are all over the place. The output was approximately 5 V.
It seems like there's an odd noise coming from under the passenger seat area whenever I turn on the ignition. Upon investigation, I found that there are only three electrical connectors—two regular blade fuses and a large blade fuse—and that they're all in good working order. Based on my observations, there doesn't appear to be any other source for this sound. It could be a red herring, but it was not present prior to installing the new relay, so it's likely linked.

I am open to recommendations from you knowledgeable people because I have exhausted all of my thoughts and the Internet has failed to provide a solution.

[CyberNinja]

Glow plugs come with two different voltage ratings.
Regardless of the plugs you've used, make sure the controller is compatible.


Proceed with caution when verifying diagnostic output. With time, the voltage at the output should decrease.

[alpaco45]

Understood 👍🏼 I will verify by cross-referencing the part numbers on EPC just to be sure, but the glow plugs I installed were reportedly for my year and model. I might simply have to buy new ones 🤦🏻‍♂️🤎.

[CyberNinja]

Every glow plug I've ever used has had the voltage printed on it.
Without glow plugs, the greatest voltage output measurement would serve as a solid reference; nevertheless, I am not very knowledgeable about controllers. Remember to turn off the ignition for that.
 

[alpaco45]

After reviewing the Bosch website, I can confirm that their duraterm plugs are rated at 11v. This makes sense considering that, with the plugs disconnected and the ignition turned on, the measured voltage at the plug connectors was around 11.5v-12v.
It appears that the glowplug side is functioning properly, which is why I am confused 🤎🤷🏻. However, it is possible that there is another defective relay.

[CyberNinja]

?? With even a little bit of excess force, these thin glow plugs will twist and deform.

[alpaco45]

It shouldn't be a problem because they were torqued to the recommended setting using a 1/4" torque wrench. However, getting them out is stressful because you're constantly waiting for them to snap. I was lucky with the number 4 plug I replaced because they haven't been in there for very long, but I still handled it with care just in case.

[MrGreedy]

I haven't snapped one yet, here's hoping the trend continues. Before taking them out, I always make sure the engine is warm, but not too hot.