W639 perplexing glow plug issues
Posted: Mon Oct 20, 2025 8:35 am
Greetings, everyone!
I am experiencing an unusual issue with the glow plug indicator light. I have conducted thorough research, both on this platform and throughout the internet, although I have been unable to locate any information pertinent to my issue. Therefore, I am optimistic that the expertise available here will prove beneficial.
Recently, the glow plug indicator has illuminated upon engine startup, remaining active for roughly 30 seconds until extinguishing. Once it shuts off, it does not restart; but, if the temperature is low and I allow the engine to cool completely, the cycle will recur.
I checked the data and identified a malfunction in glow plug number four, which was unexpected given that the Bosch Duraterm plugs are just approximately six months old. I replaced the plug, cleared the codes, and re-scanned, resulting in a new code indicating that the glow output stage is not functioning correctly. Upon conducting some investigation, it appeared that the issue was likely the glow plug relay. I disassembled it and discovered that the internal fuse for glow plug circuit number four had blown.
I procured a replacement from eBay and when installing the new module, I encountered error codes for all four glow plugs.
Upon contacting the vendor, presuming the module was only defective, they provided a replacement. However, upon installation, I received fault codes for glow plugs three and four, but no code for the relay itself.
Having prior experience with Chinese components, I recognise that receiving two defective things is not unusual. Consequently, I abandoned that option and procured one of the newer BERU type relays devoid of internal fuses.
I had believed it would resolve the issue, however following installation, I now have error codes for glow plugs three and four. 
Experiencing three consecutive defective modules, of two distinct sorts from two separate providers, appears improbable; thus, I retrieved the multimeter and conducted some tests.
The continuity test on the multi-plug leading to the glow plug connectors indicates satisfactory continuity across all four wires. The power supply to the glow plug module provides 12 V, and with the ignition activated, each of the four relay connections also registers 12 V, whereas no voltage is present with the ignition deactivated. I am experiencing variable readings on the three-pin connector to the relay, and I understand that this represents the CAN bus signal to the relay. This provided around 5 V.
Upon activating the ignition, I perceive an unusual sound resembling interference from unshielded electrical components through a radio, seemingly emanating from beneath the passenger seat. However, upon inspection, the only electrical connectors present are two standard blade fuses and one large blade fuse, all of which remain intact. I cannot identify any other source from whence this sound may originate. This may be a red herring; nevertheless, it was certainly absent prior to the installation of the new relay, suggesting a probable correlation.
I have exhausted my ideas, and the Internet has not provided a solution; therefore, I welcome advice from you intelligent individuals.
I am experiencing an unusual issue with the glow plug indicator light. I have conducted thorough research, both on this platform and throughout the internet, although I have been unable to locate any information pertinent to my issue. Therefore, I am optimistic that the expertise available here will prove beneficial.
Recently, the glow plug indicator has illuminated upon engine startup, remaining active for roughly 30 seconds until extinguishing. Once it shuts off, it does not restart; but, if the temperature is low and I allow the engine to cool completely, the cycle will recur.
I checked the data and identified a malfunction in glow plug number four, which was unexpected given that the Bosch Duraterm plugs are just approximately six months old. I replaced the plug, cleared the codes, and re-scanned, resulting in a new code indicating that the glow output stage is not functioning correctly. Upon conducting some investigation, it appeared that the issue was likely the glow plug relay. I disassembled it and discovered that the internal fuse for glow plug circuit number four had blown.
I procured a replacement from eBay and when installing the new module, I encountered error codes for all four glow plugs.
Upon contacting the vendor, presuming the module was only defective, they provided a replacement. However, upon installation, I received fault codes for glow plugs three and four, but no code for the relay itself.
Having prior experience with Chinese components, I recognise that receiving two defective things is not unusual. Consequently, I abandoned that option and procured one of the newer BERU type relays devoid of internal fuses.
I had believed it would resolve the issue, however following installation, I now have error codes for glow plugs three and four.
Experiencing three consecutive defective modules, of two distinct sorts from two separate providers, appears improbable; thus, I retrieved the multimeter and conducted some tests.
The continuity test on the multi-plug leading to the glow plug connectors indicates satisfactory continuity across all four wires. The power supply to the glow plug module provides 12 V, and with the ignition activated, each of the four relay connections also registers 12 V, whereas no voltage is present with the ignition deactivated. I am experiencing variable readings on the three-pin connector to the relay, and I understand that this represents the CAN bus signal to the relay. This provided around 5 V.
Upon activating the ignition, I perceive an unusual sound resembling interference from unshielded electrical components through a radio, seemingly emanating from beneath the passenger seat. However, upon inspection, the only electrical connectors present are two standard blade fuses and one large blade fuse, all of which remain intact. I cannot identify any other source from whence this sound may originate. This may be a red herring; nevertheless, it was certainly absent prior to the installation of the new relay, suggesting a probable correlation.
I have exhausted my ideas, and the Internet has not provided a solution; therefore, I welcome advice from you intelligent individuals.
