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Glow Light Still Flashing after Plug Replacement

Started by amilio, 27 March 2023, 10:54 AM


Hi all,

I have a '79 300 SD and in late December it started having issues starting where the glow light would blink and it might take 2-3 times to start the car in the cold. I replaced the glow plugs, as the blinking light usually indicates one or more of them have failed. Everything seemed to be fine, but the issue has started again and the GP light flashes every time the car is started cold, and it has difficulty starting. I have looked over the wires connecting the relay to the plugs and they seemed fine. Any ideas what could be going on?

My current plan is to get another W116 relay off EBay and see if that solves the issue. The glow plug fuse in the fuse box was replaced, and the 80 AMP on the relay itself seems okay, if a little old. Any help or insight is appreciated!
1979 300 SD "Liesel", 282k Miles


Here's a video of a failed start, which will hopefully assist with troubleshooting; this is with a "new" relay off a '79 300D (116 part number), which seems to present the same issue as the original relay from the car. The next step will be replacing the 80A fuse. It seems to me that the glow plugs aren't even turning on.

1979 300 SD "Liesel", 282k Miles


Since I know nothing about diesels perhaps this will help you verify that the wiring is setup correctly.

Do you have loop or newer pencil glow plugs now?
'68 250S
'77 6.9 Euro
'91 300SE,
'98 SL500
'14 CLS550,
'21 E450 Cabrio


Judging by the video, it looks like the glowplugs are working fine, it looks more like a fuel issue. I'd recommend testing the glowplugs by grounding the outside of them and turning the key to the second position. If they glow red in a few seconds, the glow plugs work fine and it is a fuel issue.


Hey all, thanks for the replies

My car came with the newer style pencil plugs stock, before I replaced them this past December (with BOSCH plugs), they had last been done in 2009. I'll look at the wiring, and I will have any shop I take it to check the glow plugs for functionality. I would do it myself, but the car is currently in my sister's care back while I am in Washington DC. These plugs are wired in parallel as opposed to the "in-series" wiring of the loop style seen in the link.

As far as a fuel issue, I'm not sure what the issue would be. There aren't any issues with fuel delivery after startup and the glow plug light flashing after an attempted start leads me to believe that it's an issue with the glow plug system, as the relay seems to be detecting some sort of anomaly still. That said, if there are any specific fuel issues that might lead to this behavior, I am happy to troubleshoot them. My primary and secondary fuel filters are relatively new. 

This behavior also seems to be somewhat random; the start up yesterday morning was rough, but then after sitting for several hours during a cool day, it started as normal. I've never had electrical issues with the car before so I am hoping this is isolated to the glow plug system.
1979 300 SD "Liesel", 282k Miles


Hey all, to conclude this thread, I actually ran into the very, very stressful issue of having the ignition tumbler fail (unrelated to the plugs). Ended up needing to have a shop replace part of the steering lock and tumbler, and while doing so, I had them check the plugs. They took the connectors off the plugs and cleaned them and the issue went away. Hopefully it will stay that way.

Long story short; Have the tumbler replaced as soon as it sticks! Luckily Liesel is now back on the road.
1979 300 SD "Liesel", 282k Miles