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  #1  
Old 01-11-2020, 03:58 PM
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1976 300D w115 - Glow Plug Indicator Light

Hey guys,

I have a 1976 300D with a “turn key” style glow plug system.

When I turn the key to accessory only (without trying to crank the car), the glow plug indicator light below the speedometer gauge should illuminate and then turn off when the glow plugs are warm enough.

Until recently, the glow plug indicator doesnt turn on usually when the car is cold like it is supposed to. I have replaced the bulb to a new one so I know it is not burned out. Also, this morning, it was below 30 degrees. When i turned accessory on, the light came on very briefly (maybe one second or so). This is the first time I have seen it on since it stopped working properly (2-3 weeks ago). So i know the light is working to some extent, its just not sensing fully when the glow plugs are warm enough it seems. Or could this be an issue with the plugs themselves?

Does anyone have any thoughts on this? Thanks

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Old 01-12-2020, 07:00 AM
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I'll assume you have the original series glow plugs and the engine is cold.



Did you do any work on the glow system, IP or master cylinder area in the last month?



Does the car start if you wait 20-30 seconds without the glow plug light coming on?


If the car does not start (these may be easier with a helper):


1) look at thermo-time switch near the #1 glow plug, the glow plug light gets it's ground from it, there should be two wires attached to it, one from the #1 glow plug completing the circuit and one for the glow plug light which goes into a black sleeve and makes it may back to the firewall; check it to make sure it is properly attached to the thermo-time switch and that the wire is not cracked or chaffed. If everything looks ok, unplug the wire for the glow plug light and ground it on the block, turn the key to the run position and see if the light comes on. If it does not it is glow plug light circuit, if it does go to #2.



2) Check the voltage at the #5 plug (rear) when you turn the the key the run position (#2) not the accessory position, you should have 10.5 volts. Any other voltage (0 or 12) mean an open circuit in the glow plug circuit: 0 before the plugs (relay circuit), 12 in the plugs or resistor wires circuit.



Let me know how you make out.
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W115 1975 300D "Geraldine"
W115 1976 300D parts/tutorial car
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  #3  
Old 01-12-2020, 02:17 PM
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So this is really odd, but the glow plugs work just fine. They warm up reliably, theres just simply no pre glow light. The inly reason im concerned is that it used to work just fine and stopped recently.

I have not done any work to the glow plug system at all. Im assuming they are still the stock series setup. But again, the car fires up just fine even in the cold. It probably takes 10-15 seconds at most. Most of the time under 10 seconds when its not super cold out. Again, its just the indicator light on the dash thats not working, the bulb is definately not burned out, and glow plugs work perfectly as they should. Let me know your thoughts. Thanks
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Old 01-12-2020, 03:22 PM
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Can you take a picture of your glow plug setup and post it?
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W115 1975 300D "Geraldine"
W115 1976 300D parts/tutorial car
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  #5  
Old 01-12-2020, 04:42 PM
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Heres a photo of my setup
Attached Thumbnails
1976 300D w115 - Glow Plug Indicator Light-523a4f13-6ba8-4edf-8f69-31466fb8a7d3.jpeg  
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  #6  
Old 01-12-2020, 05:21 PM
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Could you tell my if my setup is series or parallel? Im kind of a newbie to some of this stuff.

Also, I was messing around behind the instrument cluster and noticed that there is a good amount of oil around the gasket and dripping around the backside of the instrument. Im sure that this is a leak from the mechanical oil gauge cable which I will look into further when I have time. However, do you think the oil on the instrument may have caused a short somewhere on the instrument panel? I know this is unlikely due to oil being completely non conductive or at least very very resistant to conduction. Thanks for your help, this has been driving me nuts lol
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Old 01-13-2020, 05:12 PM
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You have a series system but the resistor between the 1st and 2nd glow plugs looks to be touching the thermo-time switch, is that the case? It looks like someone "made it fit", not sure if it would cause a fault but it is very close to the glow plug light connection and they do tend to get very hot when glowing. I would suggest you get a new one and install it correctly. Look at the resistor between plugs 3 and 4 it should look like that one. Part # 0005458018, available through Pelican parts for $10.25.



I would clean up the oil you found behind the cluster and check that the cable is tight on the guage. When you changed the bulb for the glow plug light, was there any residue?
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W115 1975 300D "Geraldine"
W115 1976 300D parts/tutorial car

Last edited by maclaveau; 01-13-2020 at 06:14 PM. Reason: added part #
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Old 01-14-2020, 12:38 AM
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I have cleaned up all the oil from the instrument cluster. I used a bit of gasket sealer on the oil line to prevent any further leakage. I think I may have cross threaded the hose the last time I worked on the instrument cluster causing the oil line to leak.

As for the pre glow bulb, I did not see any oil residue on it or the socket. When looking at the picture, im going to be completely honest. Im not sure which part the resistor is. I see the glow plugs, im just having a hard time deciphering where the resistors are that you are referring to. Could you be more detailed on the location of them? Newbie here
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Old 01-14-2020, 07:17 PM
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I placed a blue line that shows the thermo time switch. The red line shows the resistor, (sorry for the crude lines). If you look at that resistor and the one between glow plugs 3 and 4 they should be identical. I believe part of the problem is that deformed resistor. It may be grounding something out or may have melted the wire or connector that is for the glow plug light circuit.
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1976 300D w115 - Glow Plug Indicator Light-glow-plugs.jpeg  
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W115 1975 300D "Geraldine"
W115 1976 300D parts/tutorial car
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  #10  
Old 01-15-2020, 02:40 AM
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Ahh ok. Thank you for taking the time to point that out. I will take a closer look tomorrow and report my findings. Also, is there a temperature sensor somewhere for this circuit? Where would that be located? Im sure theres some way for the system to detect the temperature of the engine
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  #11  
Old 01-15-2020, 05:47 PM
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that is the item I pointed to in red
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W115 1975 300D "Geraldine"
W115 1976 300D parts/tutorial car
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  #12  
Old 01-15-2020, 11:55 PM
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I just checked my car, and the parts in the picture are not touching. It just really seems that way from the angle of the picture. Any other ideas on what may be the issue?
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  #13  
Old 01-16-2020, 11:28 PM
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Try putting a volt-meter on the output of the temperature sensor between the glow plugs. Have someone else turn the glow on and see if anything changes. And see if it changes again after 30 seconds of glow. If it cycles like it should, you'll know that the problem is between the sensor and light, but the sensor is OK. I've not done this myself so I can't verify if it's the right approach, but seems easy to check.

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  #14  
Old 01-17-2020, 12:52 AM
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Thanks for the help guys. I have an appointment with my mechanic on the 27th. Hes really well versed in these cars and should be able to figure it out. I’ll report back when I we identify the problem.
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  #15  
Old 01-17-2020, 07:54 AM
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You might consider posting in the diesel discussion part of the site. You can tell just how long a glow plug cycle is. By watching the interior light.


It should be a little dimmer until the glow plug relay opens. Ten second glow plug activation on a cold series glow plug engine sounds very short to me incidentally.


If it turns out to be that short. I would expect either the block temperature sensor for the system is bad or the relay's time constant circuit is defective.


I believe if you take it to a mechanic and the relay is tired. Or has an issue they are pretty expensive. They are located above the brake pedal I believe on the series systems.


A pick and pull one is pretty reasonable cost wise in comparison. This is a part that it does not hurt to have a cheap spare anyways.

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