1974 240d Glow Plug issue - Mercedes-Benz Forum

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post #1 of 4 (permalink) Old 06-12-2013, 09:15 PM Thread Starter
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1974 240d Glow Plug issue

I just purchased a new glow plug system from Mercedes Source for my 1974 240d. My question is that the previous owner had already changed the glow plug system to a Bosch system. From the battery the wire has a 50amp inline fuse, then the wire goes to the relay, from the relay the wire goes to another in line 50amp fuse and from there the wire goes to the first glow plug. The tests I have performed have produced interesting results. I have tested the glow plugs with a battery and they light up. At that point I took my test light and and with the key on, the relay clicks on and my test light, lights up. So now I attach the glow plug to the hot wire from the updated glow plug system, and using my test light and with the key on, it lights up. For the last test I leave the hot wire attached and I ground the glow plug out against the motor and turn the key on and the plug never gets hot not even warm and again the relay clicks on and off and my test light lights up. So first question I ask is will this new kit fix this problem, does this problem even make sense to anyone? Am I missing something?
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post #2 of 4 (permalink) Old 06-16-2013, 11:07 AM
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It makes sense - the kit will probably fix it (depends on exactly what kit you got - they sell several).

The big difference is the original glow plugs in your car are parallel that is the V+ goes from the switch / relay to the first glow plug and the output from the first goes to the second and so on - they use a twisty resistance wire to connect to each other. The new style and upgrades are wired in series - that is there are 4 wires and these go directly to each glow plug.

In the old system a single failed glow plug might cause the engine to not be able to start (depends on how it failed). The newer system this was less likely (although not impossible). Both systems are quite reliable but both will fail eventually - I only have the original on my car and it works well. (I live in California where it's warm mostly)

But I suspect your problem is the glow plugs are fouled with carbon and the pre-chamber needs to be cleaned (reamed) out (mercedes source has a good video on this - the also sell the reamer tool). Do not be surprised at the amount of crud that will come out (you cover the reamer with grease to make the carbon stick to it and not go into the cylinder - do watch the video).

When they get fouled they can short out such that they will not heat. So you might be fine just replacing the glow plugs AND CLEANING OUT the prechamber.

BUT don't mix glow plugs - the ones that are wired in series need to be wired that way and the ones designed to be wired in parallel need to be wired that way. This means of course that all your glow plugs will be one way or the other.

Good luck,

Last edited by frankflynn; 06-16-2013 at 11:10 AM.
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post #3 of 4 (permalink) Old 06-16-2013, 12:47 PM
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also have a 73 220d , the original loop glow plugs are in serie .the newer cars use a pencil type glow plug they are in parallel . from what you describe , if you are using a good plug for the test , it is no grounded properly . if it was , it would light up , and the test light would be slightly dim .redo test .
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post #4 of 4 (permalink) Old 06-20-2013, 11:35 AM Thread Starter
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Glow plugs

Well I found out the issue and it was the ground. Now the plugs work great, just have to figure out my fuel issue now. Thank y all for all the help.
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