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Old 06-13-2013, 12:13 AM
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Join Date: May 2013
Posts: 1
1974 240d Glow Plugs

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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Old 06-13-2013, 07:02 PM
katja's Avatar
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Join Date: Jul 2005
Posts: 307
I don't quite understand what you wrote...maybe a photo or diagram of the wiring would help.

There are two designs for glow plugs for these engines. The original was simply that the pull-start switch energizes the glow plug circuit, in which all the plugs were wired in series, with voltage further reduced by resistance wires between each plug and the indicator coil light above the steering wheel. There was NO relay, and each plug effectively got about 0.9V, and there is power to them as long as you hold out the start knob. This is how my 240D is set up.

The "upgrade" kit had a controller, and 11V plugs wired in parallel. You are supposed to remove the resistance wires and the ground wire and connect the new plugs with jumper wires. The controller has a temp sensor in it so when you turn the key on, it activates the glow plugs; you run another wire to wherever you want to put your glow plug indicator light; the light turns off after 3-7seconds and then the relay shuts the plugs off entirely after about 30. The start knob now only turns the car on & off and is independent of the glow plugs.

I have read of people NOT using the controller and just wiring up the plugs either with or without a simple relay. When you turn the key on, nothing happens until you ALSO pull the start knob out to activate the glow plugs, which will stay on as long as you keep pulling on the knob.

If you don't use the controller, you end up with no glow plug indicator light at all, and there's also no way to my knowledge to test which glow plug is bad unless you remove each one & test it individually.
1976 240D "Katja"
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Old 06-24-2013, 01:13 PM
Registered User
Join Date: Jun 2011
Location: Las Vegas, NV
Posts: 348
The easiest way to up grad your glow plug system is to use the relay from a 79 on Mercedes diesel.
You need pencil style glow plugs.
The relay and the wiring from the relay to the glow plugs.
And for your 240 a indicator light in the dash board that replaces the "Salt Shaker"
If you get the relay from a wrecking yard cut the wires to the small 4 prong plug.
You need the plug and the "Pigtails" to wire the relay to your car.
Post 12 in this thread has the wire designation. Fast Glow for my 76 240 D
The big wire from the relay goes directly to the battery + without a fuse.
Also you can use a 5 cylinder gp relay for your 4 cylinder 240.


76 240 D. Bought in 1998 for $25.
85 300D. Got it for free with a bad engine. ( Sold )
60 Unimog 404. What was left of it, was given to me. Now powerd by 617A.
88 560 SEL. Bought without engine and trans. Now powerd by 617A.
67 250 SE. Cuope. For resto or sale.
64 220SE. For resto.
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