115 Diesel Question - Mercedes-Benz Forum

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post #1 of 6 (permalink) Old 02-08-2007, 05:27 PM Thread Starter
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115 Diesel Question

I updated my 1976 300D glow plugs to the pencil type wired parallel. Now my GP indicator light wont come on, but it does light up when I turn the motor over. Go figure!
I didnt connect the ground wire formerly connected to old GP 1, I believe its a ground wire, don't know, its connected to a temp. sensor in the block. It also has no voltage.
I think I might have blown the GP fuse, but I cant find it. Where is it on a 115? The wire formerly conected to old GP 5 is now connected to new GP 5, but It carries no voltage at all, I checked it with a meter.

Has anyone upgraded a 300D to new style glow plugs? Am I missing something?

Any help would be appreciated
1976 MB 300D, 115.114, 617.910
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post #2 of 6 (permalink) Old 02-11-2007, 12:46 PM Thread Starter
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Anybody have an idea to fix my above problem?
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post #3 of 6 (permalink) Old 02-12-2007, 06:57 PM
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Vehicle: '77 6.9, 74 240D, 96 Ram 5.9L Cummins, 02 Civic EX, 10 Mazda 5 Sport
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I haven't done this conversion but plan to. The W115 doesn't have a strip fuse like the W123 does in the box on the left fender.

Your description is not entirely clear as the GP1 and GP5 is referred to differently from person to person. I presume GP 1 is closest to the radiator?

The grounding straps should be disconnected and the loop and pin plugs cannot be mixed and matched. The wires go from plug to plug and it should work in theory. There should be detailled instructions in the Bosch kit box.

1977 - W116 - 6.9
1974 - W115 - 240D /8 - 4(M) OM617.910 + Turbo + 300D FW - Completed 7/8/10
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post #4 of 6 (permalink) Old 02-17-2007, 03:53 PM Thread Starter
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Thanks for replying.
To clarify, Ill now call glow Plug 5 closest to the radiator and gp 1 closest to firewall.
I didnt get the Bosch kit, I got the new plugs from dieselgiant.com, Russel is a really friendly guy. He sent 10 gauge connecting wires also.
I agree the plugs should work in theory, and I can get them to work if i trick the sensor into working. This is how I do it.

With all of the new glow plugs connected, the glow plug indicator light wont come on. If i disconnect gp 5 then the indicator will come on, and will heat the first four plugs. While their heating I can then connect gp 5 and all 5 get hot, and the car cranks without a hitch! I attribute this to heating elements requiring less amps as they heat, I could be wrong. Problems is, my sensor wont feed 5 plugs at first.

My other problem is, with the factory wiring (series) there was a wire that went from gp5 to a sensor in the block. Now that ive upgraded to gps in parallel, Do I attach this wire back to gp 5?

Factory wiring starts with a 12volt positive to gp1, neg to gp2, pos to gp3, neg to gp4, pos to gp5. This would make the wire going to the sensor positive.
My upgrade to parellel gives a positive to gp5!
But connecting it doesnt fix my problem.
Im puzzled.

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post #5 of 6 (permalink) Old 03-04-2007, 07:16 PM
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Hi Ben,
Hey guy, I've got a 76' 300D as well, with 83K! She's been a Houston girl all her life and driven mostly by a "wife," which accounts for her low miles.
Well, I've a similar problem to yours that's just started the last few days. When my motors warm (like from just running into a store for a soda), the glow indicator light sometimes won't go on when I turn the key into the pre-glow position. I even checked the g-plugs & wires w/a spay to see if they're hot (with the key "left" in the pre-glow position) - but no heat.
When the motor's stone-cold, the glo-indicator light seems to yet always work properly. But even then I still get a definate smokey startup, and suspect I've a g-plug or two that are bad. But even at that, sometimes still, the pre-glow light comes on when the engines very warm.
Nonetheless, the times the glo-indicator light does not come on, I've also noted that if I just "hit the start key" anyway & turn the motor over, the glo-indicator then lights up and the motor fires up given about 10 seconds of tuning over (obviously just about long enough for the glo plugs to heat up).
As far as I know, all my g-plugs are still OEM. But also, the car began starting a bit hard and smokey just a few months ago. I of course suspected that a g-plur or two was farting out on me. Perhaps I've now hit "critical mass?"
So I guess I'm responding because my experience re: the glo-indicator light flashing when hitting the start key is exactly as you describe your situation. I thought perhaps some of my experience may help with a clue or two for your circumstance?
Nonetheless, based on your notes and the explanation I read from Diesel Giant, I'll try and find the old-style glo-plugs to replace all 5 of mine, which may take a larger energy-draw (?). If this works for me, I'll try to get back to this thread and let you know... Dale

PS: I/we would be glad to hear from the "Experts" if y'all got other insights, or better suggestions... Thanks
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post #6 of 6 (permalink) Old 03-12-2007, 10:37 PM Thread Starter
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Still havent figured out the glow plug problem. Only thing I can think of is the new-style pencil glow plugs pull more amps than the gp relay is willing to put out. Shes not a show car, so I think I might hardwire the g plugs through a solenoid switch and a push button and treat it like a starter. The positive wire for the gp indicator light is right beside the gp feed on first gp, so the I can wire it so that light will come one when push the button. I would not do this if mine was as nice as yours!

Last edited by Phylarch; 03-15-2007 at 07:25 AM.
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