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77 280E, 78 450SEL, 86 300E, 87 300E, 90 300E, 92 400E & 4 scooters
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233 Posts
Discussion Starter #1
What's wrong with my electrical system on my 1978 300D non turbo.

Some times when I turn the ignition key on the glow plug light illuminates bright BUT my three idiot lights on the far left are very dim. AND GET THIS, when this happens my throttle cable starts to glow RED HOT!

What the heck is going on here? How could all the current be directed to the throttle cable?

Obviously something is shorting out. Where do I start to look?

Could a bad glow plug relay cause this?

The battery is in great shape and I did clean the ground comming off the battery negative.

But some times it works fine. When the three idiot lights on the far left illuminate bright I know it's not shorting and it'll fire right up.

HELP ME IF YOU CAN!

thanks guys
 

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77 280E, 78 450SEL, 86 300E, 87 300E, 90 300E, 92 400E & 4 scooters
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233 Posts
Discussion Starter #2
Also, I noticed that if I pump the foot accelerator when this "shorting" condition occurs that the glow plug light flashed dim,bright,dim. As if the accelerator cable is hitting some wire going to the glow plugs.

Inspecting under the hood I don't see wires shorting. Could be between firewall and under dash some place?
 

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2003 Volvo V70 5 Speed
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1,784 Posts
Our cars have throttle linkage (no cable) which makes me think you might be looking at the wavy resistance wire that connect the old loop style glow plugs you have. Could you show us a picture of the part that is glowing red?
 

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77 280E, 78 450SEL, 86 300E, 87 300E, 90 300E, 92 400E & 4 scooters
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233 Posts
Discussion Starter #4
It's the throttle cable I swear. I'm going right now to work to take a piccture. And then try and post it here.
 

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77 280E, 78 450SEL, 86 300E, 87 300E, 90 300E, 92 400E & 4 scooters
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233 Posts
Discussion Starter #5
White arrow pointing out cable that gets red hot





 

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77 280E, 78 450SEL, 86 300E, 87 300E, 90 300E, 92 400E & 4 scooters
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233 Posts
Discussion Starter #6
Could it be that I don't have a good ground from glow plug #1 to the engine block?
 

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'89 300TE, '79 450SL, '01 ML320
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1,051 Posts
Check your chassis to engine ground strap, I'm not sure where it's at on a '78. Hope someone else can tell you.



Fish
 

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77 280E, 78 450SEL, 86 300E, 87 300E, 90 300E, 92 400E & 4 scooters
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233 Posts
Discussion Starter #9
Fish, good point, that makes perfect sence.

I'll find it for sure. Unfortunatly I'll have to work in my unheated garage, and it's 18 deg F outside :(

I'll clean every single ground I can find on the 300D just for good measure.
 

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2003 Volvo V70 5 Speed
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1,784 Posts
Ok, thats the idle control knob cable. I'd have a look under the dash to see if there is a wire coming off the relay that is shorting against it.
 

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77 280E, 78 450SEL, 86 300E, 87 300E, 90 300E, 92 400E & 4 scooters
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233 Posts
Discussion Starter #11
Dude99, that's where I was figuring would be the SNAFU. I'll locate the GP relay and and idle control thing cable under there and give a good inspection.

A short under there should be obvious to find if there is one. Once again I'll be in the drivers seat sitting upside-down with a flashlight in my mouth. That must be a mandatory position for old Benz geeks that are too cheap to pay an indy.

But first the engine to chassis ground clean-up. Much easier to work on your back while gunk falls in your face : )
 

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'89 300TE, '79 450SL, '01 ML320
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1,051 Posts
Try looking on the passenger side for a heavy cable or strap going from the chassis rail to a
trans/bell housing bolt in the area of the starter. They do decay, you're prolly pulling your
ground thru that idle control cable which is too small for the amperage that the glow plugs
pull. I would think anything under the dash that could be shorted would be fused... pulling
enough amps to light up that cable would surely blow a fuse.



Fish
 

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1980 280ce
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135 Posts
If you suspect that there's current going through that cable that is making it hot, you could easily just test the theory with a voltmeter.
 

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77 280E, 78 450SEL, 86 300E, 87 300E, 90 300E, 92 400E & 4 scooters
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233 Posts
Discussion Starter #14
Fish, I think your spot-on! Thats why I love this forum, over 100 years of expieriance at our finger tips.

Pidgy999, I'll for sure test for current first with my multimeter. That way I can pretend I know what I'm doing.

You guys are all so smart.
 

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77 280E, 78 450SEL, 86 300E, 87 300E, 90 300E, 92 400E & 4 scooters
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233 Posts
Discussion Starter #15
Fish, You are the official winner of the "fix my #@&#" award!

Replaced the ground cable from the bell housing to the chassis and BINGO!

2 points off as the ground is on the drivers side.

BUT, your award, well, I'm burnin diesel fuel once again :)

Thank you everyone for your help.
 

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'89 300TE, '79 450SL, '01 ML320
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1,051 Posts
Glad you're back up and running, now we all know what side it's on :)



Fish
 
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