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1986 W126 300sel Euro
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15 Posts
Discussion Starter #1
Hi everyone, I'm new to the forum but have been a follower for several years and had a lot of help from various threads.

I have a w126 300sel 1986 European model. It has done over 200k km and has been my daily driver for the past 6 years.

Few months back, I was dropping my son at college and on my way back it just died on a stop light. It was downhill so I barely managed to park it roadside. After a minute or two, I cranked it up and it started as if there was no problem at all.

Thereafter the lockdown began and the car stayed parked for some time. I started it few times to keep it running without any problems.

Last week I had to take it out and after about 3 km it died and didn't even sputter.
I towed it home, tried to troubleshoot, while asked my son to crank the engine, the ignition coil cracked with a crisp sound after two or three times he tried to start the engine.

I had a spare one, I replaced spark plugs, inspected CPS it read 850 ohms, tried to start the engine and in the third attempt, the second ignition coil heated up and cracked.

There are 12.5 volts at the coil with ignition on. Any help will be great before I put in another ignition coil.

Haroon
 

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1986 W126 300sel Euro
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15 Posts
Discussion Starter #2
Hi again. Sorry I missed out on the other details.
Rotor, cap and HT wires are in good shape, fairly new. Replaced them last year and have hardly done 10k KM. Fried another coil trying to check for spark from coil, while grounding spark plug, but no spark. Swapped ezl as well but nothing.
 

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1989 300SE 232k miles
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1,448 Posts
Wow. My first instinct when it stopped at a light was the fuel pump relay. But that wouldn’t be the rest.

I researched real quick. Bad alternator. But that would have a higher voltage at the coil?? Bad spark plug wires. And I expect the electrical guys will chime in on a bad ground by the radiator.
 

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1986 W126 300sel Euro
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15 Posts
Discussion Starter #4
I did check ezl ground, it's working fine. Will try to find and check the ground near the radiator, to which you have referred and report back.
 

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1989 300SE 232k miles
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1,448 Posts
Two thoughts: 1). Crank position Sensor. This will give bad info to the EZL.

2) Is it possible you’re leaking oil into the distributor cap? And that’s causing a short or something?
 

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1986 W126 300sel Euro
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15 Posts
Discussion Starter #6
Thanks for the suggestion. The distributor is dry no oil or moisture at all.

Will try to get may hands on a new or running sensor and check it out as well. But a grounded spark plug attached to the distributor should give spark, just a thought.

Instead the coil heats up so much and cracks. Yesterday I got myself an oil cooled coil to troubleshoot the problem, as I don't want to loose another OEM coil, after two/three attempts it gets hot and I disconnect everything and wait for it to cool down. It has not broken and still good, tested it on another car and ohm meter as well.

I've attached picture of busted coils. May be someone can help me out. Haven't had a problem like this in my some 35 years of car experience.
15928010639186993200216236660450.jpg



15928012080256172519358273167571.jpg
 

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1986 W126 300sel Euro
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15 Posts
Discussion Starter #9
Thank you , I will check this. Disconnect the ezl grips and try to start the car or just keep it at on position? Or may be both
 

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1990 420 SEC
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380 Posts
Key on should be enough. Problem with short circuits is that they may come and go as wiring harness is moved.
 

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1986 W126 300sel Euro
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15 Posts
Discussion Starter #11
Hi there. I did what you advised. Disconnected the grips from the ezl and switched the ignition on. The coil heated up .....
So there are two grips on the ezl. One does to sensors and the other has two wires connected to the coil on terminals one and fifteen. Third one goes to TD and fourth is connected to ground.
I've opened the harness mounted on top of the engine. Many wires are damaged and cracked.

But I understand that the issue will be only on one harness that is attached to the coil. Or may be the wiring coming from the ignition switch.

I will embark on that journey soon and post updates. Please do share if you think of anything else to help me out.
 

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1986 W126 300sel Euro
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15 Posts
Discussion Starter #12
Or I can rule out ezl harness simply by removing the connections from the coil and repeating the process. If the coil heats up then the problem lies with the other two wires, one provides current from the switch and the second one goes to the diagnostic plug.
 

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1990 420 SEC
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380 Posts
A coil is usually ground side switched. That means other pole has battery voltage any time key is on. The other pole is connected to ground by EZL for a very short period to make spark. Haven't checked the diagram but battery positive does not come to coil via EZL I believe.
 

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1986 W126 300sel Euro
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15 Posts
Discussion Starter #14
I probably was not able to explain properly.
You are right. Wire from switch provides power to coil at terminal 15. A connector from ezl harness is also attached to the same terminal which provides power to ezl.
Now two wires are connected to terminal 1 of the coil. One goes to diagnostic plug and other goes to pin 16 on ezl.
Third wire from the same harness is ground and fourth to TD.
 

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1986 W126 300sel Euro
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Discussion Starter #17
I'm back again. Took some time. I disconnected ezl connector from terminal 1 but the coil heated up. So I removed the ezl connector from terminal 15. The coil again heated up.

Then I removed the second connector from terminal 1. Now there was only one connector attached to terminal 15 of the coil that provides power to coil. This connector has two wires one goes to diagnostic plug. I switched the ignition on .... Coil still heated up. Something is missing

What should be the voltage on that connection?

I'm thinking........ Can't think right now
 

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1990 420 SEC
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380 Posts
So only one of the primary side terminals of the coil is connected to something and it still heats up? If so then it is fried.
 
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