C200 ignition coils mystery

Jan Greeff

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Bela Bela, South Africa
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C200 1997
My 1997 C200 started running on three cylinders. There was oil on no. 2 spark plug from a leaking plug tube seal so I replaced the seals and cleaned the oil, hoping that that would be it.

The car still ran on two cylinders. I tried to find the cause, changed the coils and plugs around etc but then it started ro run on two cylinders. One ignition coil tested faulty so I replaces it, but now it is running on three again, seems both coils were damaged.

What mystifies me is the fact that, according to the wiring diagram, two plus fire simultaneously from each coil, there is continuity between the two plug connector outlets but only one plug fires. Surely if one fires the other should too?

Spark plugs, leads and connectors are all fine and the feed to the coils is also OK, so why does each coil only fire on one cylinder? The one coil works on the plug direcly under its mounting and is dead on the other plug, the other coil is exactly the opposite.

I have orderer the second new coil, but the mystery remains. Another mystery is why both coils at the same time?
 

television

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2002 SL500, 216 CL500, all fully loaded
Sadly these coil packs do not like being moved around much as they get older, and the coil packs are well known to fail, strange really some fail all the time and others go on for ever
 

sixpack

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2012 w212 220
Coils

Hi Jan, most 4 cylinder / 2 coil engines work on the "wasted spark" principle. The coils secondary is floating, in that each end is connected to a spark plug, i.e. one coil feeds 2 plugs. When the coil fires, both plugs spark. The plug on the end of its compression cycle fires, producing power, the other plugs fires, I think during its exhaust cycle, doing nothing.

Peter
 
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Jan Greeff

Jan Greeff

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C200 1997
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Sorry I don't know what PMS or HFM is.

Thanks Sixpack, fair enough but how come only one plug fires via a coil that is supposed to fire both and tests show that there is continiuty between the two outlets?
 

Some guy on the internet

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A few.
Sorry I don't know what PMS or HFM is.

The different type of ignition control modules found on early W202s.

Here, PMS were dropped around '96 /'97 in favour of the less troublesome & more reliable HFM method, but you might still have one - don't know about SA cars. These units sometimes fail & result in the symptoms you've described.

If you have one it'll be under the thinnest bit of the washer reservoir bottle, mounted to the inner wheel arch.
 

sixpack

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2012 w212 220
Plugs

Hi Jan, if the insulation between part of the winding to earth was breaking down under high voltage its possible that only one plug would fire. Hard to test but you could try reducing the plug gap in the hope that the smaller gap would fire before the "fault" gap in the coil. Peter
 

grober

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W204 C200cdi estate
Check you are getting the full battery voltage to the positive low tension coil supplies [ derived from ignition activated CIRCUIT 15 - so ignition on ] The system operates by getting the coils primary supply switched to earth at the appropriate time via the engine ECU
 
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