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False ignition parts? and consequenses for ignition spark

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04 Nov 2015 10:49 #2201
Hello D-Jet friends,

After replacing my distributor in my W116 280se, I started to check the partnumbers for all ignition parts.
I checked the resistances for the blocks and saw that I had 2 times 0.4 Ohm installed, where it should be a 0.4 and a 0.6 Ohm block. Also my coil, of which I cant read a partnumber, states that it has a 1.8 Ohm coil. Where, if I am correct, it should be a coil with a resistance between 0.38 and 0.43 Ohm.

The car runs well, surprisingly, but what is the difference in spark with my parts compared to the parts that should be in my car?

Kind Regards, Sergio

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04 Nov 2015 11:23 #2202
Hi Sergio,

the task of an ignition coil is to store energy that will then be forwarded to the spark plugs by the distribution finger. Power is a multiple of voltage and current. In the 60s Bosch typically used ignition coils with 1.8 Ohm resistor. It would reduce the primary voltage on the ignition coil and create a secondary ignition voltage of around 20 kV. Main limiting factor in those years was the ignition contac that would not be able to carry too much current. In the late 60s Bosch started with transistor ignitions. A transistor can carry more current than a contact with burn-off. So they could increase the primary and therefore also secondary voltage on an ignition coil. This was done by reducing the reisitors to 0,4 and 0,6 OHms, making 1 Ohm total. This increased the energy stored in the igntion coil. They called it a high performance coild and used blue color to differentiate. Nowadys you cannot rely on those colors any more. Later with TSZ-8 ignition conttrol module in the 80s they omitted the resistors completely and also switched the ignition coil off if the engine was not running.

Now in your case you operae a traditional ignition coil with too low a resistor: 0,8 Ohm instead of 1.8 Ohm..That increases primary voltage on your coil heavily and therefore also the energy stored and run over distribution finger and spark plugs. It helps your car to start better, but at a cost. Your ignition coil is continuously overloaaded and it will burn. Also your finger will burn quicker.

So my clear advice: Buy a 0 221 122 001 ignition coil and a 0.6 Ohm resistor 0 227 901 013. Both are still available and you will find offers on the Internet. Your ignition coil is one from a Heckflosse / fin tail Mercedes.

Viele Schraubergrüße - best regards, Dr-DJet Volker
Mercedes-Benz 107 SL/C in der SLpedia Sternzeit 107

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04 Nov 2015 11:55 #2203
Thank you for your quick answer Volker.
Reading through your answer, I think you mixed up something or I didnt make things clear enough.
You write about :
quote => Now in your case you operae a traditional ignition coil with too low a resistor: 0,8 Ohm instead of 1.8 Ohm

My ignition coil is 1.8 Ohm. The resistors together are 0.8 Ohm.

Could it be that you meant to say " 0.8 Ohm instead of 1.0 Ohm " at the end of that sentence?

Best Regards,

Sergio

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04 Nov 2015 12:43 #2204
Hi Sergio,

if it says 1.8 Ohm on your ignition coil it does not mean that it has 1.8 Ohm. It rathers points out the correct pre-resistor to use on12 V systems. And that is a coil as it was typical in pre-transistor ignitions. I doubt that your ignition coild has 1.8 Ohm.

Best regards,
Volker

Viele Schraubergrüße - best regards, Dr-DJet Volker
Mercedes-Benz 107 SL/C in der SLpedia Sternzeit 107

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04 Nov 2015 12:52 #2205
Hi Volker,

I dont know either. It is just that on the coil itself is stated 1.8 Ohm. No further info to discover on it though.

So if I understand correctly, when using the coil I have, the ultimate resistance of the 2 blocks together should be 1.8 Ohm. Just hypothetically.

As I stated in my opening post, I understood from searching the internet, the original blue coil for my car is a 0.38-0.43 Ohm coil. Am I right? And if I am right, why isnt it a 1 Ohm coil, to match the original amount of 1 Ohm of the pre-resistors to be fitted alongside?

Best Regards,
Sergio

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04 Nov 2015 13:05 #2206
Hi Sergio,

no your coil needs an 1.8 Ohm pre-resistor. I do not know its own resistance by heart. But on the blue coil it is 0.4 Ohm internal plus 0.6 plus 0.4 Ohm makes 1.4 Ohm totally. So according to that it will run on a voltage of 0.4 / 1.4 * 12 V = 3.5 V on primary winding. When starting the 0,4 Ohm pre-resistor is bridged and then we have a total resistance of 0.4 + 0.6 = 1 Ohm. This is meant to compensate voltage drop when the starter runs.

So your current ignition coild demands a 1.8 Ohm preresistor and you only have 0.8 Ohm there. That is why it will have a higher primary (and secondary) voltage than desired. Higher primary voltage also draws more current and thus overloads the coil. It can burn due to that.

Viele Schraubergrüße - best regards, Dr-DJet Volker
Mercedes-Benz 107 SL/C in der SLpedia Sternzeit 107

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04 Nov 2015 18:04 #2207
Hi Volker,

Thank for your reply.
Ok, I am starting to understand the way to look at it.

Best Regards,

Sergio

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