Testing a coil

[wwheeler]

Is there anyway to test a coil to determine what ballast resistor is required for a 280SE engine? I am buying a Bremi 11821 coil which SHOULD use the .9ohm resistor. But info is scarce and I want to confirm. I am using the 051 dizzy with points. The old Bosch coils are near impossible to find for a reasonable amount. I think the .9 ohm would be the black coil.

Thanks.

Wallace 

[mdsalemi]

Is there anyway to test a coil to determine what ballast resistor is required for a 280SE engine? I am buying a Bremi 11821 coil which SHOULD use the .9ohm resistor. But info is scarce and I want to confirm. I am using the 051 dizzy with points. The old Bosch coils are near impossible to find for a reasonable amount. I think the .9 ohm would be the black coil.

Well, here is how to TEST a coil: https://youtu.be/rJYJ3KvPhhY?si=2jXVIVieWI-fpPMc
Note, it doesn't actually test the high voltage, just that it is high enough to make a spark.
If you have the wrong ballast surely you'll burn up the points in no time....right?

[wwheeler]

Interesting and thanks for posting. Since the original Bosch coils for our cars are scarce as hen's teeth (red, blue, black and tan), you have to find an appropriate aftermarket coil. Most do not publish what ohm ballast resistor to use. How then are you supposed to know what coil to buy and what ballast resistor to use? This is of course for using a points distributor.   

[mdsalemi]

Interesting and thanks for posting. Since the original Bosch coils for our cars are scarce as hen's teeth (red, blue, black and tan), you have to find an appropriate aftermarket coil. Most do not publish what ohm ballast resistor to use. How then are you supposed to know what coil to buy and what ballast resistor to use? This is of course for using a points distributor.

After burning one set of points early on (2000) with the wrong setup, (while still in the hands of the restorer) I went with an electronic ignition and didn't think anything of it. That was the Crane XR700, which worked fine for a number of years until it didn't.

In 2011 it was replaced, also without a whole lot of thought (though highly praised by Jim Villers, whom I respect immensely) with the Pertronix. It was placed with just a tiny amount of trouble, but once in and timed--it has not been touched since.

I should add that just prior to installing the Pertronix, the -051 was fully rebuilt by Glenn Ring of Long Island NY, so I started with a great condition distributor. As far as I can tell, Glenn is no longer doing these rebuilds. Parts were getting scarce even back 10 years ago and there doesn't seem to be his restoration page any longer. That leave Dan Caron (Dr. Benz) the only one I know of rebuilding Bosch distributors. Along with the Pertronix, I installed their Flamethrower II coil according to their suggestions and directions. It's a classic black in color.

 "Originality" in some cases is vastly over-rated especially if it relates to runnability, reliability or safety for me. If and when this distributor fails, it's a 123 for me.

[BobH]

This has got to be one of the most confusing subjects there are on a car, and it should be quite simple.  The more you read, the more confused you get, especially around the coil colours, it seems to depend on what colour black you search for

From what i understand a points system should have between 3 and 4.5 ohms in the primary circuit, give or take a bit. 

The original red coil windings, from what i've read are around 1.8 ohms, plus the 1.8 ohm ballast = 3.6 ohms

I've also read that the original Bosch black coil for our cars have 3.4 ohms, plus 0.9 ohms ballast = 4.3 ohms

The Bremi 11821 has 1.8 ohms, according to Bremi.  If used with the original 0.9 ohms ballast = 2.7 ohms, so a tad low, although i don't know if this would have any significant affect.  Perhaps the 1.8 ohm ballast would be a safer bet to protect the points and the coil?

[wwheeler]

Thanks Bob, this helps! Yes very confusing and not much info out there. So what you are saying is that the primary circuit needs to have between 3 to 4.5 ohm to protect the points. The closer you get to 3 ohms, the less protection for the points and coil but also a hotter spark. Correct so far?

Also new info to me is how the resistance is added to coil - coil resistance + ballast resistor = resistance in the primary circuit. I did not find any info on the Bremi nor did they respond to my question. So thank you for that. Where did you see the info? But what you are saying is that the Bremi 11821 is basically the same as the Bosch red coil?   

So the goal is to get the primary resistance between 3 and 4.5 ohms. They publish that the red coil gives 70% hotter spark, but that must be ONLY during starting? The original coil had a higher resistance that resulted in lower starting and running spark. But points would last a longer time. Good so far?

So using the Bremi and the .9 ohm resistor, I am pushing the envelope on the side of a hotter spark. Only 2.7 ohms resistance. I do have the 1.8 ohm resistor and maybe switch between that and the .9 ohm resistor. I have a distributor testing machine, so if I burn the points, changing them is a breeze. See attached.

My original issue was pitted points only after 350 miles and two years of driving. I thought having the red coil and the 1.8 ohm resistor may have been too much for the points. But I think I found the culprit and a loose screw on the point plate that also holds down the ground wire. Sounds like I was barking up the wrong tree blaming the "hotter" red coil. 

Your thoughts are greatly appreciated!

[BobH]

Hello, i'm no expert, and only guessing and surmising from what i've read, once you discard some of the tosh that's written.  The lower the resistance in the primary circuit, the higher the current, which may result in a better spark, but the coil may run warmer than designed and the points will not last, although that's not such a great deal, unless they fail when you're on a trip

Bremi confirmed the coil value when i asked them, but i'm only relaying what i've read about the red and original black coil values, when you search for the black coil, it throws up loads of different information

Seems most later coils don't need a ballast, due to cars having electronic ignition.  The problem with using these on cars with points, is that you can't then bypass the ballast (cos there isn't one!) if you need to, for better spark when starting.  When running you'll see 14 odd volts, but this could drop considerably, depending on lots of variables, as low as perhaps 8 volts, maybe less, when cranking

The annoying thing is that these comparison charts, which list alternatives, are not very accurate, the Bremi 11821 says it's a replacement for the original Bosch black coil, but if the Bosch is in fact 3.4 ohms, (to be proven) then it's almost twice the value of the Bremi, hardly a direct replacement without some fettling

Perhaps, as you say swap between the 0.9 and 1.8 ballast, and strike a compromise

Please let us know how you get on, it would be good to have some facts about replacement coils in the manual

[wwheeler]

My goal is to put my points type system back to stock. Not supercharge the spark. Back when the coils and parts were new, it was easy. Now that they are very hard to find and expensive, not so easy. And so many people have moved away from points, info is even harder to find. The reason I got looking into the Bremi 11821 is because of Niemöller. They listed this coil as the sub for the black coil and to be used with a .9 ohm ballast resistor. Sounded good enough. When I went looking for more info, Zilcho. I did find this on a Bimmer site about the Bosch coils though. See attached. Not sure how applicable this is to Mercedes of the era.

So the ballast resistor along with the coli's internal resistance keeps the points from burning. But the ballast resistor also keeps the coil running cooler. 123 recommends the red coil AND bypassing the ballast resistor. So my question is - how does the red coil keep from overheating if the ballast resistor is bypassed with a 123? I had a 123 dizzy a few years ago and bypassed the coil. Not soon after that a friend recommended that I do not bypass in order to preserve the coil. Now his recommendation makes sense what based on what we are talking about.

[BobH]

i believe transistorised ignition systems limit the voltage or current to the coil, so less voltage and less current means less heat, so the internal resistance of the coil may be adequate without an external ballast resistor, that's how i understand it

Seems plenty of owners have used the 123 on our cars without a ballast and haven't looked back

Just to add, i've checked the 123 website and it recommends a coil with at least 1.0 ohm primary, and no ballast.  At 12 volts this would draw 12A, and the coil would get very, very hot, so the 123 system limits the current to around 6A.  This was specific to a different engine than ours, but the principle should be the same.  The 123 will take into account the resistance of the coil and adjust the current accordingly

The points system using a 0.9ohms ballast and the Bremi coil giving a total of 2.7ohms would draw just under 4.5A, so may be ok without changing to the 1.8ohms ballast

[wwheeler]

That makes sense that the 123 has to have an internal mechanism to limit the amperage draw. The 123 is a popular item.

Thanks again.

[BobH]

But what you are saying is that the Bremi 11821 is basically the same as the Bosch red coil?   

So the goal is to get the primary resistance between 3 and 4.5 ohms. They publish that the red coil gives 70% hotter spark, but that must be ONLY during starting? The original coil had a higher resistance that resulted in lower starting and running spark. But points would last a longer time. Good so far?

The Bremi coil won't replicate the red coil, it has different characteristics.  The output of the red coil is twice that of the original black coil.  It's not the current of the primary circuit that dictates the strength of the spark, it's the secondary windings of the coil.  I don't know the output rating of the Bremi, but would assume it's close to the original black coil, around 13KV?  i believe the red coil is 26KV

So, the primary side of the circuit includes the ballast resistor (if fitted), the points and the coil windings, too high a current will affect the longevity of the points and may cause the coil to run hot, the ideal is to produce sufficient voltage and current to start and run the car without prematurely aging the components

The output or HT side of the coil is produced by induced voltage.  When the primary circuit is energised a magnetic field is produced inside the coil. When the points open, the field collapses, causing a high voltage, which is induced into the secondary coil windings.  The effect of the points opening and closing causes the magnetic field to build and collapse, the voltage induced into the secondary windings depends on the amount of windings, specified by the output power, in the case of the red coil, 26,000 volts odd, but with very little current, only milliamps, hence why the leads don't get hot, and need to have as little resistance as possible, which will have an effect on the spark strength

[wwheeler]

I figured it wasn't as easy as I stated, but I didn't know the rest. Good to know and thanks. I do have the Bremi coil installed, .9 ohm resistor with fresh points and a condenser. Everything is adjusted on the Sun Machine and ready to go. Ill let you know how it turns out.

Having the Sun dizzy tester is one of the reasons I went back to points. Having that, it is stupid easy to test the dizzy for wear, dwell, both advances and can even check spark quality if you add the cap and plugs. I also have a dual point dizzy in my Roadrunner and those are really difficult to adjust correctly without a machine. Plus the machine looks super cool and kind of fun to mess around with. Win/win.

Thanks again.