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  #1  
Old 11-03-2003, 07:31 PM
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Coil resistance

What should the resistance of my coil measure?
I have 1 spare I grabbed at the junkyard that measures about 0.9-1.0 ohms. (Bosch # 0 221 122 001, marked "730" and "KW 12 V). It seems to work perfectly on my car.
I have a much newer, "nicer" Bosch coil that measures 3.2-3.3 ohms. It doesn't work that well on my car. (Bosch # 0 221 119 027, marked "926" and "050" (in a circle), as well as "K 12 V").
I don't know what my current coil is - I'll have to disconnect it and check later.

What should the resistance be? (I assume it's in the m117 manual I dont have.) If I took out (or bypassed) my ballast resistors, would the "Newer" coil be a better choice? (I would tend to believe so). Please help!

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  #2  
Old 11-05-2003, 05:32 PM
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*bump* nobody knows? Is it even in the engine book?
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  #3  
Old 11-06-2003, 07:10 AM
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A coil that is intended to be used with a ballast resistor has approximately half the resistance of a non-ballast coil. So your 'nice' coil is probably a non-ballast coil.

The reason for using a coil and ballast set up is to give you a stronger spark at start up. The ballast coil is essentially a 6 volt coil and the ballast is there to give it a 6 volt supply in normal running. When you start the engine, the ballast is bypassed and the full 12 volts is applied to the coil giving roughly twice the output voltage. This helps cold starting.

When you use your non-ballst coil in this set up, you will only get half the usual voltage in normal running. This will give you a greater chance of a misfire.

The conclusion here is that if you vehicle is designed to use a coil and ballast, and it is in good working order, swapping this for a non-ballast will give you the same normal running performance but poorer cold starting.

Note: the voltages may not be exact, but it gives you an idea of how it works.
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  #4  
Old 11-06-2003, 04:16 PM
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Well, it makes sense that it runs like crap, missing a LOT, when I used it w/the resistors there.
I was hoping that, since it was newer, it'd give better spark under load though.
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  #5  
Old 11-06-2003, 08:14 PM
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Tom:

Coil resistance for the MB coil is 0.38 to 0.43 ohm across the primary winding. Don't know what the secondary should be.

Don't remove or bypass the ballast resistors or use a different coil, you may fry the switchbox, and they are almost impossible to find now.

Peter
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  #6  
Old 11-06-2003, 08:19 PM
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Both coils I have (one on there now, and other spare) both are 0.8-1.0 ohms, from terminal 1 to 15.
The main reason actually is that, Peter - the ballast resistors are 0.4 and 0.6 ohms, right? So my system has a mere 2 ohms resistance. I'm on my THIRD ignition box. (First went about 10 yrs ago, 2nd went and we parked it for 5 yrs until about 1 yr ago). I don't want this $5 ignition box to die on me (that's how much I paid for it at the junkyard!). I figured an extra ohm of resistance in my system (removing ballast resistors, replacing w/other coil) would add life and remove stress from my ignition box...
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  #7  
Old 11-06-2003, 08:32 PM
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Tom:

No, what it will do is reduce the current in the coil to the point you don't get enough spark to reliably fire the plugs under load. The spark is produced by the collapse of the magnetic field produced by current flow in the primary winding -- the secondary winding is conneted to the top spark lead and has about 1000 windings to each winding of the primary (so it produces 12,000 V instead of 12). Simple transformer. However, sufficient current has to flow in the primary to make the magnetic field first, so if you have too much resistance, the output voltage and current will be low, hence bad spark.

I'd consider a Pertronix or Crane replacement -- I'm going to spring for a Pertronix breakerless point replacement this month, I'm tired of cleaning the points all the time, and when the switchbox croaks, I'm going to put a newer design in.

The usual reason the switchbox dies is the big switching transistor, so if you still have one of the old ones, I'd take it apart and go to Radio Shack and get a replacement transistor. Someone posted the part number on the forum last year, do a search and you may have a "new" switchbox for pretty cheap! There's only about six parts in there, soldered to a board.

Peter
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  #8  
Old 11-06-2003, 09:02 PM
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No, what I mean is taking the ballast resistors out (bypassing them) and putting the coil with 3.2 ohms of resistance means I have a system (box -> coil (3.2) -> box) resistance of 3.2 ohms vs 2 (box -> resistor (0.4?) -> coil (1) -> resistor (0.6?) -> box). that would produce the same spark if the coil is working properly, possibly a stronger spark since it's a newer, "Super blue" coil (The only resistance is the coil itself) but with less wear+tear on the transistor unit (since more resistance = less amps the box has to use to fire the coil - V=IR, V is 12 volts, if R goes up then I, current, goes down - 2 ohms would require 6 amps per spark, 3.2 would require 3.75 amps per spark).
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  #9  
Old 11-06-2003, 11:03 PM
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Tom:

Still too much resistance -- the factory is 1.0 ohm plus the resistance of the coil (0.4 ohm), not 3.2. Your version of the transistor system grounds the coil via the switching unit, so be carefull. The ballast resistors are both between battery voltage and the switchbox through the coil, so total resistance is only 1.4 ohm -- the switchbox resistance to ground is minimal when the points are closed.

Best bet is a good MB coil of the correct resistance -- cheapo parts will usually get you into trouble with a Benz, and the coils off later MBs use a different switching system, the resistance won't be the same.

Or replace the entire system with a Crane optical pickup/capacitive discharge coil or Pertronix Hall Effect pickup and switching unit and coil.

Peter
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1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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  #10  
Old 11-06-2003, 11:37 PM
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My ballast resistor is bypassed...

But I went with the Crane electronic ignition, which required using a different coil with specific instructions to bypass the resistor.

Car runs like a gem, no problems at all with cold start.
Especially with the new timing chaing and valve
adjustment .

Now if I can just get that exhaust leak fixed...

PS - I'll second the post about cheapo parts getting you into trouble - go with the OEM/Benz, especially for ignition.
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  #11  
Old 11-07-2003, 12:07 AM
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As far as I know, the coil is original, and measures ~0.8-0.9 ohms, not 0.4. This is across terminals 1 and 15... that is the only way I could see how you would measure it, unless I'm mistaken?
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  #12  
Old 11-07-2003, 06:52 PM
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Resistance should be measured from 1 to 15, that's correct, but you can also measure from the secondary (high voltage) to the ground terminal to check that winding as well.

It is a blue coil? The direct point coils are not the same as the transistor switched ones, and I don't know when MB changed over. Some time in the late 60's or early 70's. Earlier coils may have too much resistance for use with the transistor setup.

Peter
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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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  #13  
Old 11-07-2003, 08:46 PM
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All 3 coils are blue coils. I do indeed have a transistor ignition, as well.

Does sweitching to a crane or petronix unit solve the issue of a bad mechanical advance in the distributor?
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  #14  
Old 11-08-2003, 10:10 AM
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Tom:

No, the switch to Crane or Pertronix only eliminates the points. I'm having to clean mine almost every time I drive the car, a bit PITA from oil getting on them

May pull the distributor and clean it again.

Mechanical advance is usually stuck from corrosion between the top and bottom shafts from not being oiled properly (there is an oil pad under the rotor, supposed to get a couple drops of oil each oil change or so). Only way to proplerly fix it is get another one or take it apart and clean it proplery. This is somewhat difficult, you need a press to drive the retainer pin out of the drive gear!

Peter
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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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  #15  
Old 11-08-2003, 03:54 PM
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Peter: Do you have the protective plastic cover on the points (it comes with the Bosch points - the odd-looking white plastic, it slips on the stationary contact)? If so, why do you keep getting oil on them? I don't see why, unless you have an oil leak somewhere - mine are perfectly fine and I haven't touched them since I put them on.

My mechanical advance has been funky since I first checked it out - right now, it seems like the springs are too weak and A: don't pull it back all the way, and B: advance it too much at lower RPMs and don't advance enough at higher (since, at idle, it's probably halfway advanced, and timing is set at 5*, probably with it already advancing it some 5*).

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