Ignition coil and ballast

Page may contain affiliate links. Please see terms for details.

Canned Benz

Member
Joined
Oct 8, 2011
Messages
59
Location
West Sussex
Car
1975 280CE (w114)
After three years of ownership I think I've finally got to the bottom of why I keep having to change my dizzy cap, rotor arm and points every year. When looking into ignition coil upgrades I realised that my w114 280CE has been fitted with a Bosch blue coil. The blue coil is a 12v coil with an internal ballast resistor. My car has two external ballast resistors designed for a 9v coil. So I've been giving my 12v ignition coil a 9v feed resulting in a weak spark causing the points, cap and rotor to burn out.

Rather than doing away with the ballast resistors I've decided to order a Bosch red coil, which is a 9v coil designed to use external ballast resistors. I've read the red coil should have 1.8 ohms of ballast resistance. My car is currently fitted with two 0.4 ohm ballast resistors, the ones with a blue clamp.

My question is are these resistors connected in series? In which case do I need two 0.9 ohm ballast resistors (the ones with the gold clamp)? Or are they for two different purposes? In which case do I need one 1.8 ohm ballast resistor (the one with the red clamp) for the coil and to leave one of the 0.4 ohm resistors where it is?

Sorry it's long winded. Any help would be much appreciated. I think anyone with experience of an m110 fitted with D-jetronic could be able to help.
 
If you have a voltmeter and you can access the terminals on the ballasts you can check if the two ballasts are in series.

If in series on one ballast you should get
On one ballast approx battery voltage on one terminal and a lower (say 10.5V) on the other terminal.
On the other ballast the same lower reading (say 10.5v) on one terminal and a lower voltage (say 9V) on the other terminal. This lowest voltage will be the same voltage as the coil.

If they are in parallel, both of the ballasts should have similar voltages to the other on each side - e.g. around 12V one side and a lower (say 9V) on the other side.
 
Had a look today. They are definitely connected in series. I have 12v in to the first resistor and 8v coming out, then 8v in to the next resistor and 4v out, and finally 4v at the coil.

The Bosch red coil turned up today and definitely states that it requires 1.8 ohms of external resistance.

So next question, in order to keep the wiring as it is, can I just replace my two 0.4ohm ballast resistors for two 0.9ohm ballast resistors? Not sure if that's how ohms work!
 
Simple answer is yes.
Make sure that the wattage is correct And remember they can get a bit warm.
 
Fitted the Bosch red coil and the two 0.9 ohm resistors. The car barely started and ran terribly. I had about 4v at the ignition coil.

I kept the red coil on but swapped the 0.4 ohm resistors back in. That gave me about 8.5 volts at the coil. Car ran better but misfires badly when set to the correct 7 degrees BTDC and idles at about 1000rpm instead of 800. To get it to idle without misfire I had to advance the timing to 13 degrees BTDC which upped the idle to 1100rpm.

Took it on a few half hour road tests and found it has a lack of power and misfires begin after about 5 minutes of driving and get progressively worse.

My theory is that the coil isn't getting the right voltage. Has anyone got and ideas on resistor combinations that might solve this? They come in 0.2, 0.4, 0.6, 0.9 and 1.8 I'm also thinking of getting an electronic distributor to get rid of the points and condenser and just in case the old dizzy's cam lobes are worn flat (mileage is 140,000).

Here is the one I'm looking at.
123ignition electronic ignition systems for classic cars

Any other ideas or suggestions would be much appreciated.
 
This suggests you have a poor connection somewhere in the low tension ignition circuit. Possibly your ignition switch. Voltage measurement can be funny. With zero current flow you will see nominal voltage but send any appreciable current through a system then poor connections start to heat up and gooble voltage across them. My first move would be to rig up an independant 12 volt supply from the battery to the supply end of the ballast resistors to see what kind of voltage you get across them and the coil in series and try that.
ps have you checked the condenser in the distributor and the distributor internal earth connection braid inside that earths the points?
 
A quick reply as I'm waiting to pick up the girls from the airport.
It doesn't already have resistive wire to the coils does it?
Usually this wire is braided.
 
For a Bosch red coil to need 1.8 ohms of external ballast resistance implies that the coils primary resistance is 5.4 ohms. That just doesn't sound right as the coil would pull little more than 2 amps. Even the old black coils pulled twice as much current. I'm not surprised it would barely run with 1.8 ohms in series.

Is it possible they meant 0.18 ohms. That would make a lot more sense as high performance coils usually have lower primary resistance than standard ones and therefore must use a lower resistance ballast. It's the ratio of primary resistance to ballast resistance that makes a 9 volt coil work on 12 volts when starting and 9 volts when running. What you really need to know is what the primary resistance of the Red coil is then you can calculate the correct ballast resistance. Without looking it up I'd guess it must be less than 2 ohms.
 
Just another thought maybe the retailers of Bosch coils are clueless and the figure of 1.8 ohms is not the ballast resistance but the actual primary resistance of the coil itself. That also makes a lot more sense and would mean you need a ballast resistance of 0.6 ohms.
 
Thanks for the response guys. First thing I'll do is measure the coils primary resistance and go from there. Hopefully I'm just using the wrong Ballast resistors.

I've never changed the condenser but I don't think that would cause such a severe misfire. All the wiring is in very good condition and I don't believe any are resistor wires.
 
Right I've measured the coil. I'll say it as I read it so as not to confuse the results. The primary resistance with the multi-meter switched to the 200 setting is 1.5 and the secondary resistance with the multi-meter switched to the 20k setting is 13.95
 
Right I've measured the coil. I'll say it as I read it so as not to confuse the results. The primary resistance with the multi-meter switched to the 200 setting is 1.5 and the secondary resistance with the multi-meter switched to the 20k setting is 13.95

They sound typical. Measuring low resistance values accurately is not at all easy so there may be some error but if we assume 1.5 ohms is correct then you need a 0.5 ohm ballast resistor.

If it was me I might be tempted to go with 0.4 ohms if I already had the resistor and keep an eye on the coil to see how warm it gets. You could always reduce the dwell a touch by increasing the points gaps slightly to reduce coil heating (if it's points ignition that is).

Also I would avoid an 'ignition on but engine not running' scenario which can overheat the coil.
 
Yeah I left the probes on for a good 30 seconds to let the readout settle. Ok I'll temporarily wire it up so that it is using a single 0.4 resistor and see how it is. If that works I'll get two 0.2 resistors to keep the original wiring intact. Thanks for the help man.
 
Honestly , none of this means anything to me .. But ...

This is old school car fixing , and hats off to all of you figuring it out.

Impressive stuff :thumb:

Hope you get it sorted.
 
I don't have direct knowledge of the Merc system but I recall (either in my old Porsche 928 or in an even older Volvo with D-Jetronic) a dual resisitor system. The resistors were used during starting the car to increase the voltage at the coil offsetting the voltage drop due to the starter cranking. They were pretty heavy duty ceramic deals - Maybe the same thing?
 
Last edited:
Right------- its possible your problems stem from a problem with the transistor ignition unit.
http://mb.bolinko.org/wis/w114/Engine/110/07_5-519.pdf
contains a useful circuit diagram

The test procedure is listed here.
http://mb.bolinko.org/wis/w114/Engine/110/07_5-520.pdf

If I have read this correctly the original coil primary resistance should be 0.35-0.45 ohms
and secondary 8-11 kohms

after reading this it becomes clearer. COILS and BALLASTS.
Its confusing because the two external ballast resistors have slightly different functions One [0.6ohm ] is in constant series with the coil- in effect its part of the coil's overall resistance. The 0.4 ohm resistance is what most of us regard as a "ballast resistor" that is one that's in circuit in "run" but switched out during starting to compensate for voltage drop.

some useful info here also but on aftermarket systems
http://www.hot-spark.com/Installing-Hot-Spark-Bosch.pdf

what emerges that to chose the correct coil its important to understand the circuit its going into?
 
Last edited:

Users who are viewing this thread

Back
Top Bottom