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Electronic ignition - shouldn't work but it does - Why??

  • Thread starter Thread starter maclariz
  • Start date Start date
M

maclariz

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My heap of junk, sorry almost complete GS400 came with a Boyer Bransden electronic ignition setup. Now I was having problems with intermittent misfiring on one cylinder which seemed to be due to a coil on the way out. Bought two ignition coils which were supposed to have the desired resistance of 3 ohms, but turned out to be 2.2 ohms (according to my multimeter). The Boyer Bransden instructions say that the resistance of the coils should be betwee 3 and 4 ohms and if a coil with a lower resistance is used then a ballast resistor should be mounted in series.

If I put a 1 ohm resistor in series the spark is quite weak and the blasted thing won't fire reliably. If I take the ballast resistor out, then the thing fires okay and ticks over sensibly.

Questions:
  1. Why do I need the ballast resistor?
    • To reduce the current through the electronic ignition box and to protect it from burning out.
    • Or more to do with time constants on the LR circuit through the coil.
  2. Will I damage the electronic ignition by running with a 2.2 ohm resistance coil and not the specified 3 ohms?
Hope someone who understands these things comes along shortly.
 
Usually ballast resistors are used for current limiting as 1(a) not 1(b). Issue 2 I have no idea, depends on the current rating of the ei's output transistors.
 
Have you checked the resistance on the 2.2 ohm coils? I have checked resistance on newly purchased 3 ohm coils and have yet to have a pair that were exactly 3 ohms. Usually, they turn out to be about 3.5 to 4 ohm.
Its plausible that measured resistance on a rated 2.2 ohm set could actually be closer to 3 ohms.

Earl
 
earlfor said:
Have you checked the resistance on the 2.2 ohm coils? I have checked resistance on newly purchased 3 ohm coils and have yet to have a pair that were exactly 3 ohms. Usually, they turn out to be about 3.5 to 4 ohm.
Its plausible that measured resistance on a rated 2.2 ohm set could actually be closer to 3 ohms.

Earl
Click to expand...

The 2.2 ohms is measured from my multimeter.
 
You may be close to 3 ohms if you include all of the wiring / connection resistances without the ballast resistor.

You may have a voltage drop issue from the battery to the + side of the coils. Check the voltage from there reference the battery -. it should be pretty close to what you get right at the battery.

You could put two 1 ohm resistors in paralell for each coil which would give you 0.5 ohms and try that.
 
Well Iv'e tried to edit my response twice and after I type all this in, it says I'm not logged in, but I had to be to be allowed to open it for revision???? WTF

1a Yes
1b No
2 It may shorten its life if you have < 3.0 ohms from the battery through the ign. sw. down to the electronic breaker (right at the breaker). I doubt that is the case. If you had < 3.0 ohms through this path and the engine stopped with a breaker closed and the ign was left on, your electronic breaker may heat up.

Now after typing all this for the third time we'll see if it will take.
 
In this case it should be fine, I would not worry about it. The electronic ignition is a transistor that switches the coils off and on. If you used low resistance coils with mechanical points the contacts would wear faster and heat more. The solid state switches in electronic ignition can take more abuse without physical wear. They may run a little hotter but the specifications are high enough that you are not going to burn them out. If you add the resistor you will end up with 8 volts across the coil instead of 12, since the 1 ohm resistor will take 1 third of the available voltage ( actually 1/3 of 14 volts when running.) and this will cause all kinds of problems. ballast resistors were necessary on some coils to limit current flow but not in this case. Kirckoffs law states the sum of the voltage drops are equal to the applied voltage. You only have 12 volts, if you have a 1 ohm and 2 ohm resistor your going to drop 4 across the 1 ohm resistor and 8 across the last 2 ohms.

Long and short of it is don't worry about it. If you come across the "correct" coils someday swap them out for peace of mind.
 
Thanks for the reassurance guys. I'll run it as is for the time being and maybe fix things later.
 
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