Ballast Resistor

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Dave Pegg

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I have a Mk 1 Commando 850 and have noticed a white/light purple wire, which when traced, is attached to the ballast resistor the other end could possibly go to the ignition switch, (it's dangling in that kind of position) , I've owned the bike for 3 years and assume it hasn't been connected, the bike is on points and has 6 volt coils.
Does anyone know where it should be connected and what damage it could cause if left disconnected
 
Dave Pegg said:
I have a Mk 1 Commando 850 and have noticed a white/light purple wire, which when traced, is attached to the ballast resistor the other end could possibly go to the ignition switch,
Click to expand...

Do not connect it to the ignition switch. There would normally be a white/red 'WR' wire with it and that should not be connected to anything either as both wires are unused.

Dave Pegg said:
I've owned the bike for 3 years and assume it hasn't been connected, the bike is on points and has 6 volt coils.
Click to expand...

Therefore, nothing to be concerned about. As it has points then you should also find white/purple 'WP' connected between the ballast resistor and both ignition coil negative terminals although not marked as such on the factory wiring diagram. The previously mentioned WP and WR wires in the area of the battery compartment are also not on the wiring diagram.

Ballast Resistor
 
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The resistor is there to reduce wear on the coils/points. Virtually every vehicle with points used a ballast resistor. Not sure why anyone would disconnect it...
 
So its purpose is to reduce 12v down to 6v running to the coil and contact breaker points as far as I understand. Running breakers at 12v works, but may incur more rapid carbon build up or erosion of the points contacts. Running 6v coils in parallel at 6 v input means the bike should continue to start fine if battery 12volts drop below normal for whatever reason (cold/sulphation etc).
 
MexicoMike said:
The resistor is there to reduce wear on the coils/points. Virtually every vehicle with points used a ballast resistor. Not sure why anyone would disconnect it...
Click to expand...
The points capacitor reduces wear at the points, the ballast resistor changes the voltage at the coils. The purpose of the 6V coils was in preparation for the failed electric start, the plan was for the solenoid to remove the ballast resistor out of circuit when the starter button was pressed, so full battery voltage (dropped by the starter motor draw) was applied to the coils giving a hot spark. This scheme was also used on the redesigned starter motor scheme fitted to the MK3. As soon as the starter button is released the ballast resistor comes back into circuit, the extra voltage for a few seconds does not harm the 6V coils.
 
Thanks everyone for your advice and replies, I can safely say to leave wiring as it is, bike was/is running fine, just slightly concerned it should be plugged in somewhere !!
 
"The points capacitor reduces wear at the points, the ballast resistor changes the voltage at the coils."

True of course, the cap is there to reduce the arcing at the points and the ballast resistor is reducing the voltage across the entire combination of coils/points. Interesting that the OP, by not having the ballast resistor in the circuit SHOULD expect to see reduced life of the downstream components but it doesn't sound like that is the case.

Since the ballast resistor is a constant draw on the electrical system when the bike is running, I would think that if the operational life of the components is not reduced by the higher voltage, it would be better all around NOT to have the ballast resistor in the circuit if you are running the OEM points/AAU. Battery charging would likely improve. Or so it strikes me...
 
MexicoMike said:
Interesting that the OP, by not having the ballast resistor in the circuit SHOULD expect to see reduced life of the downstream components but it doesn't sound like that is the case.
Click to expand...

The ballast resistor is in the circuit. The WP wire the OP was referring to is the unused ballast resistor bypass (see kommando's previous post).
 
Aha, thanks! I posted that because a couple of years ago, out of curiosity, I temporarily removed my Trispark and installed the OEM Points/AAU/ballast resistor. The first thing I noticed was that battery charging voltage was MUCH lower - battery was discharging until nearly 2k RPM. With the Trispark (no ballast resistor) the battery would charge at 1100 RPM. So it appears the ballast resistor itself was a major draw-down in available charging voltage. IOW, with the conventional ignition system, the charging system met the "generally poor" results that have always been aimed at lucas systems of that era. But remove the resistor and the SAME charging system is transformed!

On the other side of the equation - I liked the performance of the bike better with the OEM ignition; it felt a bit sharper on acceleration than the Trispark, and the TS was considerably better than the Boyer that was in the bike when I bought it.
 
MexicoMike said:
So it appears the ballast resistor itself was a major draw-down in available charging voltage.
Click to expand...
Nope. A ballast resistor is about 2 ohms, the same as a 6 volt coil. So the resistor plus one coil is 4 ohms, the same as 2 coils in series. The current drawn by either connection scheme would be the same. If you saw lower voltage using points, then I would attribute that to the longer dwell time of contact breakers vs EI.
 
Well that sounds good but the charging voltage went up when the ballast resistor was taken out of the circuit (bypassed). Not claiming I understand; just relating what happened.

Also, the ballast resistor in the circuit gets quite warm - ignition on, bike running or not running. The heat indicates (to me, anyway) that considerable power is in use. So what am I not understanding re the setup?
 
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Why are you using two 6v coils in series ? I use one 12 volt Japanese double-ended coil off a Honda CB750. I think the usual Commando setup is also 'wasted spark '. The extra wiring with two coils means more connection points which can become faulty.
 
acotrel said:
Why are you using two 6v coils in series ?
Click to expand...
Who?
Wasted spark electronic ignitions generally make use of the original (71-on) 6V coils but connected in series instead of in parallel.

acotrel said:
I use one 12 volt Japanese double-ended coil off a Honda CB750.
Click to expand...

A 12V double-ended coil can also be used (instead of two 6V coils connected in series).

acotrel said:
I think the usual Commando setup is also 'wasted spark '.
Click to expand...

The original points ignition isn't 'wasted spark'.
 
MexicoMike said:
Well that sounds good but the charging voltage went up when the ballast resistor was taken out of the circuit (bypassed). Not claiming I understand; just relating what happened.

Also, the ballast resistor in the circuit gets quite warm - ignition on, bike running or not running. The heat indicates (to me, anyway) that considerable power is in use. So what am I not understanding re the setup?
Click to expand...
Charging voltage would be a function of current & time (power) all other things being equal. I'm not sure what you mean by "bypassed", but the resistor in series with a coil wouldn't draw more current than 2 coils in series.

Power generates heat. Even though the duty cycle of a points ignition is low, for the time that the points are closed if we're drawing 3 amps (12v/4 ohms) the dissipation of the ballast is 3² * 2 = 18 watts. When the engine is off, if the points cam stops with one set of points closed the resistor will get mighty hot.
 
I'm sure @maylar will quite rightly correct my schoolboy math, but I have always seen points and electronic ignition as roughly equivalent in terms of electrical overhead on the bike.


Each 6 volt coil is around 1.8 ohms
The original ballast resistor was around 2 ohms

So for the 160 degrees when the points are closed, you are drawing around 3.7 amps for each coil.

With two coils and two points sets in play, that means you are drawing 3.7 amps for 320 degrees (assuming 14 volts from the battery)
52 watts at 89% of each rotation



The dwell of most electronic ignitions is up in the 80% region

So with two 1.8 ohm coils in series the numbers are not a million miles away:
4.4 amps for 290 degrees (again, assuming 14 volts from the battery)
61 watts at 80% of each rotation



Remember this is camshaft rotation.
The crankshaft rotates twice to every one rotation of the camshaft.

So half the percentages if you want to reflect crank rotation instead.
 
Not arguing with you guys re theory of operation! :eek:

As I said, there was a 1 volt reduction in charging voltage with the OEM ignition system vs the Trispark system. I ASSumed it was the ballast resistor that caused that (assuming that since it get's hot, it "absorbs power") but, if I understand correctly the points and E-ignition pull approximately the same power. There was no change to the charging system components when I swapped the Trispark/points/AAU. So what is causing the charging voltage to be a volt or so lower with the OEM system vs the E-ignition at the same engine RPM? o_O
 
MexicoMike said:
So what is causing the charging voltage to be a volt or so lower with the OEM system vs the E-ignition at the same engine RPM? o_O
Click to expand...
Honestly, I don't have an answer for that. Your observation implies that the OEM system is drawing more power from the battery than the EI does. If @gforce calculations are correct, there should be very little difference between them. I've never put a 'scope on one to verify this, but I was under the assumption that the "on" time of an EI is shorter than with contact breakers. That would explain the phenomenon since the average power consumption would be greater with points. It could also explain better performance at high RPM since there's more spark energy available. If my assumption is incorrect, then it's a mystery to me.
 
Well...it's really more an intellectual exercise, I guess, since the "test" with the OEM system was just out of curiosity and I reinstalled the Trispark a few days later. But I had always been under the impression that it took less power to operate the OEM since it seems pretty common to hear about how the old Boyers acted up/wouldn't start with low voltage when the points systems would start with no trouble at the same "low" voltage.
 
MexicoMike said:
But I had always been under the impression that it took less power to operate the OEM since it seems pretty common to hear about how the old Boyers acted up/wouldn't start with low voltage when the points systems would start with no trouble at the same "low" voltage.
Click to expand...

But that's because the original points ignition continues to operate down to a lower voltage than EI.
 
I hadn't thought about that but you are right, needing more voltage and needing more power (watts) are different things.
 
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