| Author | Message | ||
     Patrick Lockyer. Grand Master Username: pat_lockyer Post Number: 447 Registered: 9-2004 |
First the ballast resister bypassed lets the coil have full battery voltage while the engine is cranking. However the main purpose of the ballast resister is to compensate for the variation in primary circut current flow caused by changes in engine speed. At low engine speeds average current flow is near maximem,this results in a considerable votage drop across the resister permitting only eight to nine volts to be applied to the coil windings. At higher engine speeds average current flow and the resulting voltage drop of the resister decreases,now this then allows a higher voltage to be applied to the coil primmary at higher speeds, more voltage for higher speeds balances the primary and results in a consistently high available voltage from the coil secondary through out the normal speed range of the engine. Don't always be fooled into thinking petrol starvation is the cause when pulling away from the lights when the engine seems a little hesitant as the engine revs clime or untill a hill presents itself with the engine power dying with missfire and comming to a stop. If let to cool down the car will start and run as if all is well untill the ballast heats up and breaks down again. Why does this happen, well it seems the resister coil itself on the Shadows never break down it is the coil connections to the lucars that fail with corrosion when hot. leave to cool down and most will start and run till hot. Test for 1.3 ohms or replace every five to eight years to be on the safe side. It is the same for the opus electronic distributor setup. Maybe many fuel pumps and opus ignition systems could be blamed for this little rascal.  Pix of failing ballast still working | ||
| John Kilkenny New User Username: john_kilkenny Post Number: 4 Registered: 6-2005 |
To add to Pat’s comments on this interesting subject, though there are different opinions about ballast resistors they were most likely initially to improve starting, particularly in very cold weather when the battery voltage may be nine volts or less. Cars with the battery far from the starter motor exacerbate this problem. Early Shadows did not have a ballast resistor and starting can be a problem though a low internal resistance battery such as the Optima can help. There are two main disadvantages with using points - the current that can be switched is limited to about 4 amps, and the time represented by the dwell angle decreases as the revs increase. The diagram below shows the ignition waveforms of a Shadow at idle (600 RPM) and 3000 RPM, with the assumption (for the purpose of the argument) that it takes about 7 M Sec to reach saturation, that is when the current reaches a maximum. At idle, the dwell angle of 35 degrees represents about 19 M Secs, much longer than the time required to saturate the coil (and a waste of power). At 3000 RPM the 35 degree dwell angle provides only 4 Msec of current, not enough to fully saturate the coil, so the spark will not be as good. The only way this situation can be improved is for the rate of current increase through the coil to be speeded up. This is dependent on L/R where L is the Inductance of the coil and R is the resistance of the circuit. By using a series (ballast) resistor and a lower voltage coil, R can be increased. Additionally the initial voltage applied to the coil will be 12 volt rather than the lesser steady state voltage, which will also assist current build up. Because electronic ignition does not have these limitations, and starter motor design has greatly improved, most modern cars do not need ballast resistors. Pat’s comment about a bad ballast resistor connector causing mis-firing could be due to the bad connection effectively increasing the value of the ballast resistor.While this would reduce the field buildup time, it would also reduce the coil current, producing a poorer spark. Alternatively it could be due to intermittent interruption of the coil current.  | ||
| OhJohnny Unregistered guest Posted From: 68.121.162.114 |
So, the long and short of it? My Bentley T, SBX2442 is misbehaving at low speeds, particularly from a dead stop. The engine coughs and sputters and on occasion suceeds in dying in the middle of the intersection, much to the annoyance of the other innocent motorists. I have suspected carb issues, and am preparing for a go at them, but is this ballast resistor a possible culprit? Do I have one; I don't see one, do I need one? Thanks for your input. OhJohnny (Message approved by david_gore) | ||
| Bill Coburn Moderator Username: bill_coburn Post Number: 540 Registered: 4-2003 |
No Johnny no. You don't have one. I would be checking the pumps first, blocked filters and dirty float bowls. Replace the needle valves and clean the carburetter pistons. | ||
| John Kilkenny New User Username: john_kilkenny Post Number: 5 Registered: 6-2005 |
As well as Bill's suggestions, check that there is oil in the carburreter damper reservoirs. They should be topped up with engine oil to half an inch from the top of the tube. | ||
| Patrick Lockyer. Grand Master Username: pat_lockyer Post Number: 456 Registered: 9-2004 |
Could be the condenser or coil breaking down if this is happening after warm up. The dash pots if empty will still run ok with just the slight sneeeze when throttling up,unless the carbs are out of ballance,this can be by useing a colour tune when attempting to set the carbs up with other incorrect factors. | ||
| bobuk Unregistered guest Posted From: 62.253.64.18 |
Also the ballast does protect the coil and points from over heating should the ig be left on with the engine running and points closed. as the ballast warms up the resistance increase and the volts across the coil drops and the amps going throu the points drops. Because the time for the coil to build up a spark gets shorter as the engine revs rise this tends to limit power and revs at the top end and in the case of the shadow after 4500 revs the sparks can't really cope. This gives 4 cylinder engines 9000 revs and 6s 6750 revs. I have found that electronic switched sparks allows another 1000 revs on an 8. But 4500 is ample for the Shadow so I will leave mine as it is. (Message approved by david_gore) | ||
| Patrick Lockyer. Grand Master Username: pat_lockyer Post Number: 462 Registered: 9-2004 |
Just to clarify on John Kilkenny posting about electronic ignition greatly improved on most modern cars do not need a ballast resisters. This is correct but not on most electronic Shadows as they still run with the old coil ballast setup. | ||
| John Kilkenny New User Username: john_kilkenny Post Number: 7 Registered: 6-2005 |
The other important factor is the greatly improved efficiency of starter motors. If the electronic Shadows were still using the old Lucas starter motors I can well believe they need ballast resistors. | ||
| Patrick Lockyer. Grand Master Username: pat_lockyer Post Number: 463 Registered: 9-2004 |
The ballast resisters main purpose is for the compensation for variation of the primary voltage with engine speed as stated above. Cranking volts of less than nine volts check for the problem and there can be many, do not fit a new battery unless proved to be faulty. The lucas starter motors are bullet proof and only give poblems after many years of service. The new type motors take less drain but have been known to fail in service. | ||
| John Kilkenny New User Username: john_kilkenny Post Number: 8 Registered: 6-2005 |
I don't believe ballast resistors were used to speed up coil saturation (though they do have some effect, mainly by initially applying an over-voltage to the coil at the start of dwell time) and I would be interested to see any documentation to this effect. Surely their main purpose is to provide easier starting by being shorted out of the circuit when the starter motor is energised. In older cars, high current drain by the starter motor can drop the voltage available to the coil to ten volts or less, and even less in very cold weather. By using a six volt coil (for example) with a resistance of 1.5 ohms in series with a ballast resistor of 1.5 ohms the coil will receive a standard 4 Amps during normal running. When the battery voltage drops during starting, the shorting out of the ballast resistor applies an overvoltage to the coil for a short time, producing a more intense spark. By the same token it is unlikely that a ballast resistor is used to protect a coil from overheating if a car is left with the ignition switched on. In this condition the resistor will dissipate around 24 Watts, not enough to substantially increase its resistance. Under normal running conditions the duty cycle is around 76 per cent or up to 18 Watts dissipation which is not much different. Early Shadows did not have a ballast resistor. Their starter motor was a lighter version drawing 220 Amps for a 6.7 lb.ft torque at 1000 RPM. Later starter motors had a 14.0 lb.ft torque drawing 525 Amps. Though I have no evidence to support it I suspect this is when the ballast resistor was introduced. I was not questioning the reliability of the Lucas starter motors used in Shadows, however they are very inefficient in terms of power usage. | ||
| Richard Treacy Grand Master Username: richard_treacy Post Number: 916 Registered: 4-2003 |
A lot has been written about the purpose of the ballast resistor, and surely John's waveform sketches show it all. The ignition circuit is a tuned LRC circuit. The coil impedance, the net circuit resistance, and the capacitor are sized to give a smooth spark of a suitable duration over the required frequency range. Pumping square waves into a pure inductance cannot achieve this, hence the capacitor and added resistance of the ballast. It will not work properly or reliably over its frequency range of 4Hz to 400Hz without a resistor. Also, as John points out, the bonus of an external resistor can conveniently be short-circuited briefly, as it is on a Shadow so-equipped, to compensate for low cranking voltages. It is not there for protection, to reduce heat dissipation or anything else other than to complement the points capacitor and the coil inductance. Most of the reasons given for its inclusion are really just notes on the effects of a faulty or shorted ballast resistor. For example, the resistor is not there to prevent points arcing or coil overheating. However, it is true that the coil will overheat and the points will burn if you have no ballast in the circuit. If the vehicle does not have an external ballast resistor, it will have one built in to the coil instead. | ||
| Patrick Lockyer. Grand Master Username: pat_lockyer Post Number: 465 Registered: 9-2004 |
John, the early ignition system before car number 8742 had two sets of contacts now what does that tell you and why was the ballast resister introduced do i need to say more?. Trip to the atic to check one of your statement Quote from the book on something in all my years in the motor trade was new to me you said :the shorting out of the ballast resistor applies an overvoltage to the coil for a short time. First i must make it quite clear that you understand that the resistor is bypassed during engine cranking,this allows the lower voltage coil to receive the full battery volts for the short cranking duration only. | ||
| John Kilkenny New User Username: john_kilkenny Post Number: 9 Registered: 6-2005 |
Pat, I'm sure you know that the reason for dual points is to enable the dwell time to be more stable and contact bounce and floating to be minimised. This does not mean that removing the second set of points makes an external ballast resistor necessary. The coil can easily be designed to suit. I am not saying that a series resistor is un-necessary but that its purpose is to limit coil current and can in fact be inside the coil. Also my choice of words in writing 'overvoltage' during cranking was not the best. Perhaps I should have used 'adequate voltage'. And your last sentence is superfluous if you read my post. I think we will have to agree to differ on the reason for external coil ballast resistors. Regards, John | ||
| Patrick Lockyer. Grand Master Username: pat_lockyer Post Number: 467 Registered: 9-2004 |
John please tell me what 8 cylinder convensional not electronic assisted ignition system has just one set of contacts without the ballast system. Please tell me what conventional ht coil has an internal ballast with the three conections or whatever and what V8 car is it fitted. What publication was your diagram from. I will then get back to you on the reasons RR moved away from the two contact system to the single contact system useing the the ballast as the main purpose of improvement in greater detail. Also the publication stateing the confirmed main purpose of a ballast resister refering to my first posting. | ||
| John Kilkenny New User Username: john_kilkenny Post Number: 10 Registered: 6-2005 |
Patrick, I'm sure you know a lot more about V8 engines than I do but the service manual diagrams TSD 2481 show dual distributor points plus ballast resistor for cars 5001-6000. The original Lucas HA12 coil was replaced by the Lucas HA7 (presumably 7 volt) Also the only person who has mentioned coils with three connections has been you. | ||
| Patrick Lockyer. Grand Master Username: pat_lockyer Post Number: 468 Registered: 9-2004 |
John. To clariefy my post on your comformation of an internal ballast within the coil,i am lost to find a conventinal one. With three connectors or whatever. Please confirm what coil that is and on what vehicle. As i aways like to learn something new whatever, how about the publication of your diagram before i post my publication on my stated facts. | ||
| John Kilkenny Experienced User Username: john_kilkenny Post Number: 11 Registered: 6-2005 |
Patrick, I think you should re-read some of my posts. My diagram is not from a publication but merely uses the theory of charging an inductor to show how it applies to the Shadow ignition system. It certainly is not controversial. Because I only wanted to show the effect that reducing the dwell time has on the coil saturation level, the diagram is fairly simple. For example it does not show the back e.m.f. or the actual spark waveform which is rather more complex than a simple square pulse. The dwell time given is for a dual points distributor. There are certainly differing opinions on ballast resistors as you will see if look around the internet. For a good basic article that covers general theory, ballast resistors, dual points and so on, look at http://www.jetav8r.com/Vision/Ignition/cdi.pdf (It's about 1 Megabyte) However your 'stated facts' should explain why Rolls Royce changed from a standard 12 volt coil with a dual points distributor and no external resistor, to a 7 volt coil with an external resistor, still using the dual points, with the added facility of shorting out the resistor during cranking. Remember it was not until later that a single points distributor was used. If you can't do that, your theory has the same value as anyone's. We've probably bored everyone enough already. | ||
| Patrick Lockyer. Grand Master Username: pat_lockyer Post Number: 469 Registered: 9-2004 |
John. I hope we are not boring folk. First i must make it clear we are talking most Shadows 1 and 2 and Bentleys T1 T2.and not boats or m/cyles From my first posting i thought it was most clear. But to sumerise some of your posts regarding the above cars. First posting you stated electronic ignition does not have these limatations,and starter motor design has greatly improved,most modern cars do not need ballast resistors. We are tallking shadows here so my reply no 462 was needed as we all know that modern cars are another ball game. Your posting no 8 you said i don't believe ballast resisters were used to speed up coil saturation though they do have some effect,who said they did i din't. Read my post no 465 that was to trigger your mistaken words on the above not mine. The reason for the early two contact system running on a eight lobe dist shaft,well that was for coil saturation ,more stable dwell you say no way not in practice, that was there downfall, the slightest shaft wear and the dwell etc was all over the place. The time for the ballast system had arrived. Hope i make it clear at higher engine speeds with the ballast higher voltage is fed to the coil primary: more volts from the secondary mean consistenly high volts through the speed range. As i have said the ballast is by passed not short circuited at all on starting,we don't want folk getting that wrong. Still waiting for the details on the coil with the internal ballast, beats me? | ||
| Patrick Lockyer. Grand Master Username: pat_lockyer Post Number: 470 Registered: 9-2004 |
To give a brief workings of the distributor with two sets of contacts on how the distributor allows longer "build up" period for the primary circit. When the contacts are closed this time will decrease with the increased number of cylinders and the faster engine speed but the fitting of the two sets of contacts allows this incovenence to beovercome. The two sets are staggered in relation to the eight lobes in the distributor cam. the overlapping gives a longer closed time. This is what gives the greater primary circut saturation and as the contacts are in parralel the build up will start as soon as one set are closed but the ingnition arc will not accur untill both are open. The design is such that the position of the contacts are arranged that the second set open when the first set are already closing. This gives COIL SATURATION. Good in therory but very troublsome in practice. Hence the Ballast system with single contacts as stated "the main purpose of them". | ||
| John Kilkenny Experienced User Username: john_kilkenny Post Number: 12 Registered: 6-2005 |
Pat, I'm afraid we are on two different planets so I shan't be commenting further. However it's an interesting subject so I would encourage forum members to have a look at other opinions. A good start is to type 'Kettering Ignition' into Google. Also a short technical article is at http://home.earthlink.net/~jimlux/hv/igncoil.htm |
